812 TKANSACTlONS OP SECTION 0. 



temperature here was in rigid logical cousistency with the 'caloric* theory of 

 heat, and it carried out completely the analogy between power derived from the 

 same quantity of heat falling from a higher to a lower level, and resembling a fall 

 of water in producing its effects. For equal quantities of 'caloric,' as of 

 ' water,' temperature fall was regarded as similar to fall in space, and so an 

 accurate idea of the nature of temperature difference is attained. 



This definition, however, gave a scale greatly differing from that of mercurial, 

 air, and other thermometers, the degrees defined by it corresponding to larger 

 and larger intervals on the air thermometer as temperature increases. Professor 

 Tait pointed out also that on such a scale the temperature of a body totally 

 deprived of heat is negative-infinite. 



All these difficulties do not detract from the fundamental importance of the 

 idea here enunciated for the first time : the idea of an absolute thermometric 

 scale theoretically applicable to all bodies — solid, liquid, and gaseous. On the 

 'caloric' or 'material' theory of heat, motive power is obtained during the 

 letting down or fall from a higher to a lower level of a given quantity of heat. 

 The quantity of heat does not alter in the process; it is only its relative level 

 which alters. There is no reason, therefore, for mentally limiting the amount of 

 mechanical energy obtainable from any given quantity of caloric, just as there is 

 no reason for limiting the amount of mechanical energy to be mentally derived 

 from a given weight. Any desired quantity of energy may be derived from a 

 weight of, say, one pound, if it only be allowed to fall far enough, assuming 

 gravity to be constant through the range. 



The investigation of the work to be derived from a given quantity of heat at 

 a given temperature is thus a matter of experiment, which can be settled by 

 measurement of the properties of a few bodies. 



Reasoning, it is conceived, in this way, Thomson follows up his absolute 

 thermometric scale work with an investigation entitled ' Carnot's Theory of the 

 Motive Power of Heat,' described in a paper read in 18i9 before the Pioyal 

 Society of Edinburgh, in which he calculates from Regnault's experiments on 

 steam the power developed by a Carnot reversible engine when using one 

 centigrade heat unit; that is, the heat necessary to heat one pound of water 

 through 1° C. for temperatures from 1° to 231° C, the temperature falling in the 

 engine in each case to 0° C. 



In this paper he asks himself two questions: (1) What is the precise nature 

 of the thermal agency by means of which mechanical elfect is to be produced 

 without ettects of any other kind ? and (2) IIow may the amount of the thermal 

 agency necessary for performing a given quantity of work be estimated ? 



Using Regnault's values for the properties of steam, he calculates the lines of 

 compression and expansion without heat loss, the lines of compression and ex- 

 pansion with heat flow at the lowest temperature, and heat addition at the highest 

 temperature, and thus arrives at the work area per heat unit let down. He 

 tabulates these results, and shows that what he calls Carnot's function diminishes 

 as temperature rises, using the ordinary centigrade scale. On the caloric theory 

 the methods are rigidly logical and correct, but some inaccuracy is introduced by 

 the necessity of that theory for the discharge of the same amount of heat at the 

 third operation as is taken in on the first. The paper is of great interest, however, 

 because it shows clearly how fully the distinguished author realises the necessity 

 for re-examining the standard ideas of the nature of heat. Two paragraphs make 

 this very clear : — 



' 7. Since the time when Carnot thus expressed himself the necessity of a 

 most careful examination of the entire experimental basis of the theory of heat 

 has become more and more urgent. Especially all those assumptions depending 

 on the idea that heat is a substance, invariable in quantity, not convertible into 

 any other element, and incapable of being generated by any physical agency ; in 

 fact, the aclaiowledged principles of latent heat would require to be tested by a 

 most searching investigation before they ought to be admitted, as they usually have 

 been, by almost everyone who has been engaged on the subject, whether in com- 

 bining the results of experimental research or in general theoretical investigations. 



