554 ROSE ON THE ANHYDROUS SULPHATE OF AMMONIA. 



have been described by my brother in PoggendorfF's Annulen, 

 B. XLVIL 476. These crystals are obtained by evaporating the 

 solution ; but, like that of sulphate of oxide of ammonium, it is 

 apt to become acid during the operation, and to have its pro- 

 perties thereby difficultly recognized ; it is better to evaporate 

 over sulphuric acid in vacuo. On further evaporating the 

 mother-water another salt is formed, which differs essentially in 

 its properties from the larger crystals ; but it is difficult to se- 

 parate it from them, especially when considerable quantities of 

 the sulphat-ammon have not been operated upon. This salt 

 attracts moisture from the atmosphere, which is not the case with 

 the crystals of the parasulphat-ammon, this Avhen quite dry 

 suffering no alteration by exposure to the air. Of this salt I 

 shall treat in the foUoAving section. 



The parasulphat-ammon is rather more soluble than the 

 sulphat-ammon ; its solution is neutral to litmus paper. When 

 also preserved for a long time, so that nothing can evaporate 

 and crystallize, it remains neutral. When, however, the salt 

 is moistened with water, it acquires in a short time the pro- 

 perty of reddening litmus paper, and the solution possesses 

 qualities and acts differently with reagents from that of the salt 

 not previously moistened. 



The acid reaction, which the salt acquires by moistening, 

 probably arises from the expulsion of some ammonia by the 

 water ; the carbonic acid of the atmosphere appears also to 

 exert some action ; for if a solution of parasulphat-ammon is 

 slowly evaporated, cold, over sulphuric acid, in contact with the 

 air, it often acquires an acid reaction, which is not the case if 

 the evaporation be performed in vacuo ; when the crystals of 

 this salt are obtained, no attempt must be made to free them 

 from the solutions by washing with water ; they must be dried 

 only by blotting-paper. 



What particularly characterizes the parasulphat-ammon, and 

 distinguishes it from the sulphat-ammon is, that the solution of 

 the dry salt is not rendered turbid by the salts of barytes or of 

 lead, even whon they remain long mixed. This property, it is, 

 however, sometimes difficult to observe, partly because the 

 crystals may contain a portion of the solution from which they 

 have separated, and therefore contain the deliquescent salt; 

 and partly from having been exposed to the atmosphere alter 

 moistening, and then yielding a solution which reddens litmus 



I 



