1908] on the Sdentific Work of Lord Kelvin. 207 



When heat is allowed to flow away to a lower temperature without 

 passing through an engine, its capacity for doing work has been 

 dissipated. The opportunity for obtaining mechanical power from it 

 has vanished l)eyond recall. Can then heat be correctly measurable 

 as mechanical energy if some of the mechanical energy is lost irre- 

 coverably every time that the heat diffuses to a lower temperature ? 

 Thomson, ever attracted by tlie engineering side of things, was domi- 

 nated by Carnot's principle, as we have seen, even when as a youth 

 in 1845 he went to Paris to Regnault's laboratory. Thus he at once 

 set himself to explore its practical content by the aid of the mass of 

 exact data on gases acquired by Regnault, as soon as these results 

 appeared, in 1847, as the first instalment of the famous series of 

 experimental researches, which had been subsidised by the French 

 Government with a view to obtaining all the data that could be 

 pertinent towards the improvement of knowledge of the principles 

 of steam and gas engines. In Thomson's first paper towards this 

 end, entitled ' On an Absolute Thermometric Scale founded on 

 Carnot's Theory of the Motive Power of Heat, and calculated from 

 Regnault's Observations,' he clears the ground for exact physical 

 reasoning by elevating the idea of temperature from a mere feature- 

 less record of comparison of thermometers into a general principle 

 of physical nature, making it a measure of the dynamical potentiality 

 of heat, which is, on Carnot's principles, an intrinsic measure, i.e. 

 quite independent of the substances in which the heat happens to be 

 contained. But he cannot get rid of the impression that heat is 

 something different from energy, which may produce energy in fall- 

 ing to a lower level of temperature, or on the other hand may diffuse 

 passively so that this opportunity of creating energy is irrecoverably 

 wasted. Such a view would tend towards the caloric theory which 

 held that heat is somehow substantial ; in terms of it Carnot, in fact, 

 formulated his arguments. It has been remarked on this by Helm- 

 holtz that if Carnot had then possessed completer knowledge he would 

 possibly never have hit upon his principle ; on the other hand, his 

 rough manuscripts, published many years after, have revealed that 

 during the remaining six years of his short life he w^as inclining 

 strongly towards the correct view on the nature of heat. In a foot- 

 note, Thomson gives expression to his own doubt. The experiments 

 of " Mr. Joule, of Manchester," seem " to indicate an actual con- 

 version of mechanical effect into caloric. No experiment, however, 

 is adduced in which the converse operation is exhibited ; but it must 

 be confessed that as yet much is involved in mystery with reference 

 to these fundamental questions of Natural Philosophy." And in a 

 fuller account, soon after, of Carnot's Theory,* as further developed 

 numerically by aid of the data given by Regnault's experiments on 

 steam, he adheres substantially to this position, " although this, and 



* Trans. R. S. Edinburgh, January 2, 1849. 



