FUNDAMENTAL PROPERTIES OF MATTER 27 



The subject will be clearer from the following considerations : A 

 gas behaves much like a spring. If we put weights (force) on the 

 spring it will be compressed that is, its volume will become less, and 

 if the weights be gradually removed, the spring will expand. Sim- 

 ilarly, if we take a metallic tube closed at one end, and fitted with a 

 piston and handle and containing a certain volume of air, for 

 example, a bicycle pump, and then press down upon the handle, the 

 volume of air will become smaller and smaller as the pressure on the 

 handle is increased. It will be observed also that as the volume 

 becomes smaller it oifers more and more resistance to compression, 

 that is, its tension or elastic force becomes greater. Also, since the 

 amount of material in the original volume of air remains the same 

 during the experiment, it is plain that when the air is compressed to 

 a small volume, the amount of material in a unit volume, say 1 cubic 

 inch, is very much increased ; in other words, the density of the gas is 

 increased as the pressure is increased. If accurate measurements be 

 made of the different pressures applied and the corresponding vol- 

 umes assumed by the air, a constant relationship will be discovered, 

 such as is expressed in Boyle's Law, stated above. The meaning of 

 the phrase in the law, the volume of a gas is inversely as the pressure, 

 is that as the one quantity is increased the other is decreased in just 

 the same proportion. To illustrate, if a volume of a gas measure 1 

 cubic foot under a pressure of 10 pounds, and the pressure be 

 increased to 15 pounds, in other words, || or f times, then the vol- 

 ume will change from 1 cubic foot to 1 -s- f = f cubic foot. If the 

 pressure be decreased to say 6 pounds, or -f^ - f of its original value, 

 then the volume will become 1 -*- f = f cubic foot, that is, it will 

 expand. 



Such a relation as is expressed in Boyle's Law is known in algebra 

 as inverse proportion. Since one of the quantities is always decreased 

 in the same ratio as the other is increased, it follows that the product 

 of the two quantities thus related must always be the same, that is, a 

 constant. Hence, another way of expressing Boyle's Law is to say 

 that the product of the pressure and the corresponding volume of a 

 definite quantity of a gas is always the same. If V and V 1 repre- 

 sent the volumes of a certain amount of gas, at the corresponding 

 pressures, P and F, then V X P = V 1 X P. In the example 

 given above, Y == 1 cubic foot, P = 10 pounds, and P 1 = 15 pounds, 

 hence, 1 X 10 = V 1 X 15, or V 1 == |g- = f cubic foot. 



As will be seen later, this law is of great value in all experiments 

 where results are calculated from measurements of gas volumes. 



