CH. XVI. 



COMPRESSIBILITY OF GASES. 



129 



FIG. 18. 



so 



inventor. We have seen, however, that Guericke was the 

 first to hit upon this instrument ; Boyle only improved it, 

 and made with it many very valuable experiments upon the 

 weight and nature of air. These are too many and lengthy 

 for us to examine here ; but there is one law about the com- 

 pression of gases which you will find connected with Boyle's 

 name in all books on physics, and which you ought to under- 

 stand. Boyle knew from Torricelli's experiment that the 

 weight of the atmosphere upon the air, close down to our 

 earth is about equal to the weight of 30 inches of mercury in 

 a tube (seep. 118). Now he 

 wished to find out how much 

 air is compressed, or forced into 

 a smaller space, when more 

 weight is put upon it, and to 

 prove this he devised the fol- 

 lowing experiment. He took 

 a tube A 1 , open at the long 

 end and full of ordinary air, 

 and by putting a little mercury 

 into the tube and shaking it 

 carefully till it settled at the 

 bottom, he cut off a small quan- 

 tity of air between b and c. 

 This air was of course still un- 

 der the usual weight of the at- 

 mosphere, which pressed down 

 upon the mercury through the 

 open end of the tube. But 

 the mercury did not add to 

 the weight because it stood at the same height on both 

 sides of the tube, and so was evenly balanced. 



Pressure 



C'f" 



Atmosphere 



Pressure 



of 

 Mercury 



-r30 



-45 



--40 



-ss 



-SO 



--25 



20 



--75 



-10 



5 



