10 



ethylene in such quantities as admitted of experimental re- 

 search with them. 



Into the earlier methods of production employed by Dewar 

 it is not necessary to enter, but the later methods are so 

 applicable to the economical production of large quantities of 

 liquid gases as to lead to the belief that liquid air may play an 

 important part in commerce both as a refrigerating agent and as 

 a source of oxygen gas, while its scientific use is to give facilities 

 for studying the properties of matter at very low temperatures. 



As is well known, if a gas is compressed heat is evolved ; and 

 when the compressed gas is liberated and allowed to expand it 

 absorbs heat from surrounding bodies and itself in the process. 

 This method is familiar to you in the production of solid carbonic 

 acid. In the apparatus now used by Professor Dewar this prin- 

 ciple is made use of. 



Liquid air thus obtained is a very pale blue fluid which as 

 the nitrogen evaporates becomes deeper in colour, for since 

 nitrogen boils at a lower temperature than oxygen, liquid air be- 

 comes gradually richer in the latter by the evaporation of the 

 former. 



For the preservation of the liquid air Professor Dewar uses 

 vessels of glass constructed of double walls, the interval between 

 the two being converted into a very high vacum. So perfect is 

 the insulation from heat thus afforded that several pints of liquid 

 air have travelled from London to Oxford in these vessels, under 

 the ordinary pressure of the atmosphere. 



Further, Professor Dewar has converted this liquid air into 

 a solid transparent jelly-like mass. 



Extraordinary results have been obtained by submitting 

 different substances to the intense cold of this liquid air. The 

 property which in their ordinary state metals possess of con- 

 ducting electricity is greatly diminished. On the other hand 

 their tenacity is increased. Ivory, horn, india-rubber, and 

 other substances undergo remarkable changes. Among others 

 they become phosphorescent when exposed to the electric light. 



By means of the intense heat generated by a powerful current 

 of electricity, many metals hitherto almost impossible to obtain, 

 have been separated from their oxides — chromium and manganese 

 for example. It is by this means that aluminium is now so largely 

 manufactured and at so cheap a rate. 



The rarer metals which are fused with difficulty are found to 

 be useful in giving strength to the metals used in the industrial 

 arts. This power of obtaining the rare metals in an absolutely 

 pure state has greatly helped us in producing a compound metal 



