C().MI'()fNI)S (>! MTIJOCKN AV1TII HYIHiOGKN AM> OXYGEN 247 



Faraday employed the following method for liquefying ammonia. 

 Ammonia, when passed over dry silver chloride, AgCl, is absorbed by it 

 to a considerable extent, especially at low temperatures. 8 The solid 



the aqueous solution. In the end, the same ammoniacal solution as was originally taken 

 is re-obtained. Thus, in this case, on heating the vessel the pressure increases of itself, 

 and on cooling it diminishes, so that here heat directly replaces mechanical work. This 

 is the principle of the simplest forms of Carre's ice-making machines, shown in fig. 44. 

 C is a vessel made of boiler plates into which the saturated solution of ammonia is 

 j toured ; m is a tube conducting the ammonia vapour to the receiver A. All parts of 

 tlx 1 apparatus should be hermetically joined together, and should be able to withstand a 

 pressure reaching ten atmospheres. The apparatus should be freed from air, which 



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=5*WeEST.jT. 



FIG. 44. Carre's apparatus Described in text. 



would otherwise hinder the liquefaction of the ammonia. The process is carried on as 

 follows : The apparatus is first so inclined that any liquid remaining in A may flow 

 into C. The vessel C is then placed upon a stove F, and heated until the thermometer t 

 indicates a temperature of 130 C. During this time the ammonia has been expelled from 

 C, and has liquefied in A. In order to facilitate the liquefaction, the receiver A should 

 be immersed in a tank of water R (see the left-hand drawing in fig. 44). After about 

 half an hour, when it may be supposed that the ammonia has been expelled, the fire is 

 removed from under C, and this is now immersed in the tank of water R. The apparatus 

 is represented in this position in the right-hand drawing of fig. 44. Then the liquefied 

 ammonia evaporates, and passes over into the water in C. This causes the temperature 

 of A to fall considerably. The substance to be refrigerated is placed in a vessel G, in the 

 cylindrical space inside the receiver A. The refrigeration is also kept on for about half 

 ,n hour, and with an apparatus of ordinary dimensions (containing about two litres 

 of ammonia solution), five kilograms of ice are produced by the consumption of one 

 kilogram of coal. In industrial works more complicatecktypes of Carre's machines are 

 employed. 



8 Below 15 (according to Isambert), the compound AgCl,8NH 3 is formed, and above 

 20 the compound 2AgCl,8NH 5 . The tension of the ammonia evolved from the latter 



