SKETCH OF DR. JA3£ES P. JOULE. 105 



Transactions.' Dr. Lyon Playfair and Dr. Joule have also published 

 an account of a conjoint investigation into the volumes occupied by 

 bodies, both in the solid form and when dissolved in water, and have 

 obtained results of an unexpected nature as well as of great value. 

 Among other curious results, they found that * many salts, when dis- 

 solved in water, do not add to the bulk of the water more than is due 

 to the water actually present in the salts.' They have further shown 

 that *' when salts do add to the bulk of the water in which they are 

 dissolved, the increase of the bulk corresponds to that of a volume, or 

 some multiple of a volume of water.' " 



Dr. Joule's inventive talent was early shown in the construction 

 of galvanometers. In 1863 he described to the Manchester Society 

 his new and extremely sensitive thermometer, which was able to de- 

 tect the heat radiated by the moon. When the moonbeam passed 

 gradually across the instrument, the index was deflected several de- 

 grees, first to the right and then to the left; thus showing that the 

 air in the instrument had been heated a few ten-thousandths of a de- 

 gree by the influence of the rays. These experiments were lately 

 referred to by the present Earl Rosse in a lecture on the same subject 

 delivered at the Royal Institution. 



It was about 1840 that Dr. Joule began to direct his special atten- 

 tion to the subject of heat. He made a communication to the Royal 

 Society in that year, announcing the discovery of a principle in the 

 development of heat by the voltaic principle, in which he established 

 relations between heat and chemical affinity. This paper is recognized 

 as containing the germ of the subsequent unfoldings of dynamical sci- 

 ence in relation to chemical action. 



The old view of the nature of heat still prevailed, although the 

 declarations of Bacon and Locke, and the researches of Rumford and 

 Davy, had undermined the notion that heat was a subtile matter or 

 material agent difiiised throughout all bodies, and had prepared the 

 way for its apprehension as a mode of molecular motion. The first 

 noteworthy advance toward the establishment of the mechanical theory 

 of heat was made by Seguin, a Frenchman, in 1839, and by Mayer, a 

 German, in 1842, who had propounded the hypothesis tbat the heat 

 evolved in compressing an elastic fluid is exactly equivalent to the 

 compressing force. But the theory was not yet established upon 

 an experimental basis, so as to command the assent of the scientific 

 world. 



Independently of what had been done by others, and working in 

 his own line, Dr. Joule had established relations, as we have seen, be- 

 tween heat and chemical affinity in 1840, and, some two years later, 

 he applied the dynamical theory to steam-engines, to electro-magnetic 

 engines, to vital processes, and to chemistry. "His paper on the 

 * Electric Origin of Heat' was a first communication, in 1842, to the 

 meeting of the British Association at Manchester — the last meeting. 



