NITEOGENOUS MANURES. 233 



vapours into a receiver containing sulphuric acid, and evaporating 

 the solution obtained, so as to extract from it the sulphate of 

 ammonia by crystallization. The ammoniacal salts are decomposed 

 by lime which is added to the gas liquor before distillation. In old 

 distilling plant, working by naked fire, the use of lime required pre- 

 cautions, because it was liable to adhere to the bottom of the boilers ; 

 in modern plant heated by steam this drawback has disappeared. 

 Sometimes the two methods are combined, that is to say, the gas- 

 liquor is first distilled such as it is, and then it is redistilled after 

 adding lime. The latter is used either as quick-lime or as milk of 

 lime, and in quantity varying with the content of the ammoniacal 

 liquor. But the amount used never exceeds 5 j)er cent of the 

 material. The distillate consists mostly of a mixture of water and. 

 free ammonia ; organic bases — the most volatile — are also present 

 along with tarry matter. As amongst all these bodies ammonia is. 

 the most volatile, the principle of hot or partial condensation has been 

 applied to its distillation, a method which is especially important in 

 the distillation of alcoholic liquids. In the older plant the distillate 

 traversed one or more vessels containing reheated ammoniacal 

 liquor. By this arrangement the ammonia was separated from the 

 less volatile products which condensed on the road ; on the other 

 hand the ammoniacal liquor was brought to a high temperature 

 before being distilled. However, this method cannot be adopted 

 except by working continuously. The plant used in distilling 

 ammonia may be divided into two classes, viz. (1) nal-ed fire stills 

 and (2) stecmi stills. Although less economical and of much smaller 

 output, naked fire stills have still numerous advocates, because their 

 installation is generally less costly and their management very 

 simple. A few stills of both types will now be described. 



Naked Fire Stills— The English Still. — An old still which is 

 still in use in certain English factories where it gives excellent 

 results is that shown in Fig. 40. It consists of a small boiler A, 

 which is fed with gas liquor from a reservoir. If the liquor be 

 heated to boiling, the free ammonia, and the carbonate of ammonia, 

 the sulphide, and the cyanide of ammonium, all very volatile pro- 

 ducts, are given off with the steam, rise in the pipe a, pass into 

 pipe g, whence they pass by the pipe c into the lead-lined wooden 

 vat C containing concentrated sulphuric acid. The latter absorbs 

 the gas and the vapours with effervescence, which renders them 

 liable to return into the chamber A. To avoid this mishap a valve 

 li is fitted to the tox^ end of the pipe c, which opens from the out- 

 side to the inside and lets the air enter as soon as the acid begins 

 to rise in c, i.e. as soon as the pressure in the apparatus is lower 

 than the atmospheric pressure. When the operation is thought to 

 be sufficiently far advanced, the test tap d is opened and the 

 vapours coming from the boiler are tested with red litmus paper 



