1908] on the Scientific Work of Lord Kelvin. 211 



he had adopted this principle in connection with Joule's principle, 

 notwithstanding that he could not then resolve the apparent dis- 

 crepancy, as the basis of a practical investigation of the motive power 

 of heat in air and steam engines. " It was not until the commence- 

 ment of the present year that I found the demonstration given above. 

 .... It is with no wish to claim priority that I make these state- 

 ments, as the merit of first establishing the proposition upon correct 

 principles is entirely due to Clausius, who published his demonstration 

 of it in the month of May last year, in the second part of his paper 

 on the motive power of heat. I may be allowed to add that I have 

 given the demonstration exactly as it occurred to me before I knew 

 that Clausius had either enunciated or demonstrated the proposition. 

 .... The reasoning in each demonstration is strictly analogous to 

 that which Carnot originally gave." 



Once Thomson gets thus under weigh, as we have seen, by his 

 own unaided efforts though anticipated by Clausius, he develops 

 rapidly the thermal aspects of the subject, concurrently with Clausius 

 and Rankine, but with wider generality, in particular avoiding their 

 hypotheses connected with perfect gases. So little was he prepared 

 to trust to a permanent gas thermometer as giving practically the in- 

 trinsic dynamical scale of temperature, that the following year he had 

 already begun Avith Joule their series of laborious joint experiments 

 to determine exactly how much the gas thermometer differs from the 

 absolute scale. Their procedure was to deduce the result sought 

 from observation of the slight cooling or heating produced by 

 driving the gas under high pressure through a porous partition ; with 

 a perfect gas the process would be isothermal. When we consider 

 that the results were to lead straight into the very core of molecular 

 dynamics, the investigation may well rank to this day as one of the 

 most striking advances in the record of physical science. It is note- 

 worthy that Thomson in his own work kept on with the symbol for 

 the unknown Carnot's function, until the dynamical scale had thus 

 been experimentally investigated ; though a gas thermometer was 

 •doubtless adequate to give to Clausius and Eankine indications of 

 absolute temperature, so far as required for their preliminary approxi- 

 mate investigations ever limited range. We have only to think of 

 the modern physical undertakings steadily pushed downward toward 

 the absolute zero of temperature, to realise that, except on the basis 

 ■of Thomson's dynamical scale of 1847 and his method conjointly 

 with Joule of exactly realising it in 1852, there could be no such 

 thing as temperature in a scientific sense, and low temperature 

 research would be devoid of most of its significance. These essen- 

 tial foundations for the scientific treatment of Energy were laid 

 firmly in 1852, in a way that has held good without substantial 

 modification ever since. 



Perhaps this point, the rigorous scientific generality of the founda- 

 tions on which he built from the beginning, could not be enforced 



P 2 



