148 Selections from Foreign Science. 



very fine purple colour, exactly like that of the vapours of indigo. 

 This property belongs to the pure anhydrous sulphuric acid, which 

 alone will dissolve the indigo in a similar manner, whilst 

 anhydrous sulphurous acid has no action upon it. When 

 the purple solution is exposed to the air, or when water or com- 

 mon sulphuric acid is added to it, it becomes blue though always 

 retaining a trace of purple. M. Bussy concludes, that in the 

 purple solution the indigo is more finely divided than in the blue 

 solutions, and that for the same reason the vapour appears purple, 

 that being the true colour of the substance. 



Finally, with reference to the method of obtaining the fuming 

 acid of Saxony : as it diflPers from common acid only in containing 

 less water, it is evident it may be obtained in all proportions. 

 Some dry persulphate of iron was distilled in a coated glass 

 retort, and the produce received in distilled water ; it gave an acid 

 of specific gravity 1.16; this was repeated several times with the 

 same portion of liquid, and ultimately it became very fuming 

 sulphuric acid. It is, however, evident, that it would be far more 

 economical to receive the produce of the distillation of persulphate 

 of iron at once into acid of the specific gravity 1.848. 



That processes for the preparation of this acid in the large way 

 may be successful, it is requisite to multiply the points of contact 

 between the liquid acid and that in vapour. The vapours should 

 pass by a small orifice. M. Bussy joined to his retort an adapter, 

 with the extremity drawn out ; to this was joined a glass balloon 

 with a pointed tube, and to that a second, tubulated. The acid to 

 be saturated was then divided between these vessels, and in this 

 manner about 4lb. 7oz. of dried sulphate of iron converted 

 lib. 10^ oz. of common sulphuric acid into 2lb. 3Loz. of very 

 fuming acid. 



When the acid is made as concentrated as possible, it crystal- 

 lizes at common temperatures. The specific gravity of these crys- 

 tals could not be taken, but that of the liquid about them was 

 1.907, and this M. Bussy thinks was not so high as the pure sul- 

 phuric acid would have had, for it was ascertained by direct expe- 

 riment that sulphurous acid added to sulphuric acid, dimijiished 

 its density. This eflPect influences materially the specific gravity 

 of the Nordhausen acid which may vary from 1.848 to 1.896. 



M. Bussy's results are, 1. That Nordhausen sulphuric acid is 

 only common sulphuric acid, containing a certain quantity of 

 anhydrous acid to which it owes its properties ; and that the sul- 

 phurous acid is accidental, and has no important influence. 

 2, That this anhydrous acid may be separated by distillation, and 

 that it has among other remarkable properties, that of making a 

 red solution of indigo. 3. That all those sulphates which are 

 decomposed by heat give oxygen, sulphurous acid, and sulphuric 

 acid, which is essentially characterized by the white vapours pro- 



