352 THE MECHANICAL EQUIVALENT OF HEAT. 



Customary language, according to which gravity is called 

 a moving force and heat a substance, occasions, on the one 

 hand, the significance of an important natural object, falling- 

 space, or the space through which a body falls, to be kept as 

 much as possible out of sight, and, on the other hand, heat to 

 be removed to the greatest possible distance from the vis viva 

 of motion. The sciences are thus reduced to an artificial sys- 

 tem, over whose fissured surface we can advance in safety 

 only by the powerful aid of the higher analysis. 



Without doubt the fact that so simple and obvious a mat- 

 ter as the connection between heat and motion could remain 

 unperceived up to the most recent times must also be attrib- 

 uted to the same defect. Nevertheless, as has been already 

 pointed out, the quantitative determination of chemical heat- 

 ing-effects and of galvanic actions, as well as researches into 

 vital phenomena, instituted in the spirit of those of Liebig, 

 must soon have led to the law, not difficult to discover, of the 

 equivalence of heat and motion. 



In reality this law and its numerical expression, the me- 

 chanical equivalent of heat, were published almost simulta- 

 neously in Germany and in England. 



Starting from the fact that the amount of chemical as well 

 as of galvanic effect is dependent only and solely on the 

 amount of material expenditure, the celebrated English phys- 

 icist Joule was led to the principle that the phenomena of 

 motion and of heat rest essentially upon one and the same 

 foundation^ or, as he expressed himself, in the same way as I 

 have done, heat and motion are transformable one into the 

 other. 



Not only did this philosopher indisputably make an inde- 

 pendent discovery of the natural law in question, but to him 

 belongs the credit of having made numerous and important 

 contributions towards its further establishment and develop- 

 ment. Joule has shown that when motion is produced l>y 

 means of electro-magnetism, the heating effect of the galvanic 



