29 



abstracted, some sawdust was treated with acid strong enough to char 

 it slightly* ; and gas, which instantly reddened the clean sawdust and 

 dilute acid, was passed first through the black and then through the 

 clean acid sawdust. No colour was produced in the latter, though 

 the flow of gas continued for an hour. 



Hydrochloric may be substituted for sulphuric acid, so far as that 

 gas colours sawdust moistened with it, but it is liable to a consider- 

 able disadvantage. If gas contain ammonia, the vapour of the acid 

 unites with it in the tube before the gas comes into contact with the 

 sawdust, and the result is a deposit of chloride of ammonium on 

 the surface of the sawdust where the colour commences, which ren- 

 ders the observation less precise and easy. Olefiant gas likewise 

 does not redden this acid sawdust, and therefore .cannot be estimated 

 by it. 



Nitrogenized compounds in coal-gas present the greatest difficulty 

 in the way of efficient purification, and the almost impossibility of 

 obtaining them in a state fit for examination, renders their investiga- 

 tion laborious and unsatisfactory. Much nitrogen is contained in 

 gas as cyanogen, which can be separated from the clay used in purifi- 

 cation. Probably not much less exists as sulphocyanogen, which 

 can be separated from the foul clay with ease, and the presence of 

 further quantities in combination with sulphuretted hydrocarbons 

 and tar can be demonstrated. The bodies formed by this combina- 

 tion of elements are, I believe, unknown at present. 



By placing clay in a purifier through which crude gas passes from 

 the condenser of a gas-works, and treating the saturated clay with 

 spirit, a solution is obtained, of a brown colour, which has no effect 

 upon litmus, turmeric or lead-paper, which decolorizes a solution of 

 iodine, and from which nitrate of silver throws down a white or 

 brownish white precipitate, and acetate of lead a white precipitate. 

 The aqueous solution possesses the same properties, and, like the so- 

 lution in spirit, is always neutral. Litmus paper, immersed in either 

 of the solutions and exposed to the air, becomes quickly, strongly, 

 and permanently reddened. Soluble sulphides have been tested for 

 repeatedly with nitroprusside of sodium, as well as with acetate of 

 lead, but have never been found ; yet a sulphur compound exists 

 in solution which possesses the power of forming a sulphide with 

 * This acid was of the same strength as that used in some gas-works. 



