164 Seat and Organization. 



shall trace this kind of action to a greater extent ; but it may 

 now be stated that a certain rate of composition and decompo- 

 sition is essential to organic life, and that heat is one of the 

 influences by which such actions are stimulated and controlled. 



Temperature determines whether bodies shall be solid, 

 fluid, or gaseous. Organic life, as we know it, demands the 

 co-operation of matter in these three states, and, consequently, 

 too much or too little heat is fatal to its existence. 



When heat is applied to any substance, it acts in two ways. 

 It effects a certain change in the arrangement and mutual dis- 

 tance of its particles, and it also warms it. That portion of 

 the heat which is employed in changing the internal condition 

 of the substance does not directly help to make it hot. When 

 the fact was first discovered that the temperature of a body, as 

 measured by a thermometer, or made manifest to our sensa- 

 tions, was not augmented in direct proportion to the quantity 

 of heat communicated to it, the term latent heat was employed ; 

 but the new discoveries concerning the correlation of forces 

 afford a better explanation, and we no longer regard the heat 

 as hidden, but as having taken another form, or become an- 

 other force. It can be shown, for example, that a certain quan- 

 tity of heat is convertible into an equivalent quantity of mechani- 

 cal force, and, on the other hand, that a certain quantity of me- 

 chanical force is convertible into an equivalent quantity of heat. 

 Firing off a rifle is a good illustration of this matter. The 

 heat arising from the combustion of the powder expands the 

 solid materials of which it is composed into an enormously 

 larger bulk of composite gases. By suddenly occupying more 

 room, the powder expels the ball with great velocity, and the 

 ball moves because heat has been converted into motion. On 

 reaching the target, the ball stops, and both it and the target 

 are heated. In this case motion has been converted into heat. 

 In firing great guns at iron targets a flash like lightning 

 appears at the moment when the motion of the projectile is 

 arrested. If the earth's centrifugal motion ceased, it would 

 fall into the sun, and the " amount of heat generated by the 

 blow would be equal to that developed by the combustion of 

 560(» worlds of solid carbon. "* 



The temperature of a body is, as we have said, not an in- 

 dication of the quantity of heat it has received. " To raise a 

 pound of water, for example, 1 J , would require thirty times the 

 amount of heat necessary to raise a pound of mercury V. Con- 

 versely, the pound of water in falling through 1°, would yield 

 up thirty times the amount of heat yielded up by the pound of 

 mercury." The heat which is not engaged in producing 

 hotness, if we may use the word, has been employed in in- 

 * Tyndall's Beat as a Mode of Motion, p. 43 ; ibid, p. 143. 



