CHEMICAL FORCE AND HEAT. 915 



raise its temperature 1 Centigrade ; and these 1000 heat-units will lift the 

 same weight of water, viz., 1 kilogramme, 424. metres high; or, conversely, 

 424 kilogrammes, 1 metre high ; 212 kilogrammes, 2 metres ; or 106 kilo- 

 grammes, 4 metres high, and so on. The mechanical equivalent of 1000 heat- 

 units is expressed, therefore, as 424 metre-Mogrammes (met. kils.). 



In English works the scale of temperature employed is that of Fahrenheit, 

 of which 1.8 are equal to 1 Cent. ; the weight is the Ib. av., of which 2.2 are 

 equal to 1 kilogramme, and the measure of height is 1 foot, of which 3.28 are 

 equal to 1 metre. The mechanical equivalent of a given quantity of heat is 

 expressed in foot-pounds. Thus the heat which will raise the temperature of 

 1 Ib. of water, 1 Fahr., will lift that weight, viz., 1 Ib., to a height of 772 feet, 

 or 772 Ibs. to a height of 1 foot, and so on ; hence the mechanical coefficient of 

 heat in this system is 772, and its mechanical equivalent is expressed as 772 

 foot-pounds (ft. Ibs.). To reduce the English ft. Ib. into the French met. kils., 

 divide the former by 7.216, which is the number obtained by multiplying the 

 number of pounds in a kilogramme by the number of feet in a metre, viz., 2.2 

 X 3.28. On the other hand met. kils. multiplied by 7.216, are changed into ft. 

 Ibs. 



Not only is heat convertible into mechanical work, but mechanical work may 

 be reconverted into heat, and the same equivalents, as before, express the ratio 

 between them. Thus a mechanical force equal to lifting 1 kilogramme of water 

 424 metres high, will, when employed in friction, blows, or otherwise, develop 

 a temperature equal to 1000 heat-units, or will raise the temperature of 1000 

 grammes, i. e., of 1 kilogramme of water, 1 Cent. In friction, for example, 

 according to the physical theory, so ably expounded by Tyndall, that heat is 

 a vibratory motion amongst the molecules of matter, the resistance arrests, 

 to a certain degree, the motion of masses of matter rubbed against each other ; 

 but the visible motion so disappearing, is transferred to the molecules, and so 

 causes the invisible motion known to us as heat. The frequency of these vi- 

 brations increases in proportion to the sensible heat produced. 



Quantities of Heat developed ~by the Chemical Process of Combustion. 



In order to be able to determine the relation between chemical force and me- 

 chanical work, it remains to be ascertained what is the amount of heat evolved 

 by the combination of proportional quantities, or atomic weights, of two or 

 more elementary substances. The heat evolved in the combustion of charcoal 

 with oxygen, is thus measured, the heat itself being supposed to be the result 

 of an almost infinitely rapid motion of the combining molecules, through al- 

 most infinitely minute distances. Experiments made on the amount of heat 

 imparted to water in a calorimeter, have established the fact, that very differ- 

 ent amounts of heat are given off by burning equal weights of different com- 

 bustible bodies. The mode of estimating these varying quantities is, by 

 measuring the heating effects produced by the combustion in oxygen, of 1 

 gramme weight of each substance, upon the standard gramme of water em- 

 ployed in the calculation of the heat-units. (Favre and Silbermann.) In this 

 way, 1 gramme of carbon, in combining with oxygen in the act of perfect 

 combustion, to form carbonic acid, evolves as much heat as will raise the tem- 

 perature of 8080 grammes of water 1 Cent. ; in other words, it evolves 8080 

 heat-units. Again, 1 gramme of hydrogen, in uniting with oxygen to form 

 water, evolves 34,462 heat-units. Now, carbon and hydrogen are the two 

 chief combustible or oxidizable elements of the food, the blood, and the solid 

 tissues ; the nitrogen passes out of the body un oxidized, but combined in the 

 urea. Most of the carbon and hydrogen escape from the system completely 

 oxidized, as carbonic acid and water ; but some of each of those elements, es- 

 pecially of the carbon, appear combined with a little oxygen, and also with the 

 nitrogen in the urea. Moreover, the quantity of heat given out by combusti- 

 ble bodies appears to be the same, whether they are oxidized slowly or rap- 

 idly. The physical data just explained may therefore be applied to the quan- 

 titative examination of the relations between the chemical changes occurring 

 in the human body, and the amount of mechanical and calorific work performed 

 in it. 



