32 CHEMISTRY, AS APPLIED TO THE 



unites with alkalies and earthy bases, forming compounds denominated hydro- 

 sulphurets : on this property is founded the method of purifying gas by lime. 

 It precipitates most of the metallic oxides from their solutions, forming hy- 

 drosulphurets of the metal, and liberating the acid with which it was pre- 

 viously combined. The compounds are generally coloured : with salts of 

 lead we have a black precipitate, with those of antimony an orange one. 

 This property is useful, in enabling us to ascertain readily whether the gas 

 we have prepared is sufficiently purified for the purposes of illumination. We 

 have only to pass a portion of it through a solution of a salt of lead, (say the 

 acetate, or sugar of lead) when the smallest quantity of sulphuretted hydrogen, 

 if present, is rendered perceptible ; or we may moisten a piece of white paper 

 with the solution, when, by exposing it to a jet of gas, a blackened surface 

 will be the consequence if the gas be impure. 



It is not necessary for the sulphuretted hydrogen to be in solution before 

 combining with the earthy bases : if we merely moisten the lime in the puri- 

 fiers, the effect is the same, the gas being fully absorbed as it passes through 

 them, provided the surface of lime is great enough. 



Sulphuretted hydrogen is constituted of one volume of the vapour of 

 sulphur and one of hydrogen gas. Its specific gravity is generally estimated 

 at 1-178. 



NITROGEN is one of the constituents of coal, and its compounds are conse- 

 quently among the products of the decomposition of that substance. It has 

 the property of extinguishing burning bodies, and is not absorbed by water ; 

 its specific gravity is O9760, being lighter than common air, of which it 

 forms a constituent part. Its compounds are of great interest to the gas en- 

 gineer : ammonia is one of the most important. 



AMMONIA is formed during the distillation of coal, and of all organic sub- 

 stances containing nitrogen. In such distillation the nitrogen unites with hy- 

 drogen in the proportion of one to three, the formula being N H 3 , and ammo- 

 nia is the result. It is a colourless gas, very pungent, acting strongly on the 

 eyes and nose when respired. It dissolves in a very small portion of water, 

 one volume taking up about 750 of the gas, forming a liquid possessed of simi- 

 lar properties, and sold in the shops under the name of Spirits of Harts- 

 horn. Ammonia is strongly alkaline, uniting readily with all the acids, and 



