86 SCIENTIFIC APPARATUS. 



aluminium in water. The bodies which pass through such a 

 septum are called crystalloid, those retained are colloid, the 

 wall in this case being colloid. Colloid bodies when solid are 

 amorphous. 



The energy of diffusion of a solid into a gas, and that of a liquid 

 into a gas or vacuum is usually measured by the vapour tension, 

 or thrust which the vapour of the solid or liquid exerts, as exhibited 

 by the depression of the barometric column when the body is intro- 

 duced into a partial or perfect vacuum. The vapour tension so 

 measured always increases with the temperature, and is the same 

 with whatever gas the vacuum may be partially filled. But the 

 vapour tension does not measure directly the rate at which vapor- 

 isation takes place, when a continually renewed current of a gas 

 sweeps over a vaporisable solid or liquid. Here the relative 

 natures of the gas and body are concerned. Thus alcohol eva- 

 porates more rapidly under like conditions in a current of olenant 

 gas than in one of air; and the difference depends upon the 

 relative ratio of solubility of the gas in the liquid. Moreover, 

 while the vapour tension of water is continuous through its maxi- 

 mum density, the evaporation shows a tendency to diminish at 

 that point, overwhelmed, but not obliterated by the temperature. 



The liquefaction of a gas by absorption into a liquid is also 

 conditioned by their relative chemical natures, as well as by the 

 common pressure and temperature. The solubility increases with 

 the pressure, and unless chemical union between the gas and the 

 liquid ensues, the solubility of the gas in the liquid runs parallel 

 with the evaporation of the liquid into the gas. 



The diffusion of a gas into a gas when they are in direct contact 

 with one another is almost immeasurably rapid, but when sepa- 

 rated by a permeable septum a diffusion ensues similar to that 

 between liquids. The less dense gas moves through the wall with 

 the greater nimbleness, and this difference of rate may give rise to 

 great force. 



FREDERICK GUTHRIE. 



