434 M. BECQUEREL ON CHEMICAL DECOMPOSITION AND 



odour peculiar to the combinations of sulphur with chlorine and hydro- 

 gen. If the liquor is tested with an acid, there is an escape of sul- 

 phurous acid. The lower part of the plate of lead has become brittle, 

 in consequence of the combination of the lead with mercurj'. In order to 

 explain these several results, it must be recollected that when the lead 

 is in contact with an alkaline or earthy chloride, such as that of magne- 

 sium, a double chloride is formed and magnesia is precipitated. In con- 

 sequence of this reaction, the lead becomes negative and the solution 

 positive ; the circulation of the electric fluid is then carried on through 

 the medium of the infinitely thin layer of liquid which adheres to the sur- 

 face of the glass. Under the same circumstance, the sulphuret of mer- 

 cury being dissolved in the chloride, is subjected to the action of the small 

 pile : the lead, which is the negative pole, attracts the mercury, and the 

 sulphur is attracted by the double chloride of lead and magnesium. One 

 portion of the sulphur combines with the lead, whence there arises a sul- 

 phuret of lead, which crj'stallizes without any trace of mercury, while 

 the other portion combines with the chloride of magnesium, and the 

 chlorine which was combined with the lead ; and thus there is produced 

 a sulpho-chloride of magnesium and a chloride of sulphur. 



The operation being continued for several months, the liquor in the 

 part adjacent to the sulphuret of mercury assumes a reddish tint, which 

 is probably owing to the presence of chloride of sulphur. It is neces- 

 sary to observe, that no trace of lead is found in the liquor ; a fact 

 which proves that it is precipitated as soon as dissolved. * 



The action which determines the formation of the sulphuret of lead 

 being very complex, it would be difficult to say, without having 

 analysed the liquor, how the several decompositions and combinations 

 first mentioned are effected in definite proportions. 



Sulphuret of Iron and Zinc. — It is very difficult to form these com- 

 pounds by the processes which we have been describing, because of 

 their great liability to be aff'ected by contact with air and with water. 

 We have succeeded nevertheless in obtaining the first, by means of the 

 alkaline hyposulphite, in small yellow crystals of great brilliancy, which 

 became rapidly decomposed in contact with the air. As to the sul- 

 phuret of zinc, we have not been yet able to obtain it crystallized. 



Sulphuret of Cadmium. — The sulphuret is obtained, of an orange 

 yellow, in a crystalline form, by employing the second process ; namely, 

 that which gives the sulphuret of lead. 



From the facts set forth in this chapter, we may conclude that, in 

 order to obtain the crystallization of an insoluble substance by means 

 of electro-chemical reactions, it will be sufficient that we bring it into 

 combination with another substance which is soluble, and then produce 

 a very slow decomposition. The same object may be attained, but 

 with far greater difficulty, by the ordinary resources of chemistry, as 

 the following observation will show : 



