454 Prof, Williamson on the Dynamics 



production of force in such a process, only a transfer of motion 

 from masses to atoms. Picture to yourselves a heavy train at the 

 top of a railway incline ; let the train run freely down the incline 

 impelled by its own weight alone, and while it rushes down with 

 headlong speed let the breaks be applied with the utmost force. 

 The train is stopped ; yet the motion which is taken from it is 

 not destroyed, it is merely transformed into heat, for the friction 

 at the breaks evolves such intense heat that it is no easy matter 

 to prevent it setting fire to the wooden parts above them. 

 Again, if we drive the piston of a steam-engine up and down 

 by the pressure of steam, first below, then above it, and if this 

 piston is connected by suitable machinery with moveable masses, 

 we get those masses set in motion as effectually as if they were 

 drawn by hundreds or even thousands of horses. And yet in this 

 case the force is not created any more than in the former instance 

 it was destroyed; for the steam which presses upon the piston 

 loses heat by the effort, and the heat is transformed into work, 

 from which it can be again recovered. 



One of the most important steps of modern science has been 

 the exact measurement, by Mr. Joule, of the quantity of heat 

 which corresponds to a given amount of mechanical force. The 

 force of one pound weight falling 772 feet makes what we call the 

 unit of heat, viz. that quantity needed to raise the temperature of 

 a pound of water 1° F. Whenever mechanical force is expended 

 in heat, or heat transformed into mechanical force, we now know 

 exactly the proportion of the one which can be got from the 

 other. You perceive from this that we not only know heat to 

 be a motion of atoms, but we have even a definite measure of it, 

 as we have of the weights of the atoms themselves. 



Philosophers had long since suspected that matter must be built 

 up of atoms; but it is within the last half century that chemistry has 

 given actual proof that such is really the case. No doubt there 

 are some eminent men, for whom everyone entertains very great 

 respect, who speak of the atomic theory as a mere hypothesis ; 

 and there are also eminent men who speak of the existence of 

 matter in the same sceptical terms. There is no reasoning upon 

 physical questions without assuming the existence of matter, and 

 our friends only deceive themselves when they fancy they can do 

 without it. And, in like manner, there is no reasoning upon 

 chemical transformations without the atomic theory. No other 

 theory has ever been found to explain the facts ; and without it 

 chemical science, such as it now exists, and I may say flourishes, 

 would be transformed into a confused and unmeaning chaos of 

 facts. Every chemist uses the atomic theory when he attempts 

 to explain chemical facts; and chemists are bound to inform 

 those who study nature and her forces from other points of view, 



