114 



SCIENCE. 



[N. S. Vol. VIII. No. 187. 







/^«-<T'.--^-^^^-t-!-.--Z.,,^Il 



tremes as true surviving ancestral types, as 

 the table might seem to suggest. 



It is merely justice to him to add that he 

 regarded the whole subject as a problem 

 for further investigation, and that he by no 

 means looked upon his own conceptions at 

 that time as final. Chaeles S. Minot. 



RECENT EXPERIMENTS ON CERTAIN OF TEE 



CHESIICAL ELEMENTS IN RELATION TO 



HEAT* 



The discovery that diiferent substances 

 have diiierent capacities for heat is usually 

 attributed to Irvine, but there can be no 

 doubt that Black, Crawford and others con- 

 tributed to the establishment of the idea. 

 The fact that equal weights of different sub- 

 stances, 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 



*A Lecture delivered before the Royal Institution 

 of Great Britain on May 13, 1898, by Professor W. A. 

 Tildeu, D.ac, F.E.S. 



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 specific 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 produced 

 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 con- 

 tact 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 



