278 THE KINETIC THEORY OF GASES. 



attempt will be made to show how the thoughts of many men, each 

 striving to "increase natural knowledge," as the formula of admission 

 to .the Royal Society runs, have led to a discovery of some interest — 

 that of a hitherto unsuspected constituent of atmospheric air. 



The Roman poet Lucretius, a .friend and contemporary of Cicero, 

 was the author of a poem entitled "De Rerum Natural ("On the 

 Nature of Things"). In this poem, which treats of almost all things 

 in heaven and earth, he argues that the atoms, the existence of which 

 is obvious because one sees them in a cone of light passing through a 

 dark room, fall rapidly together in their dancing course throughout 

 the spheres, and by their collision engender all known things. Their 

 paths are, however, not directed, but fortuitous; and, therefore, the 

 world is the product of chance. 



Passing over many centuries, we find Boyle, in the reign of 

 Charles II, suggesting that the difference between different kinds of 

 matter is to be explained by the nature and the motion of the particles 

 or atoms of which they are composed. The region of speculation was 

 narrowed when Daniel Bernoulli, in 1738, attempted to account for the 

 law, due to Boyle, that the volume of gases varies inversely with the 

 pressure to which they are exposed; and similar attempts were made 

 by Herapath in 1821, and by Joule in 1851. Their ideas were systema- 

 tized by Clausius in 1857 under the name of the "Kinetic Theory of 

 Gases." 



Briefly stated, the theory is this : Granted that in gases the par- 

 ticles — or, as they are now termed, the molecules — of which they con- 

 sist are widely separated from each other, and that the pressure which 

 the gas exerts on the sides of any vessel in which it maybe confined — 

 a pressure which may be realized by pumping away the air outside the 

 vessel, when, if the vessel is constructed of yielding material, such as 

 bladder, it will distend, and ultimately burst — is caused solely by the 

 bombardment of the molecules of gas on the walls. It is at the first 

 blush not very easy to conceive of a steady pressure being due to an 

 enormous number of impacts irregularly delivered. But there are 

 many analogies which help to form the conception. For instance, a 

 musical note, which may strike us as of the utmost smoothness and 

 uniformity, is in reality the result of a succession of blows on the 

 tympanum of the ear, each following the preceding one too rapidly for 

 our ears to distinguish the break in continuity. In a similar manner 

 the pressure of a gas is accounted for. And the temperature, a rise in 

 which also increases the pressure of a gas on the walls of a vessel con- 

 taining it, is attributed to the increased velocity of the molecules of 

 the gas. Now, for simplicity's sake, considering a blow given by only 

 one molecule, the force of the blow — to use a rough expression which 

 will serve the purpose — will depend not merely on the rate at which 

 that molecule is moving, but also on the weight of that molecule. So 

 that a light molecule with a high rate of motion may deliver as forcible 



