40 5 NINETEENTH CENTURY. PT. IIL 



Liquefaction of the Permanent Gases, 1877-8. 



But though we cannot follow out the more subtle advances 

 in chemistry, there is yet one so interesting and important 

 that it must be mentioned. In 1823 Faraday first liquefied 

 chlorine and several other gases, but oxygen, hydrogen, and 

 nitrogen refused to yield to liquefaction, and were therefore 

 called 'permanent gases,' and Dr. Andrews of Belfast ex- 

 plained the reason why they could not be overcome. 



If we picture to ourselves a gas as being composed of free 

 molecules flying about in all directions, we can understand 

 that if these are pressed very closely together they may 

 come sufficiently near to be held together by cohesion, and 

 in fact many gases, such as chlorine, hydrochloric acid, 

 and ammonia, can be reduced to liquids merely by being 

 compressed at the ordinary temperature of our air, or very 

 little below it. But Dr. Andrews has shown that by heating 

 liquids we can always arrive at a critical point of tempera- 

 ture (differing for different substances), at which no amount 

 of pressure will keep them liquid. If we heat a liquid with 

 its vapour in a sealed tube, as we arrive near to this par- 

 ticular temperature the surface of the liquid loses its natural 

 curve in the centre, and flattens more and more, thus show- 

 ing that the molecules are held together less and less firmly, 

 until at the critical point the force of cohesion loses its 

 power altogether, and the whole contents of the tube become 

 vapour. In fact, the energy of movement of the molecules 

 has become so great that it quite overcomes the attraction 

 which each molecule has for the other, and so long as the 

 vapour is kept at this temperature, no amount of com- 

 pression will force it to become a liquid. Thus, for example, 

 carbonic acid at a temperature of 3 5 -5 Cent, may be com- 

 pressed under 108 times the weight of our atmosphere, till 

 430 pints of it are forced into a vessel only holding one 



