December 2, 1898.] 



SCIENCE. 



769 



and by their collision enge;ider all known 

 things. Their paths are, however, not di- 

 rected, but fortuitous ; and, therefore, the 

 world is the product of chance. 



Passing over many centuries, we find 

 Boyle, in the reign of Charles II. , suggest- 

 ing 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 ac- 

 count for the law, due to Boyle, that the 

 volume of gases varies inverselj^ 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 systematized by Clausius in 1857 under 

 the name of the ' Kinetic Theory of Gases.' 



Briefly stated, the theory is this : Granted 

 that in gases the particles — or, as they are 

 now termed, the molecules — of which they 

 consist are widely separated from each 

 other, and that the pressure which the gas 

 exerts on the sides of any vessel in which 

 it may be confined — a pressure which may 

 be I'ealized by pumping away the air out- 

 side the vessel, when, if the vessel is con- 

 structed of yielding material, such as blad- 

 der, 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 containing 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 pur- 

 pose — 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 a blow as a heavy 

 molecule with a slower rate of motion. By 

 Clausius's hypothesis the temperatures of 

 two gases are believed to be equal when the 

 products of their masses into the square of 

 their rates of motion are equal. This is 

 not quite the same thing as saj'ing ' when 

 the force of the blows they give is equal,' 

 but it may be taken as connected with it. 



Supposing, then, that two gases are at the 

 same temperature — that when placed in 

 contact neither gives up heat to the other^ 

 then the product above mentioned must be 

 equal for both. For it is obvious that the 

 specifically lighter gas must have the higher 

 velocity ; that is, the molecules must be 

 endowed with a higher rate of motion. 



What is that rate of motion ? Clausius 

 was able to answer that question : A mole- 

 cule of hydrogen, the lightest gas known, if 

 it moved in a straight line, unimpeded in 

 its motion bj^ coUison with any other mole- 

 cules or with any solid body, would pass 

 through no less than a mile and a quarter 

 in a second. And a molecule of oxygen 

 equally free to move would travel through 

 space with a velocity of rather less than 

 one-third of a mile per second. The rela- 

 tive rates of motion are, therefore, in inverse 

 proportion to the square roots of the densi- 

 ties of the gases. Thus, as oxygen is six- 

 teen times as heavy as hydrogen, a mole- 

 cule of hydrogen would move through space 

 in a straight line, were it free to do so, at a 

 rate four times as great as that at which a 

 molecule of oxygen moves. 



