270 



rt W3. 



PNEUMATICS. 



CHAPTER XXIII. 



TIIK PHYSICS OF GASES AND THEIR APPLICATION IN 



RESPIRATION. 



THE gaseous state, Gas is a fluid, and possesses 

 those properties that have been seen to belong to 

 other fluids and to liquids. Chief among these is the 

 mobility of the particles of which gas is composed. 

 It is, of course, this extreme mobility of the gaseous 

 particles that permits movement in air to be so 

 readily effected. Like liquids (Pascal's law), gases 

 transmit pressure equally in all directions, and so a 

 body surrounded by gas is pressed upon equally on all 

 sides. Gases, however, differ from liquids, in their 

 greater elasticity and compressibility. 



The elastic force or expansibility of gas is 

 due to a repulsive action exercised between the 

 molecules of the fluid. In virtue of this property, 

 gases always tend to expand and fill the space in 

 which they are placed ; and they exert, in con- 

 sequence, pressure on anything which contains them, 

 and offers itself as an obstacle to their continued 

 expansion. This is easily proved by partly tilling a 

 bladder with air, and placing it under the receiver of 

 an air-pump. As soon as one begins to exhaust the 

 air from the receiver the air within the bladder finds 

 itself unopposed by air outside, and its pressure is 

 thus sufficient to distend the bladder. As the 

 exhaustion goes on the bladder will become more 

 and more inflated, till the resistance, developed in the 



