288 HISTORY OF SCIENCE. 



And as the quantity of heat which the hotter water parts with raises 

 the other 'from 40 to 50, it follows that the same quantity of heat 

 which raises the temperature of i Ib. of water from 40 to 50 will raise 

 it from 50 to 60. The like experiment may be tried for other tem- 

 peratures between 32 and 212, and the results always show that the 

 quantities of heat required to raise the temperature of a given weight 

 of water are proportional to the number of degrees through which the 

 temperature is raised, the temperature of a mixture of equal weights of 

 water at different temperatures being always the mean. But if we mix. 

 i Ib. of water at 150 with i Ib. of turpentine at 50, the resulting tem- 

 perature of the mixture will be not the mean, but 120; therefore the 

 heat lost by the water suffices to raise the same weight of turpentine 

 through nearly double the range of temperature, for the experiment 

 shows the same quantity of heat which would raise i Ib. of water 30, 

 would raise i Ib. of turpentine 70. The difference of the effects pro- 

 duced by the same quantity of heat is still greater in the case of mer- 

 cury and water. If i Ib. of mercury at 66 were shaken up with i Ib. 

 of water at 100, the resulting temperature would be 99; hence, the 

 quantity of heat which would raise i Ib. of water from 99 to 100, 

 raises i Ib. of mercury from 66 to 99. Here, then, are instances of 

 the same quantity of heat producing very different temperatures, and 

 from these it is obvious, also, that to produce the same increase of 

 temperature in different bodies different quantities of heat are required. 

 Thus, the last case shows that, to raise the temperature of a given 

 weight of water 1, requires thirty-three times as much heat as suffices 

 to raise the temperature of the same weight of mercury i. Black ex- 

 pressed the differences of bodies in this respect by saying each sub- 

 stance has its own special capacity for heat. We may compare the 

 difference with respect to heat between i Ib. of water and i Ib. of 

 mercury, to the difference between two vessels, one of which holds 

 thirty-three times as much as the other. The phrase specific capacity 

 for heat does not here necessarily involve any particular views as to 

 the nature of heat ; but the phrase was also applied about this period 

 to another class of facts, for the rise of temperature observed in bodies 

 when submitted to pressure was attributed to the decreased capacity of 

 the body for heat. Heat was thus regarded as some subtle fluid which 

 rilled the pores of bodies, and was squeezed out by pressure, just as 

 pressure causes the water to ooze from a saturated sponge. The term 

 specific capacity for heat has been superseded by the shorter one " specific 

 heat" which is altogether free from any theoretical suggestion.. 



Black did not himself follow out the path of research he had opened, 

 but for the most part left the determination of the numerical values of the 

 specific heats of bodies to his friends and disciples. One of the methods 

 employed in such determinations has been exemplified, namely, the 

 method by mixtures. This method is not here presented as that by 

 which Black originally observed the specific heats, but as giving the 



