112 CHEMICAL DISCOVERY AND INVENTION 



Down to quite the end of the eighteenth century it was sup- 

 posed that heat was a kind of substance which existed in all 

 sorts of matter and was squeezed out of it when subjected to 

 pressure or friction. The material of heat was called caloric, and 

 when associated with certain kinds of matter in sufficient 

 quantity the liquid or gaseous state was produced. Lavoisier, 

 for example, spoke of oxygen gas as made up of the basis of 

 oxygen combined with caloric. But very soon after this time 

 Rumford showed that a given mass of any metal, such as brass, 

 can give out heat in indefinitely large quantity when subjected 

 to friction in the process of boring. Sir Humphry Davy a few 

 years later demonstrated that ice can be melted by merely 

 rubbing it, though kept all the time in an atmosphere below the 

 freezing point of water. Finally, in 1843, Joule of Manchester 

 showed that there is quantitative relation between the work 

 done by a body falling under the influence of gravity and the 

 heat which is produced in the process. Joule's experiments led 

 to the result that a mass of 772 Ibs. falling through 1 foot or of 

 1 Ib. falling through 772 feet may produce, by friction or other- 

 wise, heat enough to raise the temperature of 1 Ib. of water 

 1 Fah. This expresses in ordinary English weights and measures 

 the mechanical equivalent of heat. It may be expressed in grams, 

 metres, and degrees centigrade, or otherwise, and the work done 

 by the falling mass, attracted by the earth, may be utilised 

 either to produce heat by friction directly, or it may first produce 

 an electric current which may then be converted into heat, but 

 the same quantitative relation is maintained. Hence we have 

 the theory that heat is a " mode of motion " and that this 

 motion is convertible into heat, light, electricity or magnetism, 

 production of steam, and so mechanical force, or finally into 

 chemical action. In any one of these cases the body concerned 

 is said to possess energy, and the work it can do while changing 

 its state is a measure of the amount of energy available. The 

 energy of a body in motion is what is called kinetic energy (KLVGW, 

 to move), while that which it owes to its position or chemical 

 state is called its potential energy. The one being convertible 

 into the other, the sum of these two quantities is constant. 

 These facts have been studied from many sides by a large 

 number of physicists and engineers, beside those whose names 

 have already been mentioned. Among those of the past are the 

 names of Carnot and Meyer, while those of Helmholtz, Kelvin, 



