Chap, xxiii.] LIQUEFACTION OF GASES. 287 



the walls of the cotyloid cavity. As soon as a hole 

 was drilled through the pelvic wall into the depth of 

 the cavity the femur fell away. 



Cupping- instruments exhibit very well locally 

 the effects of diminished pressure. A small glass cup, 

 exhausted of air, is closely applied to the skin, and at 

 once the part bulges out into the cup, becomes red 

 and congested by the afflux of blood. If the part 

 have been previously scarified a copious flow of blood 

 is produced. Dry cupping is the phrase applied to the 

 use of the instruments without scarification. It pro- 

 duces merely a local determination of blood. The 

 exhaustion is accomplished by moistening the inner 

 surface of the cup with spirit, setting fire to it, and 

 immediately applying it ; or a cup may be used, con- 

 nected with an aspirator, for withdrawing the air after 

 it is applied. 



Liquefaction of gas. It has been observed 

 that gases resemble liquids in many respects, but differ 

 from them in the mutual repulsion of their molecules, 

 in virtue of which they tend to expand and fill what- 

 ever space may enclose them. Diminution of pressure 

 permits the expansion to take place, and increased 

 temperature encourages it. On the other hand, 

 increased pressure and diminished temperature would 

 both alike hinder the rarefaction and produce a con- 

 densation. It might be expected that if the pressure 

 could be sufficiently increased and the temperature 

 sufficiently lowered the condensation might be so 

 great as to reduce the gas to the liquid state. Both 

 increased pressure and diminished temperature can 

 liquefy certain gases, a combination of both being 

 often used. Thus sulphuric acid gas, carbonic acid 

 gas, and nitrous oxide gas were early liquefied by 

 pressures varying from 2| to 45 atmospheres ; but till 

 recent years air, oxygen, hydrogen, nitrogen, nitric 

 oxide, and marsh gas had resisted. Lately, however, 



