162 Thermal Expansion of Pure Nickel and Cobalt. 



Society.* Since the determinations made by Fizeau in the year 1869, 

 a large amount of additional knowledge has been accumulated with 

 reference to nickel and cobalt, including the discovery of the liquid 

 nickel carbonyl, which places processes of puiification in the hands of 

 the chemist of a character so superior to the older methods, as to 

 render it highly desirable that re-determinations of the physical con- 

 stants of these interesting elements should be carried out with speci- 

 mens of the metals thus purified. By the kindness of Professor 

 Tilden, who has prepared such specimens with infinite care for the pur- 

 poses of the investigation of other physical and chemical characters, 

 the author has been enabled to carry out determinations of the thermal 

 expansion with rectangular blocks varying in thickness from 8 to 

 13 mm. The blocks were furnished with parallel and truly plane 

 surfaces by the makers of the dilatometer, Messrs. Troughton and 

 Simms. The range of temperature of the observations was from 

 6° to 121°. 



The results of the determinations of the coefficients of linear expan- 

 sion a are as follows : — 



a = a + 2bt. 



For nickel cc = O000 012 48 + 0-000 000 014 St. 



For cobalt a = 0-000 012 08 + 0-000 000 012 8L 



Nine different determinations were carried out for each metal, three 

 in each of the three rectangular directions, in order to eliminate any 

 slight error due to directional strain in the metallic blocks. As the 

 metals crystallise in the regular system, the expansion should be the 

 same in all directions. The metal in each case had solidified after 

 fusion in an oxy-hydrogen flame in presence at the last of excess of 

 oxygen. The individual results are highly concordant, the highest 

 result for cobalt being still lower than the lowest of the nine values 

 obtained for nickel. Hence there can be no doubt that the above 

 coefficients represent the true relationships. 



The main result of the investigation may be summarised as fol- 

 lows : — The coefficients of linear expansion a of pure nickel and cobalt 

 exhibit a slight but real difference, the coefficient of nickel being 

 distinctly greater than that of cobalt. This is true with respect to 

 both the constant a, the coefficient for 0°, and the increment per 

 degree, 2b, of the general expression for the coefficient at any tempera- 

 ture t, cl = a + 2bt. The difference is consequently one which augments 

 with the temperature ; at 0° it amounts to 3*2 per cent., while at 120°, 

 the upper limit of the temperatures of the observations, it attains 4"5 

 per cent. Similar rules apply naturally to the cubical coefficients. 

 The metal possessing the slightly lower atomic weight, nickel, is thus 

 found to expand to a greater extent than the metal, cobalt, which is 

 endowed with the higher atomic weight. 



* 'Phil. Trans.,' A, vol. 191, p. 313; 'Koy. Soc. Proc.,' vol. 63, p. 208. 



