r 4 HER OES OF INVENTION AND DISCO VER Y 



it may be considered as, for most practical purposes, annihilated. 

 Still a certain quantity, as we have said, remains, be it ever so 

 small ; and this quantity continues, just as at first, to be 

 diffused over the whole space within the receiver. From this 

 circumstance Boyle deduced some striking evidences of what 

 seems to be the almost indefinite expansibility of the air. He 

 at last actually dilated a portion of air to such a degree that it 

 filled, he calculated, 13,679 times its natural space, or that 

 which it occupied as part of *^he common atmosphere. But 

 the usual density of the atmosphere is very far from being the 

 greatest to which the air may be raised. It is evident that, if 

 the two valves of the air-pump we have already described be 

 made to open inwards instead of outwards, the effect of every 

 stroke of the piston will be, not to extract air from the receiver, 

 but to force an additional quantity into it. In that form, 

 accordingly, the machine is called a forcing-pump, and is used 

 for the purpose of condensing air, or compressing a quantity of 

 it into the smallest possible space. Boyle succeeded, by this 

 method, in forcing into his receiver forty times its natural 

 quantity. But the condensation of the air has been carried 

 much further since his time. Dr. Hales compressed into a 

 certain space 1522 times the natural quantity, which in this 

 state had nearly twice the density, or, in other words, was 

 nearly twice as heavy as the same bulk of water. Of the air 

 thus condensed by Dr. Hales, therefore, the same space 

 actually contained above twenty millions of times the quantity 

 which it would have done of that dilated to the highest degree 

 by Mr. Boyle. How far do these experiments carry us beyond 

 the knowledge of Aristotle, who held that the air, if rarefied so 

 r.s to fill ten times its usual space, would become fire ! 



On leaving Oxford, in 1668, Boyle came to London, and 



