XXVI INTRODUCTION. 



the attraction of the sun, it were to impinge on the latter (whereby, according 

 to J. R. Mayer, its final velocity would be 85 geographical miles per second), the 

 amount of heat produced by the collision would be equal to that produced by the 

 combustion of a mass of pure charcoal more than 5000 times as heavy (Julius v 

 Robert Mayer, Helmholtz). 



Thus, the heat of the sun itself can be produced by the collision of masses of cold 

 matter. If the cold matter of the universe were thrown into space, and there 

 left to the attraction of its particles, the collision of these particles would ulti- 

 mately produce the light of the stars. At the present time, numerous cosmic bodies 

 collide in space, while innumerable small meteors (94,000-188,000 billions of kilos, 

 per minute) fall into the sun. The force of gravity is perhaps, in fact, the only 

 source of all heat (J. R. Mayer, Tyndall). 



We have a homely example of the transformation of kinetic energy into heat in 

 the fact, that a blacksmith may make a piece of iron red-hot by hammering it. Of 

 the conversion of heat into kinetic energy, we have an example in the hot watery 

 vapour (steam) of the steam-engine raising the piston. An example of the conver- 

 sion of potential energy into heat occurs, in a metallic spring, when it uncoils and 

 is so placed as to rub against a rough surface, producing heat by friction. 



4. Chemical Affinity : Relation to Heat. Whilst gravity acts upon 

 the particles of matter without reference to the composition of the 

 body, there is another atomic force which acts between atoms of a 

 chemically different nature ; this is chemical affinity. This is the force, 

 in virtue of which the atoms of chemically-different bodies unite to 

 form a chemical compound. The force itself varies greatly between the 

 atoms of different chemical bodies ; thus, we speak of strong chemical 

 affinities and weak affinities. Just as we were able to estimate the 

 potential energy of a body in motion, from the amount of heat which 

 was produced when it collided with an unyielding body, so we can 

 measure the amount of the chemical affinity by the amount of heat 

 which is formed, when the atoms of chemically-different bodies unite to 

 form a chemical compound. As a rule, heat is formed when separate, 

 chemically-different atoms, form a compound body. When in virtue 

 of chemical affinity, the atoms of 1 kilo, of hydrogen and 8 kilos, of 

 oxygen unite to form the chemical compound water, an amount of heat 

 is thereby evolved which is equal to that produced by a weight of 

 47,000 kilos, falling and colliding with the earth from a height of 

 1000 feet above the surface of the earth. If 1 gram, of H be burned 

 along with the requisite amount of O to form water, it yields 34,460 

 heat-units or calories; and 1 gram, carbon burned to carbonic acid 

 (carbon dioxide) yields 8,080 heat-units. Wherever, in chemical processes, 

 strong chemical affinities are satisfied, heat is set free i.e., chemical affinity 

 is changed into heat. Chemical affinity is a form of potential energy 

 obtaining between the most different atoms, which during chemical 

 processes is changed into heat. Conversely, in those chemical processes 

 where strong affinities are dissolved, and chemically-united atoms 

 thereby pulled asunder, there must be a diminution of temperature, or, 



