July 29, 1898.] 



SCIENCE. 



117 



as tbe two forms of antimony and the silver- 

 zinc alloy of Heycock and Neville, and thej' 

 may be left out of account. Bunsen com- 

 pared the so-called allotropic tin, obtained 

 by exposing the metal to cold for a long 

 time, and found it .0545 against .0559 for 

 the ordinary kind.* In dimorphous sub- 

 stances there is often no difference. Eeg- 

 nault found for arragonite .2086 and for 

 calcite .2085 respectively. The differences 

 between metals hammered and annealed, 

 hard and soft, were also found by Eegnault 

 to be very small. f 



Hard steel ... .1175. Same, softened ... .1165. 

 Hard bronze ... .0858. Same, softened ... .0863. 



Kopp came to the conclusion, first that 

 each element in the solid state, and at a 

 sufficient distance from its melting point, 

 has one specific or atomic heat, which varies 

 only slightly with physical conditions ; and 

 secondly, that each element has essentially 

 the same specific or atomic heat in com- 

 pounds as it has in the free state. This 

 last is practically identical with the state- 

 ment which is known as Neumann's law. 

 "With Kopp's conclusion I agree, but from 

 some of Regnault's results, coupled with 

 my own, the effect of small quantities of 

 carbon, and perhaps of sulphur, upon the 

 specific heats of metals is greater than has 

 been supposed. If we take the results of 

 Eegnault and of Kopp and combine them 

 VFith the most accurately known atomic 

 vreights the products are still not constant. 



The ' Law ' of Dulong and Petit is there- 

 fore only an approximation ; but this may 

 perhaps be due to inaccuracy in the estima- 

 tion of the specific heat, owing to impuritj' 

 in the material used. That is the problem 

 which I have endeavored to solve. 



The introduction by Professor J. Joly of 

 a new method of calorimetry, vrhich de- 

 pends upon the condensation of steam upon 

 the cold body, and the excellent results ob- 

 tained by the author in the use of the dif- 

 ferential form of his instrument, J led me to 

 think that with due attention to various pre- 

 cautions — such as exact observations of the 

 temperatures, and practice in determining 

 the moment at which the increase of weight 

 due to condensation is completed — results 

 of considerable accuracy might be obtained. 



The problem is to find two elements, very 

 closely similar in density and melting point, 

 which can be obtained in a state of purity, 

 and then to determine with the utmost pos- 

 sible accuracy the specific heat of each under 

 the same conditions. 



The two metals cobalt and nickel were 

 selected for the purpose. They were exam- 

 ined by Eegnault, but the metals he used 

 were very impure. 



The cobalt employed in my experiments 

 was prepared by myself. For the nickel 

 I am indebted to Dr. Ludwig Mond. Both 

 were undoubtedly much more nearly pure 

 than any metal available in Eegnault's 

 time. The results obtained are as follows : 



Atomic Weights Most Accurately Knowit (1897) Combined with Specific Heats. 



*Pogg Ann. 141, 27. 



tAnn. Chim. [3], IX. 



X Proo. E. S. 47, 241. 



