34 Prof. Tyndall's Notes on Scientific History. 



effect in the case of gunpowder, and finds in certain cases that 

 9 per cent, of the force of the consumed charcoal is expended on 

 the projectile. He gives various illustrations of the generation 

 of heat by mechanical power, and describes some observations of 

 his own, made in a paper-mill, in which were four pulping 

 machines, each containing about 80 lbs. of paper and 1200 lbs. 

 of water. The surrounding temperature being 15° C, the pulp 

 rose in thirty-two minutes from 14° to 16°. The highest observed 

 temperature, which remained uniform for several hours, was 30°. 

 Assuming that in one minute a horse can raise 27,000 lbs. a foot 

 high, the heating of 1280 lbs. of water 1 degree in sixteen minutes 

 (not taking into account the heat communicated to the apparatus) 

 is equivalent to 3'16-horse power. The estimate in the factory 

 was, that the pulping machines were worked with 5 -horse power. 

 Does the mechanical action of the five horses become nothing in 

 the machine ? Fact replies, it becomes heat. 



21. He then goes on to show the relationship of mechanical 

 work to electricity and magnetism, and passes to the con- 

 sideration of chemical processes as compared with mechanical 

 operations. A weight at such a distance from the earth that 

 the attraction is insensible, he regards as in a state of mechanical 

 separation; the falling of the weight to the earth as a case of 

 mechanical combination. Such a weight would reach the earth's 

 surface with a velocity of 34,440 feet a second, and the heat 

 generated by its collision would raise the temperature of an 

 equal weight of water 17,356°. Chemical combination is in 

 principle the same. The chemical combination of 1 gramme of 

 carbon and 2*6 grammes of oxygen is equivalent to the mechani- 

 cal combination of a weight of \ a gramme with the earth. The 

 chemical combination of 1 gramme of hydrogen with 8 grammes 

 of oxygen is equivalent to the mechanical combination of a weight 

 of 2 grammes with the earth. The heat here developed is equal 

 to 34,700 thermal units*. 



'* In 1843 Mr. Joule wrote the following remarkable passage : — " I had 

 before endeavoured to prove that when two atoms combine together, the 

 heat evolved is exactly that which would have been evolved by the electric 

 current due to the chemical action taking place, and is therefore propor- 

 tional to the chemical force causing them to combine. I now venture to 

 state more explicitly that it is not precisely the attraction of affinity, but 

 rather the mechanical force expended by the atoms in falling towards one 

 another, which determines the intensity of the current, and consequently 

 the heat evolved" (Phil. Mag. 1843, vol. xxiii. p. 442). I cite this as one 

 of the points of osculation between these two remarkable men. They 

 thus touched each other repeatedly, Joule being in advance sometimes, 

 and Mayer sometimes. But their main achievements lie in distinct fields ; 

 and these are, in my opinion, so balanced as to render them a kind of 

 " double star, the light of each being, in a certain sense, complementary 

 to that of the other." (Phil. Mag. S. 4. vol. xxvi. p. 67.) 



