240 MAEIOTTE'S LAW. 



be to press the body together and crush it. But the soft 

 fleshy parts of the body are everywhere penetrated by 

 liquids, which are incompressible, while the cavities of 

 the body, as the mouth, lungs, ears, are virtually in 

 connection with the external air : the internal pressure 

 is therefore the same as the external. Finally, the 

 solid parts of the skeleton are sufficiently strong to 

 resist even a higher pressure. The effect of atmo- 

 spheric pressure upon the human body is only rendered 

 sensible when it is removed from any part. If the 

 mouth of a small glass-funnel (3 or 4 cm in diameter) 

 be firmly pressed upon the back of the hand, or the 

 fleshy part of the arm/ and the air sucked by the 

 mouth out of the funnel, the pressure of the air upon the 

 surface covered by the funnel becomes diminished, the 

 external pressure distends the skin and raises it, while 

 at the same time the blood is driven to the part where 

 there is less pressure, and it is reddened. 



26. Mariottds Law. It has been shown in art. 3, that 

 gases alter their volume if the pressure varies, and it 

 remains now to investigate more closely the relation 

 between the volume of a gas and the pressure upon it. 

 Experiments on these relations may be made by an 

 apparatus of which fig. 166 represents the most essen- 

 tial parts. Two glass tubes, a and &, are connected 

 by a stout india-rubber tube, which is additionally 

 strengthened by being covered with wool or cotton 

 fabric ; the tube a can be closed by a well-ground air- f 

 tight stop-cock of glass, the tube & is open at the top ; 

 a is fixed permanently along a wooden scale, b is 

 attached to a bar which slides in a groove by the side 

 of the scale, and may be fixed at any point of it ; 



