350 Professor Helmholtz on the Law of the [April 12, 



that it becomes glowing and luminous, by only beating it continuously 

 with a hammer. Now, if mechanical power is produced by heat, we 

 always find that a certain amount of heat is lost ; and this is pro- 

 portional to the quantity of mechanical work produced by that heat. 

 We measure mechanical work by foot-pounds, and the amount of heat 

 we measure by the quantity of heat which is necessary to raise the 

 temperature of one pound of water by one degree, taking the centi- 

 grade scale. The equivalent of heat has been determined by Mr. Joule, 

 of Manchester. He found that one unit of heat, or that quantity of 

 heat which is necessary for raising the temperature of a pound of water 

 one degree centigrade, is equivalent to the mechanical work by which 

 the same mass of water is raised to 423i metres, or 1389 English feet. 

 This is the mechanical equivalent of heat. 



Hence, if we produce so much heat as is necessary for raising the 

 temperature of one pound of water by one degree, then we must apply 

 an amount of mechanical work equal to raising one pound of water 

 1389 English feet, and lose it for gaining again that heat. 



By these considerations, it is proved, that heat cannot be a pondera- 

 ble matter, but that it must be a motive power, because it is converted 

 into motion or into mechanical power, and can be either produced by 

 motion or mechanical power. Now, in the steam-engine we find that 

 heat is the origin of the motive power, but the heat is produced by 

 burning fuel, and therefore the origin of the motive power is to be 

 found in the fuel, that is, in the chemical forces of the fuel, and in the 

 oxygen with which the fuel combines. 



You see from this, that the chemical forces can produce mechanical 

 work, and can be measured by the same units and by the same 

 measures as any other mechanical force. We may consider the 

 chemical forces as attractions, in this instance, as attraction of the 

 carbon of the fuel for the oxygen of the air ; and if this attraction 

 unite the two bodies, it produces mechanical work just in the same 

 way as the earth produces work, if it attract a heavy body. Now the 

 conservation of force, of chemical force, is of great importance for 

 our subject to-day, and it may be expressed in this way. If you have 

 any quantity of chemical materials, and if you cause them to pass 

 from one state into a second state, in any way, so that the amount 

 of the materials at the beginning, and the amount of the materials at 

 the end of this process be the same, then you will have always the 

 same amount of work, of mechanical work or its equivalent, done 

 during this process. Neither more nor less work can be done by the 

 process. Commonly, no mechanical work in the common sense is done 

 by chemical force, but usually it produces only heat ; hence the 

 amount of heat produced by any chemical process must be independent 

 of the way in which that chemical process goes on. The way may be 

 determined by the will of the experimenter as he likes. 



We see, therefore, that the energy of every force in nature can be 

 measured by the same measure, by foot-pounds, and that the energy 

 of the whole system of bodies which are not under the influence of 



