HISTORY OF DYNAMICAL THEORY OF HEAT. 335 



sessed as to the fact appears for the most part in the invariability 

 of the ratio of any dynamic magnitude of a definite kind which dis- 

 appears to that of another kind which is thereby produced, and the 

 numerical value of which, for a particular transformation, depends 

 only on the relative magnitude of the characteristic units as compared 

 by the same standard system of dynamic units. That is, that the 

 conversion of one manifestation of energy into another takes place 

 with as great certainty and absence of waste, and with the same 

 integrity of the elementary magnitude, as the more formal conversion 

 of foot-pounds into kilogrammetres, or British thermal units into 

 calorics. To the experimental establishment of this principle as 

 involved in transformations between heat and work, and which is 

 called the First Fundamental Law of Theruio-Dyn amies, we shall 

 return hereafter. 



But in the transformation of heat into mechanical effect or work, 

 an additional principle has been found to hold, respecting the trans- 

 formable quantities of these two magnitudes as influenced by tem- 

 perature, and which is known in like manner as the Second Funda- 

 mental Law of Thermo-Dynamics. 



Experience has not as yet encountered any phenomena at variance 

 with these fundamental laws; which furthermore agree with the 

 strictest requirements of intuitive science, and illustrate, respectively, 

 the axioms that nothing is by natural means creatable from nothing, 

 and that things are equal to the same thing only which are equal to 

 each other. In the development of these two principles, and the ap- 

 plication to them of empirical laws with reference to the behavior of 

 bodies under the action of heat or mechanical effect, consists the first 

 principal division of the subject in which the results obtained are 

 generally reliable. 



But in assuming a complete analogy between molecular and mass 

 energy, and in tracing the consequence of this assumption through 

 the different forms of material aggregation, the conclusions reached 

 are generally much beyond the present power of experimental science 

 to explicitly confirm, and, although many of the results obtained in 

 these investigations are of great probability, they yet are of inferior 

 certainty to those properly included in the first division. 



In short, although the laws which govern the relations of molar 

 energy to heat are in the abstract positively known, yet in endeavor- 

 ing to trace the distribution and precise condition of energy when it 

 becomes absorbed within a body, or vice versa,ihe mode and minutest 

 detail of its transformation into gross mechanical effect, the most 

 consistent theories have heretofore depended on the hypothesis that 

 actual or real heat is a condition of molecular kinetic energy, and that 

 the various latent heats are due to potentialities of molecular arrange- 

 ment. 



The full extent to which this principle of the indestructibility of 



