250 



PRINCIPLES OF CHEMISTRY 



is continually evolving ammoniacal vapour, and so has the characteristic 

 smell of ammonia itself. It is a very characteristic and important fact 

 that ammonia has an alkaline reaction, and colours litmus paper blue, 

 just like caustic potash or lime ; it is therefore sometimes called caustic 

 ammonia (volatile alkali). Acids may be saturated by ammonia water 

 or gas in exactly the same way as by any other alkali. In so doing, 

 ammonia combines directly with acids, and this forms the most essential 

 chemical reaction of this substance. If sulphuric, nitric, acetic, or any 



give off a part of their ammonia, in accordance with the laws of the solution of gases in 

 liquids already considered by us. But the ammoniacal solutions at the same time 

 absorb carbonic anhydride from the air, and ammonium carbonate remains in the solu- 

 tion. 



Solutions of ammonia are required both in the laboratory and in practice, and have 

 therefore to be frequently prepared. For this purpose the arrangement shown in fig. 46 

 is employed in the laboratory. In works the same arrangement is used, only on a larger 

 scale (with earthenware or metallic vessels). The gas is prepared in the retort, from 



FIG. 46. Apparatus for preparing solutions of ammonia. 



whence it is led into the two-necked globe A, and then through a series of Woulfe's 

 bottles, B, C, D, E. The impurities spurting over collect in A, and the gas is dissolved 

 in B, but the solution soon becomes saturated, and a purer (washed) ammonia passes 

 over into the following vessels, in which only a pure solution is obtained. The bent 

 funnel tube in the retort preserves the apparatus from the possibility both of the pres- 

 sure of the gas evolved in it becoming too great (when the gas escapes through it into 

 the air), and also from the pressure incidentally falling too low (for instance, owing to a 

 cooling effect, or from the reaction stopping). If this takes place, the air passes into the 

 retort, otherwise the liquid from B would be drawn into A. The safety tubes in each 

 Woulfe's bottle, open at both ends, and immersed in the liquid, serve for the same purpose. 

 Without them, in case of an accidental stoppage in the evolution of so soluble a gas as 

 ammonia, the solution would be sucked from one vessel to another for instance from E 

 into D, &c. In order to clearly see the necessity of the safety tubes in a gas apparatus, 

 it must be considered that the gaseous pressure in the interior of the arrangement must 

 exceed the atmospheric pressure by the height of the sum of the columns of liquid 

 through which the gas has to pass. 



