358 THE POPULAR SCIENCE MONTHLY. 



lute electrometer, which serves for reducing the scale-readings of other instru- 

 ments to absolute measure, and which was used by Thomson in his measurement 

 of the electrostatic force producible by a Daniell's battery and in many other 

 investigations. Those who have seen the collection of electrometers in the Loan 

 Collection at South Kensington will not think it too much to say that to Sir "W. 

 Thomson is due our present system of practical electrometry. 



" But while thus engaged in investigations in electrostatics and magnetism, 

 there were many other branches of science that were receiving from him 

 advancement in a not less remarkable way. There is no part of his work of 

 higher importance than his investigations on the Dynamical Theory of Heat. 

 These were communicated in a series of papers to the Eoyal Society of Edin- 

 burgh, the first of which was given in 1849. It was a critical account of Car- 

 not's memoir of 1824, 'Eeflexions sur la Fuissance Motrice du Feu.' Though 

 Eumford and Davy had, in the beginning of this century, experimentally dis- 

 proved the material theory of heat, their experiments and arguments were un- 

 heeded and nearly unknown; and it was only after 1843, when Joule actually 

 determined the dynamical equivalent of heat, that the great truth that heat is a 

 mode of motion was admitted and appreciated. Thus Carnot, although dissatis- 

 fied with it, was obliged to adopt the material theory of heat in 1824; and, re- 

 garding heat as indestructible, spoke of the letting down of the heat from a 

 higher to a lower temperature, and looked on the production of work by the 

 heat-engine as a phenomenon analogous to that in which water, descending from 

 a higher to a lower level, does work by means of a water-wheel. Thomson, 

 among the first to appreciate the importance of Joule's results, set himself to 

 alter the theory given by Carnot into agreement with the true theory ; and in 

 the series of papers referred to, placed the whole science of thermodynamics 

 on a thoroughly scientific basis. In 1846 he first suggested the reckoning of 

 temperature on an absolute thermodynamic scale independent of the properties 

 of any particular substance. Subsequently, in consequence of experimental in- 

 vestigations of the thermodynamic properties of air, and other gases, made in 

 conjunction with Joule, he showed how to define a thermodynamic scale of tem- 

 perature having the convenient property that air-thermometers and other gas- 

 thermometers agree with it as closely as they agree with one another. This 

 system of reckoning temperature gives great facility for the simple expression 

 of thermodynamic principles and results. 



"Having here mentioned Joule and Thomson together, we cannot omit to 

 remark that somo of the most admirable researches in thermodynamics were 

 those undertaken in conjunction by these two attached friends. 



"Among the many important results of Sir W. Thomson's investigations in 

 thermodynamics, one of the most remarkable was his discovery of the principle 

 of dissipation of energy, announced by him in 1852. During any transforma- 

 tion of energy of one form into energy of another form there is always a certain 

 amount of energy rendered unavailable for further useful application. No known 

 process in Nature is exactly reversible, that is to say, there is no known process 

 by which we can convert a given amount of energy of one form into energy of 

 another form, and then, reversing the process, reconvert the energy of the sec- 

 ond form thus obtained into the original quantity of energy of the first form. 

 In fact, during any transformation of energy from one form into another, there 

 is always a certain portion of the energy changed into heat iu the process of 

 conversion; and the heat thus produced becomes dissipated and diffused by ra- 

 diation and conduction. 



