TIEPORT ON MINERAL AND TtiERMAL WATERS. 53 



preferable to any other substance, as the only combinations 

 formed are the chloride and the sulphuret, of which the former 

 is soluble in liquid ammonia, whilst the latter is not acted upon 

 by it. 



I have already stated, that M. Anglada considers the sul- 

 phuretted hydrogen of the Pyrenean springs to be combined 

 with an alkali. In order to determine whether this be the case 

 or not, the test he employs is a solution of arsenious acid*, 

 which gives a yellow precipitate with the free acid, but does 

 not affect solutions of the hydrosulphuretsf . 



Azote is usually detected by negative trials, but an ingenious Azote. 

 method of directly proving its presence has lately been sug- 

 gested. 



This is, to melt a piece of potass in contact with a slip of 

 zinc in the air suspected to contain it, suspending over the two 

 a piece of turmeric paper, moistened. 



The water of the potass will thus be decomposed, its oxygen 

 passing over to the zinc, and the hydrogen being liberated. 

 The latter, at the moment of its separation, unites with any 

 azote that may be present, forming ammonia, which produces 

 its characteristic effect upon the test paper. 



The fabrication of factitious mineral waters, being entirely On factid- 

 dependent on the knowledge we may possess of their chemical ""* mineral 

 constitution, seems to claim a place immediately after the con- ^^'*'^'^^' 

 sideration of their analysis. 



The subject is one which has excited considerable interest on 

 the Continent, in consequence of the labours of Dr. Struve of 

 Dresden, who has devoted himself, for a number of years past, 

 to the imitation of those natural springs which possess the 

 highest reputation amongst his countrymen. 



To do this completely, considerable skill in manipulation, 

 and a minute attention to several apparently unimportant cir- 

 cumstances, ai'e found to be requisite. 



As the first step of the process, the water intended to be 

 mineralized, must be impregnated with the same amount, of car- 

 bonic acid, and the other gases which its natural prototype 

 possesses ; and, in order to effect this object, the whole of the 

 atmospheric air existing in the water must be pi'eviously ex- 

 pelled, and the carbonic acid added, under a pressure, neither 

 greater nor less, than that to which it is subjected in nature. 



All this time the fluid must be kept at the exact temperature 



* Memoires pmir scrvir, &c., vol. ii. 



t Prof. Johnston mentions in his Rpport on Chemislnj another method, 

 p. 460. 



