690 THE POPULAR SCIENCE MONTHLY, 



pressure, and the proportions of suoh increase which result from heat- 

 ing a gas. Similarly, Charles's important law, that the volume of a 

 given mass of gas under a constant pressure varies directly as its tem- 

 perature, follows obviously from the hypothesis. 



Priestley was the first to remark that gases diffuse through each 

 other. This fact is familiarly illustrated by the passage of odorous 

 gases through the atmosphere. If a bottle of ether is opened in a 

 room, its vapor diffuses through the air, and its presence is soon recog- 

 nized by the sense of smell. In this case, the ether-molecules may be 

 figured as issuing from the bottle with great velocity ; and, if their 

 course were not interrupted by striking against the molecules of the 

 air, the room would be instantaneously permeated by their odor. But 

 the molecular particles of both air and ether are so inconceivably nu- 

 merous, that they can not avoid striking one another frequently in 

 their flight. Every time a collision occurs between two molecules, the 

 paths of both are changed ; and the course of each is so continually 

 altered that it is a long time in making any great progress from the 

 point at which it set out, notwithstanding its great velocity. 



We must next inquire how these velocities are measured, and what 

 is their amount. We have seen that the pressure exerted by a gas is 

 due to what may be approjDriately called the molecular bombardment 

 of the walls of its containing vessel ; and, knowing this pressure, we 

 can calculate the velocity of the projectiles, if we can ascertain their 

 weight, just as we can estimate the speed of a bullet when its weight 

 and mechanical effect are known. Now, a cubic centimetre of hydro- 

 gen at a pressure of one atmosphere weighs about one thousandth part 

 of a gramme ; we have, therefore, to find at what rate this mass must 

 move whether altogether or in separate molecules makes no differ- 

 ence to produce this pressure on the sides of a cubic centimetre. The 

 result gives six thousand feet per second as the velocity of the mole- 

 cule of hydrogen, while in other gases the speed is much less. 



The question of molecular weights brings us face to face Avith the 

 chemical aspect of the hypothesis ; and we have now to examine the 

 support which is given to it by chemical phenomena, and show how 

 wonderfully these are correlated with the physical proofs. Bearing in 

 mind the distinction between physical and chemical changes, we know 

 that we can make a mixture of finely divided sulphur and iron, for 

 example, in any proportion. But these bodies when heated combine 

 chemically to form a new substance called sulphide of iron ; and the 

 two classes of products exhibit great differences, which are indicated 

 by a most remarkable characteristic. Chemical combination, unlike 

 mechanical mixture, always takes place in certain definite proportions. 

 Thus, fifty-six grains of iron combine with exactly thirty-two grains 

 of sulphur ; and, if there is any excess of either substance, it remains 

 uncorabined. This principle is known as the law of definite combin- 

 ing proportions, and the atomic theory, which, in one shape or an- 



