1894.] on Scientific Uses of Liquid Air. 397 



white. Chromic acid, dilute solution of iodine in alcohol, strong 

 solutions of ferric chloride and other coloured solutions become 

 greatly changed. Such facts are sufficient to prove that the specific 

 absorption of many substances undergoes great changes at the 

 temperature of — 190° C. 



The tranquil atmosphere of air above the surface of the liquid in 

 cylindrical or spherical vacuum vessels is a convenient place to cool 

 very fragile bodies. During the slow ebullition of the fluid, gas 

 between — 190° C. and — 180° C. is given off, which has three times 

 the density of ordinary air, and which falls slowly over the mouth of 

 the vessel in a heavy stream. On dipping into this atmosphere small 

 soap bubbles, they contract rapidly and then freeze. If a soap film 

 is made on a circle of thin wire about 2 inches in diameter, and 

 allowed to stand until it shows the various orders of coloured bands, 

 and is then carefully dipped into the cool air, it freezes, showing all 

 the original colours. The black band is, however, always broken. 

 Speaking of films, an interesting experiment may be made with a thin 

 stretched sheet of india-rubber, such as is used for making balloons. 

 It is well known that stretched india-rubber contracts when heated 

 and expands when cooled. Now this can be shown very easily by 

 covering a glass funnel or the end of a cylindrical vessel with a 

 stretched sheet of rubber as thin as the walls of balloons. Such a 

 surface is quite flat and fairly transparent. If a sponge of liquid air 

 is drawn across the surface, the course is marked by a series of 

 wrinkles, due to the temporary expansion of the rubber caused by the 

 extreme cold. The sheet of rubber being extremely thin, soon 

 regains the ordinary temperature, and the surface then is as flat and 

 tense as before. During the continuous motion of the cotton wool 

 liquid air sponge over the rubber surface, it is followed by wave-like 

 depressions which disappear almost as quickly as they are formed. 

 The elasticity of india-rubber, after cooling to — 182° 0. and re- 

 heating, seems unimpaired. 



Organic substances that only become solid at very low tempera- 

 tures may be divided into two classes : those which crystallise, and 

 those which form glasses. Thus bisulphide of carbon, tetrachloride 

 of carbon, methyl alcohol, hydride of amyl, all form crystals, whereas 

 ethyl alcohol, amyl alcohol, turpentine, ethyl nitrate, chinoline, pico- 

 lin, are glass-like. If a few drops of bisulphide of carbon are added 

 to alcohol and the mixture cooled to —180° C, a white solid emulsion 

 is formed, whereas the addition of tetrachloride of carbon to the 

 alcohol resulted in the production of a clear solid without any separa- 

 tion. In the same way pure methyl alcohol crystallises easily, but the 

 addition of a few drops of ethyl alcohol prevents crystallisation and 

 causes a glass to be formed. Thus the examination of the behaviour 

 of organic bodies at low temperatures may be a fruitful means of 

 organic investigation. 



For many purposes of investigation it is necessary to keep liquid air 

 without evaporation. This is readily done by the use of two vacuum 

 test-tubes, fitting freely one inside the other, arranged as in Fig. 4, 



