29© GREAT MEN OF SCIENCE 



second paper in 1845; it was Joule's endeavour to test the 

 agreement by means of measurement in the most varied 

 manner possible, and this occupied him for thirty-five years 

 with experiments of continually increasing perfection and 

 refinement, lasting from 1843 to 1878.^ 



It was a curious fact in this connection that Joule did not 

 at first (1843) regard it as particularly necessary to carry out 

 more elaborate experiments after what had already been dis- 

 covered. He says rather - after he had also forced water in 

 a calorimeter through narrow tubes, with measurement of 

 the mechanical work required, and again obtained a fair 

 agreement of the heat produced (423 mkg/cal) - that he 

 did not wish to waste further time on experiments, in the 

 conviction 'that the grand agents of nature are indestructible 

 by the Creator's fiat.'^ 



It was only gradually, and also as the result of requests 

 from outside, that he became more and more absorbed in the 

 investigation, with great exactitude, of further processes of 

 transformation. Thus in the period mentioned he meas- 

 ured the work done and the heat produced by compression 

 of air, by friction of a paddle in water, by friction in oil, and 

 in mercury, and finally once more by electrical heating, the 

 latter with greatly refined apparatus. All the values for the 

 equivalent agree with one another and with Mayer's value as 

 far as the accuracy of measurement of the time allows us to 

 expect, and the result of the two most careful investigations 

 (425 mkg/cal) could for long be regarded as the best value 

 known for the mechanical equivalent of heat, whereby Joule, 

 originally a man of practical affairs with an enthusiasm 

 for the highest type of scientific investigation, came to be 



^ From later and more exact measurements of the specific heats of air 

 the figure 424 mkg/cal was found by Robert Mayer's reasoning, while 

 later and more refined measurements by Joule of current heat gave 425 

 mkg/cal. The closer agreement in the two figures corresponds to the 

 improvement in the methods of measurement. 



^Philosophical Magazine series 3, Vol. 23, 1843; Collected Papers of 

 J. T. Tou/e, Vol. i., pp. 157-158. 



