130 SEVENTEENTH CEbTURY. PT. III. 



He next added more mercury, till it stood 30 inches higher 

 in the long end than in the short one (as seen in A* 2 ). The 

 air between b and c was now pressed down twice as much as 

 before, for it had the 30 inches of mercury weighing upon 

 it, as well as the atmosphere, which equalled another 30 

 inches. Boyle found that this dotible pressure had squeezed 

 it into half the space (b c, fig. A 2 ) ; in other words, by 

 doubling the pressure he had halved the volume of the air. 

 He then poured in 30 inches more mercury, making the 

 pressure three times as great as at first, and he found the air 

 was now compressed into one-third of the space it had filled 

 at first. And this he proved to be the law of compression of 

 air and of all gases, that the volume of a gas (that is, the 

 space it fills) is decreased in proportion as the weight upon it is 

 increased. If you double the pressure you halve the 

 volume ; if you halve the pressure you double the volume. 



This law of the compressibility of gases is known as 

 Boyle's Law, or sometimes as Marriotts Law, because a 

 Frenchman named Marriotte also discovered it some years 

 later without knowing that Boyle had done so. It is not 

 always absolutely true, but we cannot stop to discuss the 

 exceptions here ; you will find them in books on physics and 

 chemistry. 



Boyle and Hooke both gave much time to the study of 

 chemistry. Hooke published a theory in 1665 that air acts 

 upon substances when they are heated, and so produces fire ; 

 for, said he, in making charcoal, although the wood is in- 

 tensely heated and glows brightly, yet so long as the air is 

 kept away it will not be consumed. Boyle also proved that 

 a candle will not burn, nor animals breathe, without air. 

 He found that when he put mice and sparrows into his air- 

 pump, and then drew out the air, they died ; and that flies, 



