ARSENIOUS ACID 



219 



arsenical spots of a cinnamon-red and dissolves them almost immediately. On anti- 

 monial spots it has no visible action within three or four hours. Solutions of the 

 hypochlorites (chlorides) of soda and lime and chlorine-water dissolve arsenical spots 

 instantaneously, leaving those of antimony. A concentrated solution of the chlorate 

 of potash gradually acts upon arsenical spots, but not upon those of antimony. The 

 nitroprusside of potassium, on the other hand, slowly dissolves antimony, producing 

 no perceptible effect upon arsenic. The statement of Bischoff, that arsenical spots 

 were soluble, antimonial insoluble, in a solution of tho chloride of sodium, could not 

 bo verified, as, after repeated trials, it was found to leave both not perceptibly affected. 

 The chloride of barium, the hypochlorate and the sulphite of ammonia, afforded like- 

 wise no distinguishing action. The nitrate of ammonia dissolves arsenical more rapidly 

 than antimonial stains. Of these reactions tho most decisive are those of iodate of 

 potash, hypochlorites of soda and lime, and fresh chlorine-water. 



It is well known that fluids mixed with glutinous matter are very liable to froth up 

 when hydrogen is disengaged in them, from the mutual action of zinc and a dilute 

 acid ; and that the froth obstructs the due R , 



performance of the experiment of Marsh. 

 A committee appointed by the Prussian 

 Government contrived an ingenious modi- 

 fication of Marsh's apparatus, the annexed 

 form {fig. 84) representing a convenient 

 simplification of it by Dr. Ure: A, is a 

 narrow glass cylinder, open at top, about 

 10 inches high, and l^or 1| inch diameter 

 inside; B is a glass tube, about 1 inch dia- 

 meter outside, drawn to a point at bottom, 

 and shut with a cork at top. Through tho 

 centre of this cork the small tube c passes 

 down air-tight, and is furnished at top with 

 a stopcock, into which the bent small tube 

 of glass (without lead) E is cemented. Tho 

 bent tube F is joined to the end of E with a 

 collar of caoutchouc, or a perforated cork, 

 which will be found more convenient. 



The manner of using this apparatus is 

 as follows : Introduce a few oblong slips 

 of zinc, free from arsenic, into B, and then 

 insert its air-tight cork with the attached 

 tubes. Having opened the stopcock, pour 

 into A as much of the suspected liquid, 

 acidulated with dilute hydrochloric or sul- 

 phuric acid (each pure) as will rise to tho 

 top of the cork, after B is full, and imme- 

 diately shut the stopcock. The generated 



hydrogen will force down the liquid out of tho lower orifice of B into A, and raise the 

 level of it above the cork. The extremity of the tube F being dipped beneath the 

 surface of a weak solution of nitrate of silver, and a spirit-flame being placed a little 

 to the left of the letter E, the stopcock is then to be slightly opened, so that the gas 

 which now fills the tube B may escape so slowly as to pass off in separate small bubbles 

 through the silver-solution. By this means the whole of the arsenic contained in the 

 arseniuretted hydrogen will be deposited either in the' metallic state upon the inside 

 of the tube E, or with the silver in the characteristic black powder. The first charge 

 of gas in B being expended, tho stopcock is to be shut till the liquid be again ex- 

 polled from it by a fresh disengagement of hydrogen. The ring of metallic arsenic 

 deposited beyond E may be chased onwards by placing a second flame under it, and 

 thereby formed into an oblong brilliant steel-like mirror. It is evident that by tho 

 patient use of this apparatus the whole arsenic in any poisonous liquid may be 

 collected, weighed, and subjected to every kind of chemical verification. If F be 

 joined to E by means of a perforated cork, it may readily be turned about, and its 

 taper point raised into a position such as when the hydrogen issuing from it is kindled, 

 the flame may be made to play upon a surface of glass or porcelain, in order to produce 

 the arsenical mirror, 



7. ReinscKs Test. Professor Eeinsch proposed an entirely different method of 

 detecting arsenic, which consists in acidulating any suspected fluid with hydrochloric 

 acid, heating in it a thin plate of bright copper, upon which the arsenic is deposited in 

 tho form of a thin metallic crust, and then separating the arsenic from tho copper in 

 tho state of oxide by subjecting tho copper to a low rod heat in a glass tube. Organic 



