376 Contributions' to Experimental Chemistry, 



it would most likely have approached still nearer to exactness, 

 if the washing of the precipitate had been repeated oftener. 

 At any rate this method of analysis deserves by far the pre- 

 ference, where smaller quantities of lime are to be separated 

 from larger ones of magnesia. It scarcely need be men- 

 tioned that the solution of sulphate of lime must be of such a 

 concentration, as not to yield a spontaneous precipitate by 

 standing, and that the filter must be covered to prevent evapo- 

 ration. 



The calcined residue of a similar solution, if dissolved in 300 

 grains of water (instead of the solution of sulphate of lime), did 

 not leave any weighable traces of sulphate of lime. 



On the Influence of Gum Arabic on the Precipitation of Lead 

 by Sulphates. 

 Sulphate of soda produces, after a few minutes, a precipitate 

 in a solution of crystallized acetate of lead, when the latter is 

 Wiyo of the liquid. But when the water at the same time 

 contained /^^ of its weight of gum arabic, a precipitate was only 

 obtained with -joVo of the acetate. With -j;-^^^ of the acetate 

 there was no precipitation, even after a few hours, and the same 

 was the case when the liquid contained ^^^ of gum arabic, and 

 the acetate amounted to toVo* The cause of this anomaly 

 cannot be ascribed to the suspension of the precipitate by the 

 viscid fluid; for neither standing for a few days, nor boiling, 

 assists the efficacy of the precipitant, whilst a few drops of 

 acetic, nitric, or sulphuric acid, instantly occasions precipitation. 



On the Reduction of Arsenic by a Single Galvanic Circuit, 

 Professor Buchner found this method of detecting small 

 quantities of arsenic, objectionable, on account of the arsenic 

 being volatilized in a great measure in the shape of arseni- 

 uretted hydrogen. Repeating these experiments in a small 

 apparatus, such as Professor Dobereiner recommended for 

 similar purposes^', I conducted the arseniuretted hydrogen 



♦ A glass tube, open at both ends, is closed at its lower end by a bladder tied over 

 it, the upper aperture being hermetically sealed with a good cork, furnished with a 

 syphon-like tube, of very small bore, to conduct the gas, through which a copper 

 or platinum wire is passed. The tube being filled with the acid solution, is put, 

 together with a piece of zinc foil, attached to the upper end of the wire, jqto a 

 larger vessel containing a solution of muriate of j^mmonia or soda. 



