1898.] on Experiments on Certain Elemeuts in relation to Heat. 737 



attend the acceptance of the statement of Petit and Dulong in the 

 form in which they gave it. He then discussed the three principal 

 experimental methods : viz. (1) fusion of ice ; (2) mixture with water 

 or other liquid ; and (3) cooling ; and decided in favour of the 

 second, which he used throughout his researches. The general form 

 of the apparatus used by the great physicist has been a model for 

 the guidance of successive experimentalists since his time. 



Another quarter of a century elapsed before the question of the 

 specific heats of the elements was resumed by Hermann Kopp. His 

 results were communicated to the Royal Society, and are embodied 

 in a paper printed in the ' Philosophical Transactions ' for 1865. After 

 reviewing the work of his predecessors, he described a process by 

 which he had made a large number of estimations of specific heat, 

 not only of elements, but of compounds of all kinds in the solid state. 

 Concerning his own process, however, he remarks that " The method, 

 as I have used it, has by no means the accuracy of that of Regnault " 

 (p. 84). 



In 1870 Bunsen introduced his well-known ice calorimeter. This 

 is an instrument in which the amount of ice melted by the heated 

 body is not measured by collecting and weighing the water formed, 

 but by observing the contraction consequent upon the change of 

 state. The results obtained by Bunsen himself are uniformly slightly 

 lower than those of Regnault for the same elements. 



Since that time, experiments have been made by \A'eber, Dewar, 

 Humpidge and others, in connection especially with the influence of 

 temperature in particular cases. 



. Setting aside the elements, carbon, boron, silicon and beryllium, 

 as providing an entirely separate problem, the question is whether the 

 law of Dulong and Petit is strictly valid when applied to the metals. 

 Kopp, in the discussion of his subject, came to the conclusion that it 

 is not ; but the grounds for this conclusion are unsatisfactory, since 

 neither the atomic weights nor the specific heats were at that time 

 known with sufficient accuracy. It has been customary to assume 

 that the divergences from the constant value of the product, At. Wt. 

 X Sp. Ht., are due partly to the fact that at the temperature at which 

 specific heats are usually determined, the different elements stand in 

 very different relations to their point of fusion : thus, lead at the tem- 

 perature of boiling water is much nearer to its melting point than 

 iron under the same conditions. The divergences have also been 

 attributed to temporary or allotropic conditions of the elements. As 

 to the relation to melting point, the specific heats of atomic weights 

 seem to be practically the same in separate metals and alloys of the 

 same which melt at far lower temperatures. For example, the atomic 

 heat of cadmium is 6 * 85 ; of bismuth 6 • 47 ; of tin, 6 • 63 ; and of lead, 

 6*50; while the mean atomic heat in alloys of bismuth with tin and 

 lead with tin ranges from 6 '40 to 6*66 (Eegnault), which is practi- 

 cally the same. Again, while the melting point of platinum is at a 

 white heat, the metal becomes plastic at a low red heat, and yet tho 



