The Preparation of Nickel

At the beginning of the nineteenth century, P. Berthier, J. B. Bichter, and O.L. Erdmann prepared the nickel in the laboratory by heating nickel oxide or carbonate, alone, or mixed with charcoal, or made into a stiff paste with oil, with or without a layer of glass, in a covered crucible, and at the temp, of a blast-furnace. The nickel dissolves a small quantity of carbon. If the temp, be kept as low as possible, a grey, porous mass of nickel is produced with very little carbon. J. von Liebig and F. Wohler, A. Laurent, G. Charpy, G. Magnus, G. Grassi, W. N. Ipateeff, and C. Klinzel obtained nickel in an analogous manner. E. W. von Siemens and J. G. Halske reduced the oxide with carbon in an electric furnace. E. D. Clarke observed that nickel oxide is reduced to the metal in the oxyhydrogen flame. Nickel oxide is also reduced by heating it in a current of hydrogen, and, according to W. Miiller, a suboxide - vide infra - is formed at 194°, and the reduction is complete at 270°. M. Mayer and V. Altmayer showed that nickel reduced in hydrogen absorbs much gas - vide infra. W. N. Ipateeff found that soln. of nickel salts are reduced to the metal when heated under press, in hydrogen. Thus, with hydrogen at 100 atm. Press. 0.2N-NiSO₄ deposits nickel at 186°, and of a 0.22V-soln. of nickel acetate, at 168°. J. von Liebig and F. Wohler, and A. Laurent reduced the oxide by heating it in a current of carbon monoxide; and O. L. Bell noted that at a high temp, much carbon is formed.

J. W. Dobereiner, and J. J. Berzelius prepared the metal by heating to redness nickel oxalate, or ammonium nickel oxalate under a layer of powdered glass free from heavy metal. E. Peligot, P. Schiitzenberger, and H. Bose obtained lustrous crystals of nickel by heating nickel chloride in a current of hydrogen; and F. Vorster by heating the chloride in a current of dry ammonia. L. Mond and co-workers, and M. Berthelot prepared the metal by heating nickel carbonyl to 200°. A. C. Becquerel found that copper in a soln. of sodium nickel chloride precipitates nickel; Z. Roussin, that magnesium precipitates nickel from slightly acidified soln. of nickel salts; A. Commaille, that magnesium precipitates nickel from soln. of nickel sulphate; and A. Merry, and J. L. Davis, that nickel is precipitated by zinc from ammoniacal soln. of its salts. H. Moissan obtained the metal by distilling nickel-amalgam in a current of hydrogen. A. Damour found that zinc amalgam precipitates nickel from neutral soln. of zinc salts, and forms an amalgam. C. Meme, and F. Stolba found that a soln. of nickel sulphate and zinc chloride deposits nickel on iron in contact with zinc; but F. M. Baoult observed no deposit of nickel from acidic or neutral, hot or cold soln. of nickel salts in contact with a nickel-gold couple. For the production of nickel from nickel carbonyl, vide supra. H. E. Fierz and H. A. Prager, A. E. van Arkel, and C. Miiller and W. Schubardt used this process.

Nickel is deposited on the cathode during the electrolysis of ammoniacal, neutral, or acidic soln. of its salts, and, as shown by B. Bottger, J. M. Merrick, and C. Winkler, the metal so obtained has a high degree of purity, but, as shown by J. M. Merrick, it may be contaminated with oxides.

In 1842, B. Bottger showed that dense and lustrous deposits of nickel could be obtained by the electrolysis of a soln. of nickel and ammonium sulphate, or of an ammoniacal soln. of nickel sulphate. He also emphasized many valuable qualities of the nickel deposits, which attracted attention only after B. Bottger's work had been forgotten, and the facts discovered anew. In 1868, W. H. Remington patented a process for the electrodeposition of nickel; in 1869, I. Adams patented the use of nickel ammonium sulphate as an electrolyte in conjunction with cast nickel anodes; and in 1871, N. S. Keith patented the use of tartrates - e.g., Bochelle salt - in the electrolytic bath. At ordinary temp., the use of a current density of about 5 amp. per sq. ft. was considered to give the best results. The corresponding rate of deposition was comparatively slow. The use of higher temp, hastened the rate of deposition, but this was considered inconvenient under manufacturing conditions. In order to produce a higher concentration of nickel in the bath than is possible with ammonium nickel sulphate, which has a relatively low solubility, as pointed out by A. Brochet, a portion of the complex salt was replaced by the simple salt nickel sulphate. In 1878, E. Weston showed that the ammonium sulphate could be advantageously omitted if boric acid be added along with nickel sulphate. A higher rate of anodic corrosion was then needed to enable higher rates of deposition to be employed. W. D. Bancroft pointed out that a relatively small proportion of a soluble chloride in the bath would greatly improve anodic corrosion, and electrolytes containing nickel sulphate, boric acid, and a soluble chloride came into favour. In 1916, O. P. Watts found that with soln. containing per litre 240 grms. of nickel sulphate, 20 grms. of boric acid and 20 grms. of nickel chloride gave good deposits with a current density of 200 amps, per sq. ft., and under commercial conditions, 50 to 100 amps, per sq. ft. The subject was studied by V. I. Lainer and co-workers, N. V. Hybinette, C. C. Downie, R. L. Suhl and co-workers, W. A. Mudge, A. H. W. Aten and co-workers, G. A. Guess, N. R. Laban, and C. G. Fink and F. A. Rohrman discussed the conditions favourable to the preparation of pure, electrolytic nickel.

Although the equilibrium potential of nickel in N-NiSO₄ at room temp, is about -0.25 volt, a large excess of anodic polarization is necessary to inaugurate the dissolution Ni+2⁺→Ni^••. Thus, E. P. Schoch found a nickel anode in N-NiSO₄ at 26° dissolves only at current densities below 0.036 amp. per sq. dm. If this value be exceeded, the anode becomes passive, and oxygen is evolved when the potential is about +0.28 volt, i.e., an excess polarization of 0.53 volt. The passivity decreases as the temp, is raised or in the presence of Cl'-ions or H^•-ions. Hence, by raising the temp., or in acidic soln., or soln. with a soluble chloride, a nickel anode can be subjected to greater current densities without the anode assuming the passive state. As shown by O. W. Brown, the potentials.at the anode vary with the physical state of the metal; thus electrolytic nickel requires a high potential, and cast nickel with its surface roughened dissolves more readily than any other form of anode. Wrought iron of uneven structure may yield slimes containing undissolved metal. Since the equilibrium potential of nickel in N-NiSO₄ is –0.26 volt, nickel cannot be deposited from soln. containing even moderate quantities of free acid. The work of E. P. Schoch, and A. Schweitzer shows that the hydrogen overvoltage of nickel at room temp, is at least 0.2 volt, but this is counterbalanced by the fact that the slowness of the reaction Ni^••→Ni+2⁺ at the cathode necessitates a higher cathodic polarization than corresponds with the equilibrium value. N. A. Isgarisheff and C. M. Ravikovitsch found that the cathode polarization with nickel chloride is affected by the addition of various chlorides in the order CdCl₂ < AlCl₃ < LiCl < CoCl₂ < MgCl₂ < Ni Cl₂ < NaCl < KCl < CaCl₂ < SrCl₂ < NH₄Cl < BaCl₂ < ZnCl₂. At room temp., 16°, A. Schweitzer found that the following cathode potentials are needed for the deposition of nickel from N-NiCl₂ in an atm. of hydrogen, and an equilibrium potential about -0.31 volt:

Current density	0	0.01	0.03	0.06	0.11	0.9 amp. per sq. dm.
Cathode potential	-0.31	-0.462	-0.486	-0.510	-0.535	-0.645 volt.

Hence, the excess polarization increases with the current density, but it decreases with a rise of temp., being only about 0.1 volt at 90°. It therefore follows that the H^•-ion conc. must be low, or the current efficiency for the deposition of nickel will be low. There is a Scylla to this Charybdis because if the H^•-ion conc. be too low, basic salts may be formed, the bath fouled, and the quality of the deposit impaired.

The effect of the H^•-ion conc. on the deposition of nickel has been studied by M. Ballay, J. Barbaudy and A. Petit, J. Barbaudy and co-workers, W. Blum and N. Bekkedahl, W. Blum and M. R. Thompson, A. E. Brewer and G. H. Montillon, C. E. Clindinin, A. K. Graham, L. E. and V. E. Grant, H. E. Haring, R. Harr, J. C. Krotchmer, H. Kurrein, W. Lockerbie, D. J. MacNaughtan and R. A. F. Hammond, D. J. MacNaughtan and co-workers, H. B. Maxwell, G. H. Montillon and N. S. Cassel, W. A. Mudge, J. B. O'Sullivan, H. C. Parker and W. N. Greer, L. C. Parn, W. H. Phillips, W. M. Phillips, K. Pitschner, A. Regmunt, R. G. Suman, W. A. Taylor, M. R. Thompson, S. Triandafil, and O. P. Watts - vide infra.

W. Wernicke observed that nickel peroxide is formed at the anode in alkaline soln. - say, an alkaline soln. of sodium tartrate and nickel hydroxide. The reaction in the nickel-plating bath was studied by A. Brochet, H. S. Carhart, F. Forster and F. Kruger, R. L. Dorrance and W. C. Gardiner, F. Forster, P. K. Frolich and F. L. Clark, and F. E. Lathe. C. Russo studied the rate of anodic solution in N-H₂SO₄.

There is a difficulty in obtaining thick deposits of nickel at ordinary temp. With other than low current densities, the deposit peels off in thin flakes, and a coherent film cannot be obtained. In the case of iron, a similar effect has been attributed to the irregular occlusion of hydrogen by the metal producing strains in the deposit; but K. Engemann showed that the solubility of hydrogen in nickel is probably too low to produce such an effect, and he attributed the phenomenon to the presence of traces of iron in the electrolyte depositing on the cathode more readily than nickel, so that the first layers contain a higher proportion of iron than the subsequent layer. As a result, strains are set up and flaking occurs. He obtained no flaking with electrolyte and anode quite free from iron. A low temp., a low H^•-ion conc., and a high current density all favour an irregular deposition of iron and also flaking. F. Forster obtained good thick deposits at a higher temp., say 50° to 90°, even when the current density is high, say 2.5 amps, per sq. dm. Hot soln. were also recommended by M. Kugel, G. Langbein, W. S. Barrows, R. F. Clark, W. G. Horsch, and A. E. Shepherd.

F. Forster showed that compact and thick deposits, used with crude nickel anodes, can be obtained at temp, between 50° and 90°. A. Classen recommended hot, neutral soln. of nickel sulphate and ammonium oxalate for the electrodeposition of nickel. F. Forster obtained much better results with sulphate soln. than with chloride soln. as electrolytes. Very little separation was effected with nickel, iron, and cobalt, but carbon, silicon, copper, and manganese could be eliminated. When the proportion of iron is high, the deposit curls off the cathode. With chloride soln. bubbles of hydrogen form more readily on the cathode, and these tend to produce warty deposits. The heavy deposits are due to the formation of basic salts, and this is particularly marked with neutral soln. of nickel chloride.

" Conducting " salts are added to the bath to decrease the electrical resistance of the bath. The resistance of the bath was studied by R. L. Dorrance and W. C. Gardiner, A. E. Nicol, H. E. Haring, L. D. Hammond, C. W. Heil, and E. F. Kern and M. Y. Chang. The effect of superposing an alternating current on the direct current employed in the deposition of nickel, and its alloys, was studied by H. C. Cocks, A. Copperado, N. A. Isgarisheff and S. Berkmann, W. G. Ellis, V. Kohlschutter and H. Schodl, and S. A. Tucker and H. G. Loesch; and the effect of high current densities by N. R. Laban. G. Langbein considered that potassium, sodium, magnesium, or ammonium salts are best suited for this purpose, but the ammonium chloride is not good, and that sodium acetate, barium oxalate, ammonium nitrate, ammonium-alum, and the like are unsuitable. In general, the additions of chlorides, and baths prepared with nickel chloride or nitrate, are not suited for the solid nickeling of iron. P. A. Nichol and O. P. Watts found the use of nitrates objectionable. D. W. Robinson emphasized the use of magnesium sulphate in raising the conductivity of the soln., improving the character of the deposit, increasing the weight deposited in a given time, dissolving the anodes more evenly, and avoiding pitting. Small proportions of fluorides or chlorides – sodium, ammonium, magnesium, or nickel - however, act by reducing the tendency of the anode to become passive, and by favouring the corrosion of the anode. The use of baths of nickel cyanide in soln. of potassium cyanide, recommended by R. H. Marshall, and P. and Q. Marino, were found by G. Langbein to give poor deposits. Whilst the presence of a dil. acid is desirable, the addition of strong acids is to be avoided, but weak acids, like citric or acetic acid, etc., can be recommended. The salts of the organic acids act by regulating the acidity of the bath, by dissolving basic salts of nickel, iron, etc., and reducing the rate of deposition of nickel by more positive metals as in the electroplating of zinc and of its alloys. J. Powell recommended benzoic acid, and E. Weston, J. Barbaudy and A. Petit, E. M. Baker, etc., boric acid. The boric acid has a favourable effect in producing silvery-whiteness; prevents the formation of basic salts, and it makes the deposit more adherent, softer, and flexible. The yellowness of electrodeposited nickel is attributed to the presence of basic nickel salts, but this has not been proved. G. Langbein, and F. Forster said that the deposit with boric acid is harder than it is with a free organic acid, and that they cannot be made so thick as is readily attained with the nickel ethyl sulphate baths. L. D. Hammond showed that boric acid acts by maintaining more uniformly the H'-ion conc. of the soln., and that although good deposits can be obtained from soln. slightly acidified with strong or weak acids, none of the acids were so good as boric acid for continued service. The addition of glycerol is claimed by P. and Q. Marino, J. A. Murphy, and H. Gardner to act as a depolarizer and allow lustrous nickel deposits to be produced of great homogeneity. G. Langbein was unable to produce better deposits in the presence of glycerol than in its absence. The addition of carbon disulphide to nickel baths is said to prevent the nickel deposit from becoming dull when it has attained a certain thickness, but G. Langbein found no advantage attends the use of this agent. The general subject, and the use of glycine were discussed by G. Fuseya and co-workers; and the effect of foreign metals, by B. Setlik.

According to I. Adams, in the search after solutions from which metals can be practically deposited, the rule-of-thumb method prevailed and had to prevail. In the search for a practical silver-plating solution, doubtless hundreds of silver salts were tried before the rather out-of-the-way double cyanide of silver and potassium was found. In the case of untried metals, the experimenter could not safely draw inferences from similar combinations. There were no rules, and few theories to guide, and it is much the same to-day. All this means that in the quest for solutions suitable for electroplating, the method of trial and failure, hit or miss, has to be followed. J. M. Merrick reported trials with ammoniacal soln. of nickel chloride, sulphate, and nitrate, as well as with the complex salts with the corresponding ammonium or potassium salts. A. Smee, and J. M. Merrick tried nickel acetate; J. M. Merrick, potassium nickel cyanide; E. F. Kern and F. G. Fabian, nickel fluosilicate, dithionate, chloride, and sulphate; J. Powell, L. E. Stout and C. L. Faust, J. H. Potts, and A. Watt, acetates, benzoates, chlorides, citrates, lactates, tartrates, and formates; G. Gore, selenate; A. Hollard, dichromates, and pyrophosphates; M. le Couteulx, ammonium persulphate; D. J. MacNaughtan and R. A. F. Hammond, chromic acid or chromic sulphate; C. P. Madsen, W. G. Ellis, and the Madsenell Corporation, hydrogen dioxide, and chlorine; E. A. Ollard, ozone or ozonized air; W. Blum, and Q. Marino, fluorides; P. and Q. Marino, sulphanilic acid, borotartrates, glyceroborates, glycerobenzoates, boro- benzoates, phosphates, bromates, glucose, borocitrate, and borotartrate; A. Classen, liquorice root; Y. Garin, gelatin or albuminous substances; E.N. Todd and W. R. King, gum tragacanth; N. A. Isgarisheff and S. Berkmann, gelatine; and B. H. Divine, glue. E. F. Kern studied the function of addition agents in electrolytes, and K. Pitschner, the buffer action.

Numerous recipes for baths for the electrodeposition of nickel have been published. For instance, W. R. Barclay and C. H. Hainsworth recommended (NH₄)₂Ni(SO₄)₂.6H₂O, 375 grms.; NiSO₄.7H₂O, 94 to 125 grms. made up to 5 litres with water at 20°, using a current density of 5 amps, per sq. ft. initially at 5 volts and subsequently falling to 3 volts or even less. They also recommend (NH₄)₂Ni(SO₄)₂.6H₂O, 312 grms.; NiSO₄.7H₂O, 125 grms.; potassium or sodium chloride, 31 to 47 grms. made up to 5 litres with water, and worked at a current density of 10 amps, per sq. ft. 0. P. Watts recommended a soln. containing NiSO₄.7H₂O, 240 grms.; NiCl₂.6H₂O, 20 grms.; and boric acid, 20 grms. per litre. E. A. Ollard gave NiSO₄.7H₂O, 300 grms.; H₃BO₃, 25 grms.; NaF, 6 grms.; and NiCl₂.6H₂O, 2 grms. per litre, with a current density of 15 to 70 amp. per sq. ft., and pH=5.7 to 5.9.

In the so-called black nickelling, instead of trying to obtain a silver-white deposit, a black or dark brown coloured deposit is produced. The bath then employed contains some thiocyanate and arsenic. Thus, E. Blasset recommended a bath of ammonium nickel sulphate 12 ozs., potassium thiocyanate 2.75 ozs., copper carbonate 2 ozs., and arsenious acid 2 ozs., made up with 95 galls, of water. The subject was discussed by G. B. Hogaboom and co-workers, J. Haas, A. Classen, W. S. Barrows, K. Tamaki, H. B. Maxwell, C. H. Proctor, R. H. Slater, J. Haas, O. P. Watts, and W. Blum. H. Kersten and J. Maas found that the structure of black nickel is amorphous.

In analytical work, nickel, as well as cobalt, can be deposited electrolytically from soln. of their double cyanides or oxalates, or from soln. of the sulphates mixed with alkali acetates, tartrates, or citrates, or from ammoniacal soln. E. F. Smith said that ammoniacal soln. are best adapted for the electrodeposition, and the presence of ammonium sulphate or sodium phosphate favours the separation. A rotating cathode may be employed. In illustration, H. Fresenius and F. Bergmann commended using as electrolyte 50 c.c. of a soln. of nickel containing 0.1233 grm. Ni, 100 c.c. of aq. ammonia of sp. gr. 0.96,10 c.c. of a soln. of 305 grms. of ammonium sulphate per litre, and 100 c.c. of water. The electrodes can be 0.5 to 0.67 cm. apart; the current density 0.5 to 0.7 amp. per 100 sq. cm. at 2.8 to 3.3 volts at ordinary temp.; and the time required for a complete separation of the nickel was 4 hrs. The separation of nickel from copper, cobalt, and iron was studied by C. G. Fink and F. A. Rohrman. A. Glazunoff and J. Krieglstein discussed the iron content of electrodeposited nickel.

O. Meyer showed that nickel may be deposited from a soln. of the chloride in alcohol; R. Taft and H. Barham, from soln. of nickel salts in liquid ammonia; R. D. Blue and F. C. Mathers, and L. Y. Yntema and L. F. Audrieth, from soln. of salts in formamide or acetamide, although H. Rohler obtained no deposit from soln. in formamide; and H. S. Booth and M. Merlub-Sobel obtained a deposit from a soln. of nickel thiocyanate in liquid ammonia.

According to G. Lambris, some carbon may contaminate the deposit of nickel, and he concluded that the absorption of carbon is entirely due to a gas reaction, and that carbon dioxide or acetylene may introduce carbon into nickel or cobalt. Oxalic acid is partly reduced to acetylene on platinum and nickel, but not when copper, iron, or tin cathodes are employed. The carbon in electrolytic nickel is present in the form of a carbide. The subject was discussed by P. K. Frolich, and C. P. Madsen.

L. E. and Y. E. Grant studied the variations in the thickness of electrodeposited nickel. W. Blum showed that the crystals of electrodeposited nickel decreased in size as the current density increased up to 2 amps, per sq. dm.; the size of the crystals decreased with temp.; and with a decrease in the conc. of the metal ion. According to G. L. Clark and P. K. Frolich, a low current density, a low temp., and the presence of gelatin favour a fibrous or oriented structure - the evolution of hydrogen is unfavourable. The orientation of the structure is parallel to that of the cathode metal in the case of platinum. W. S. Barrows studied the effect of the nature of the anodes. The microstructure of the deposits was studied by R. Audubert, H. Stager, W. D. Bancroft, G. Eger, A. K. Graham, A. N. Kuznetzoff and S. A. Baranoff, W. G. Burgess and W. Elenbaas, W. E. Hughes, G. L. Clark and P. K. Frolich, W. A. Wood, M. Z. Wolfmeyer, C. P. Madsen, C. Upthegrove and E. M. Baker, H. Kersten and J. Maas, Y. Kohlschutter and co-workers, and B. Waser and E. H. Schultz. X-radiograms were observed by R. M. Bozorth, R. Glocker and E. Kaupp, J. D. Hanawalt and L. R. Ingersoll, H. Hirata and H. Komatsubara, and S. Procopiu. Most of the heavy metals - copper, silver, zinc, and lead - are normally deposited from simple acid soln. in a coarsely crystalline form, but with nickel, cobalt, and iron, under similar conditions, the metal is deposited in a finely crystalline form. Owing to its electropositive character, nickel is always deposited along with some hydrogen; this is not the case with copper, silver, or lead, and only to a small extent with zinc. Consequently, V. Kohlschutter suggested that the liberated hydrogen interferes with the growth of the crystals of nickel. S. Glasstone suggested that nickel is deposited in an unstable atomic form which is rapidly transformed into a more stable modification with the consequent interference of crystal growth. V. Kohl- schiitter and H. Schecht showed that the presence of colloidal metal hydroxides may have reduced the size of the crystals of electrodeposited nickel so as to produce a mirror-like surface; and H. J. S. Sand suggested that a similar effect may be produced by inorganic colloids in the course of electrolysis. According to J. B. O'Sullivan, the electrodeposits from buffered nickel sulphate soln. become smoother and more fine-grained as the pH of the bath is raised so that neither the hypothesis of V. Kohlschutter nor that of S. Glasstone explains the results. He suggested that the effect is due to the presence of colloidal nickel hydroxide or basic salt in the cathode film. This is supported by the fact that electrodeposits contain a little oxygen. H. T. S. Britton found that nickel hydroxide is precipitated from nickel sulphate soln. when pH=6.66, and nickel borate probably behaves similarly; and D. J. MacNaughtan and A. W. Hothersall observed that on adding a soln. of sodium hydroxide to a nickel sulphate-boric acid plating bath, permanent precipitation begins when pH=6.6, but in an ammonium nickel sulphate bath it begins when pH=7.6. In general, J. B. O'Sullivan found that as the value of pH is raised the deposits become smoother and finally more brittle - vide supra. J. B. O'Sullivan continues the discussion as follows:

According to P. K. Frolich, colloids affect the structure of an electrodeposit. by forming a more viscous layer at or near the cathode, so that the diffusion thither of metal ions is hindered, and he visualized two ways in which this might take place. Firstly, the colloid may become pressed against the cathode owing to its particles carrying a positive charge. The metal ions will then have to pass through the channels of the sponge-like mass produced, and will be discharged immediately they reach the cathode surface, irrespective of the orientation of the metal atoms previously deposited. Thus the growth of existing crystals will be interfered with, and there will be a tendency for the metal to be deposited in needle-shaped crystals in the channels of the colloid mass until this becomes embedded in the metal deposit. If, however, the colloid is so firmly held against the cathode that this can take place, it is, to all intents and purposes, being electrolytically deposited with the metal. Also the quantity of colloid enclosed in the deposit would be so great as to constitute an extreme case, such as those described by W. D. Bancroft and T. R. Briggs, and by R. Marc, who obtained slimy deposits from solutions containing organic colloids. The latter describes his deposits as consisting of extremely thin flakes, not needles, of metal, set in a collagenous mass. The other case suggested by P. K. Frolich is that in which the colloid has an amphoteric character, and the pH of the cathode film is distinctly higher than that of the bulk of the solution. Such a case would be that of gelatine in a nickel or zinc plating bath. If the bath is sufficiently acid, the positively charged colloid will migrate towards the cathode, but at a definite distance from this it will encounter a region where the pH is the same as its isoelectric point; beyond this region the pH will be higher, so that the colloid would migrate away from the cathode. In the intermediate region, the colloid will be held, forming a diaphragm which will offer a resistance to the passage of metal ions, so that the colloid free liquid between this diaphragm and the cathode may become depleted of metal ions. But, according to W. E. Hughes, decreased concentration of metal ions in a simple solution results in slower deposition of metal, and consequently in the formation of a more coarse-grained, not a more finegrained deposit. On the other hand, if there is produced at or near the cathode a viscous layer which is difficultly permeable to metal ions, the effect will be that the concentration of these in the cathode film will diminish, so that a greater number of hydrogen ions will be discharged, and the PH of the cathode film will rise. Consequently, more colloidal metal hydroxide will be formed, and if it is produced in the cathode film more rapidly than it is being electrolytically deposited, the conditions will continue to be accentuated until all the available metal has been converted into hydroxide, whereafter this can only be replenished as more metal ions migrate or diffuse to the cathode film.

L. R. Ingersoll, F. Kirchner, and R. Glocker and E. Kaupp showed that the fibre axis represents the direction in which the velocity of deposition is a maximum. If the current density is too high, the crystals lose all regular orientation. R. M. Bozorth thought that the orientation of the crystals of iron, cobalt, nickel, during their electrodeposition is associated with strains. The subject was also studied by K. N. Oesterle. E. S. Hedges observed that a periodic electrodeposition could be obtained with a soln. of potassium nickel cyanide.

K. Okimoto discussed the electric melting of nickel. As with iron (q.v.), L. Jordan and co-workers found that crucibles made from commercial, fused zirconia, magnesia, or alumina are not suitable for melting the purified metal; but crucibles made from the purified oxides gave good results. They preferred those made with purified magnesia. A. Dingwell and co-workers referred to the contamination of nickel by molybdenum when fused in a molybdenum resistance furnace.

G. Magnus found that when the oxide is reduced at a low enough temp., pyrophoric nickel is formed, and W. N. Ipateeff showed that in hydrogen the nickel is pyrophoric if reduced at temp, below 270°. G. Grassi obtained the pyrophoric metal by heating the oxalate or oxide in hydrogen at about 280°. According to H. Moissan, pyrophoric nickel is formed below 270° as a black powder which does not burn so brightly as iron reduced at 440° when it is exposed to air. If reduced at 270° to 280°, it is oxidized by dry air or oxygen only. G. Tammann and N. Nikitin discussed the effect of grain-size on the pyrophoric quality above 350°; moisture favours the oxidation so that in moist air or oxygen the powder is oxidized at a lower temp.

L. Graf prepared single crystals of nickel. L. R. Ingersoll, and A. C. G. Beach prepared nickel mirrors and nickel films by spluttering nickel cathodes in hydrogen, nitrogen, and argon, or neon, with a direct current generator of 1000 volts, and allowing the metal to deposit on a cooled surface. These films, as well as films obtained electrolytically - vide supra - were studied by K. Lauch and W. Ruppert, J. Hanawalt and L. R. Ingersoll, L. R. Ingersoll and co-workers, R. M. Bozorth, K. M. Oesterle, O. G. Keiko, R. K. Cowsik, H. Bracchetti, F. Kirchner, F. H. Constable, C. Miiller, J. Jolist, E. O. Hulburt, J. H. Howey, J. Strong and C. H. Cart- wright, A. W. Gauger, R. Glocker and E. Kaupp, H. Hirata and H. Komatsubara, S. Procopiu, and Y. Kohlschiitter and co-workers - vide iron mirrors. F. W. Laird, C. F. van Duin, and C. G. Fink and W. G. King found that by admitting nickel carbonyl vapour suddenly into an evacuated flask uniformly heated to 180° to 200° the walls may be coated with a bright, adherent, uniform deposit of nickel. The presence of traces of air, oxygen, chlorine, etc., is to be avoided because it leads to dark deposits. A preliminary flushing of the flask with hydrogen is effective in producing bright deposits, but is unnecessary if the vessel is evacuated to 5 to 10 microns press. Z. Debinska found that mirrors obtained by vacuum distillation exhibit orientation of the microcrystals. J. E. Henderson obtained a mirror of nickel by evaporating that metal from a heated tungsten filament. J. Manning prepared nickel membranes for ultra-filtration.

According to M. Kimura, when metallic nickel is heated to incandescence and quickly plunged into distilled water, a colloidal solution of nickel and of nickel hydroxide is formed. G. Bredig did not have success in preparing the hydrosol by electrically spluttering the metal under water, but D. Zavrieff, A. H. Erden- brecher, F. Ehrenhaft, J. Billitzer, E. Thoren, C. H. von Hoessle, T. Svedberg, O. Scarpa, and E. Thoren obtained the colloid in this manner. D. Zavrieff also prepared colloidal soln. by electric disintegration. C. Paal found that colloidal nickelous hydroxide is reduced to a colloidal soln. of nickel by hydrogen in the presence of a hydrosol of palladium. The stability of the colloid is increased if sodium protalbinate be present as protective colloid. The hydrosol so obtained is a brilliant chestnut-brown in transmitted light, and black in reflected light. The corresponding hydrogel forms blackish-brown, brittle lamillse, and its state is reversible. L. Hogounenq and J. Loiseleur used glycogen as protective colloid. According to C. Kelber, a soln. of nickel formate and gelatine in glycerol at 200° to 210°, when submitted to the action of a stream of hydrogen, assumes a chestnut- brown colour. The colloidal soln. remains unaltered in the air and is miscible with alcohol, but on treatment with water and centrifuging, it deposits the colloidal metal as a dark brown solid, containing 25 to 30 per cent, of nickel, which can again yield colloidal nickel soln. in dilute acetic acid, acidified water, glycerol, or alcohol. Other reducing agents can be applied to the same purpose; nickel formate at 220° in glycerol soln. in the presence of gelatine is reduced by hydrazine hydrate with formation of a colloidal nickel solution of similar properties to that just described. Formaldehyde, hydroxylamine, and hypophosphorous acid can also be applied as reducing agents for the purpose, whilst gum arabic can be used in place of gelatin. The nickel formate can be replaced by nickel acetate or freshly-precipitated nickel hydroxide. B. C. Soyenkoff prepared colloidal nickel soaps. H. Freundlich and W. Seifriz studied the electric charge of the colloid. According to Wo. Ostwald, when a soln. of nickel carbonyl in benzene is boiled, carbon monoxide is evolved, and a violet-grey organosol, or benzenosol, of nickel remains. T. Svedberg obtained an alcoholsol by the electrical spluttering of nickel in isobutyl and other alcohols, and an ethersol using ethyl ether as the dispersion medium; S. R. Rao, and C. G. Montgomery, with isopropyl alcohol; and S. J. Folley and D. C. Henry, an acetonesol, where traces of water reduce the stability of the sol, and traces of sulphuric, hydrochloric, or m-hydroxybenzoic acid, or m-nitroaniline, make the sol more stable. E. Hatshek and P. C. L. Thorne obtained sols by dissociating nickel carbonyl in benzene, toluene, and amcymene with rubber as protective colloid; and F. Haurowitz obtained a stable sol of nickel in benzene. H. B. Weiser and G. L. Mach discussed the subject. W. E. Gibbs and H. Liander prepared aerosols of nickel.