Page:Popular Science Monthly Volume 5.djvu/115

Rh 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 dissolved 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 detect the heat radiated by the moon. When the moonbeam passed gradually across the instrument, the index was deflected several degrees, 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 degree 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 attention 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 science 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 diffused 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 Séguin, a Frenchman, in 1839, and by Mayer, a German, in 1842, who had propounded the hypothesis that 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, between 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,