ON THE LAW OF THE CONSTANCY, &C. 



41 



by définition a quantity of heat, and the équation expresses the law that 

 the total heat is constant. It is obvions that, if one was anxious to avoid 

 theory as long as possible, he might apply the formula i. e. use the law 

 of mixing and solve ail problems in calorimetry altogether without the 

 notion of quanti ty of heat. 



The theory assumée! a still more definite form, when tlie constancy of 

 heat was regarded as indicating its nature as a substance. Tins addition? 

 though from our point of view unessential and unnecessary, was, as we 

 know, for many years looked upon as being the very essence of the the- 

 ory and gave rise to the most phantastic developmenls. With this 

 however we are not concerned at présent. 



The heat-phenomena, accompanying the change of substances from 

 one physical state into another, naturally led to an extension of the 

 theory. In a transformation of that nature heat, in the above sensé, 

 appears — is created — , or disappears — is destroyed — , the quantity 

 being pro|)ortional to the amount of substance transformed. Thèse results 

 inay be writteu in the form: 



Tmc(t—T) + 2J//=0/ (/*) 



where M stands for the mass of a substance transformed, and / is a 

 constant depending upon the nature of the substance. 



From the point of view of the quantity-of-heat-theory it was now 

 necessary to extend the définition of quantity of heat, so that it would 

 represent either a terni me (/ — T) or a terni Ml, and the équation then 

 a gain expressed the fact, that heat could not be destroyed or created. 

 The terms Ml were called latent heat, to indicate that they did not dé- 

 pend upon a change of température. 



In this manner the dogma, that beat was indestructible, was upheld, 

 and gradually became looked upon as equally well established as the 

 dogma of the indestructability of matter, or of the constancy of mass. 

 As we know, it was later on further developed into that of the constancy 

 of energy, a development which was simply another such step as that 

 lying between équations {A) and (B). 



Leaving cases in which heat is transformed into energy or vice-versa 

 out of the question, it may be asked: what is exactly the expérimental 

 évidence for the truth of tins law of the constancy of heat? The obvions 

 answer will probably be : the test should be the accuracy of équation 



