CH. XVI. 



COMPRESSIBILITY OF GASES. 



127 



Pressure 



of 

 Mercury 



I ss 



E-JO 



z-45 

 = -0 

 = -35 



to understand. 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 (see p. 1 16). 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 discover this 

 he devised the following ex- 

 periment. Retook 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 quantity of air 

 between b and c. This air 

 was of course still under the 

 usual weight of the atmo- 

 sphere, 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. 



He next added more mer- 

 cury, 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 weigh- 

 ing upon it, as well as the atmosphere, which equalled 

 another 30 inches. Boyle found that this double pressure 

 had squeezed it into half \hs space (b c, Fig. A 2 ); in other 



Pressure 



equals 



-r90 



--s 



-15 



-10 



FIG. 19. 



