1898.] Prof. W. A. Tilden on Experiments on Certain Elements. 735 



WEEKLY EVENING MEETING, 



Friday, May 13, 1898. 



LuDWiG MoND, Esq. Ph.D. F.R.S. Vice-President, in the Chair. 



Professor W. A. Tilden, D.Sc. F.R.S. 



Becent Experiments on Certain of the Chemical Elements 

 in relation to Heat. 



The discovery that different substances have different capacities for 

 heat is usually attributed to Irvine, but there can be no doubt that 

 Black, Crawford and others contributed to the establishment of the 

 idea. The fact that equal weights of different substances, in cooling 

 down through the same number of degrees, give out different amounts 

 of heat, may be illustrated by the well-known experiment, in which 

 a cake of wax is penetrated with different degrees of rapidity by 

 balls of different metals heated to the same temperature. But, for 

 the quantitative estimation of the amounts of heat thus taken up and 

 given out again — that is, the specifiG heats — the physicist must resort 

 to other forms of experiment, each of which presents difficulties of 

 its own. Broadly speaking, three principal methods have been used 

 in the past for this purpose. The first is based upon the observation 

 of the exact change of temperature j^roduced in a known mass of 

 water, by mixing with it a known weight of the substance previously, 

 at a definite temperature above or below that of the water. The 

 second consists in determining the quantity of ice melted, when the 

 heated body is brought into contact with it in such a way that no 

 heat from any other source can reach the ice. And the third method 

 consists in observing the rate at which the temperature of the heated 

 body falls through a definite range of degrees, when suspended in a 

 vacuous space, as compared with the rate of cooling of another body 

 taken as the standard. 



The process of intermixture with water w^os used by the earlier 

 experimenters in the last century, and some of the best results extant 

 have been obtained by this method, which, however, is not so easy 

 as it appears when the highest degree of accuracy is desired. 



Lavoisier and Laplace, in 1780, devised the ice calorimeter which 

 bears their name ; and in a most interesting memoir, which is re- 

 printed among Lavoisier's works, they show that they were familiar 

 with the idea which in modern times is expressed as the principle 

 of the conservation of energy. In this memoir they give the results 

 of experiments, in which the specific heats of iron, mercury and a 



