APPARATUS 89 



just as heat is economised in a regenerative steel furnace. 

 In some machines the compressed air is cooled before escape by 

 means of liquid carbon dioxide, but this is not necessary, as in a 

 well-constructed machine liquid air begins to drip into the 

 receiver half an hour after setting the pumps to work at atmo- 

 spheric temperatures. 



The liquid collected in this way has a very pale blue colour, 

 and it boils at about -181 C. To attempt to collect it in any 

 ordinary bottle would be like trying to catch water in a red-hot 

 vessel. For when the liquid is poured into any vessel in the open 

 air it boils furiously, and continues to do so till the two hundred 

 degrees of difference of temperature has been abolished by the 

 evaporation of a portion of the liquid. Even then heat reaches 

 it from the outside far too rapidly to permit of keeping it for 

 more than a very short time. To meet this difficulty Sir James 

 Dewar's device of a vacuum jacketed glass vessel is everywhere 

 adopted. The device is now familiar in the ordinary " Thermos " 

 flask, which consists of two vessels, one inside the other, with a 

 space between from which the air has been removed as completely 

 as possible. A vessel of this kind is shown in section, page 95. 



The vacuum vessel is rendered still more efficient by coating 

 it with a thin but bright deposit of silver. 



The low temperature of liquid air is demonstrable by many 

 curious experiments. A tube full of mercury, which freezes at 

 -39, when plunged into liquid air, sets almost instantly to a 

 solid resembling silver in appearance, and as malleable as lead. 

 As is well known the mercurial thermometer is replaced in some 

 latitudes such as the north of Russia, by a thermometer con- 

 taining absolute alcohol. This liquid dipped into liquid air at 

 first assumes the consistency of oil, becoming more and more 

 viscous till it sets into a glass-like solid. Such materials as fruit, 

 flesh, and india-rubber cooled in liquid air become as brittle as 

 glass, and when at this temperature they are struck with a 

 hammer they fly to pieces like that substance. 



The commercial production of liquid air has not only provided 

 a valuable agent for the investigation of the properties of matter, 

 but has led to some important practical results which will be 

 described later on. 



The history of the liquefaction of the gases would not be 

 complete without reference to the two cases which have pre- 

 sented the greatest difficulty, namely, hydrogen and helium. 

 The principles involved were perfectly well understood, the 



