128 SEVENTEENTH CENTURY. FT. in. 



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 

 ams (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 Marriotlcs 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, 

 bees, and even worms, became insensible ; while fish, 

 though they lived longer than the mice, soon turned on their 

 backs and ceased to live. He also put a bird under a glass 

 vessel full of air, and it died after three-quarters of an hour. 

 It was clear, therefore, that fresh air is necessary to life, and 

 Boyle began to think that just as a candle-flame cannot be 

 kept up without air, so there must be some vital fire in the 

 heart which is extinguished when air is shut out from it 



