8 



L. F. Nilson, 



there is no difficulty, after a little practice, in passing the anhydride into 

 the other part of the tube, where it is obtained, now quite pure, in colour- 

 less, acicular, long crystals or, if the quantity is larger, in solid, firm cry- 

 stal-cakes. The impurities remain as a melted, green mass, which, besides 

 acid, contains the oxides of iron and copper. The pure anhydride being 

 dissolved in water — which is most easily done by immersing that part 

 of the tube into a somewhat wider tube with water — the same tube may 

 be employed several times. Above all, it is indispensable in these sublima- 

 tions to keep up a uniform current of air through the tube; selenious acid 

 will, otherwise, obstruct it and cause its bursting, together with the inconvenien- 

 ces consequent thereupon. If it be neglected to filter the air through cotton, 

 a little part of the anhydride is reduced by organic dust-particles, and the 

 sublimate coloured red by selenium. When large quantities of acid are 

 employed, it easily happens that the acid, in an impure state, retains a 

 little nitric acid, owing to imperfect evaporation; it has therefore been found 

 very convenient, before beginning the sublimation, slightly to warm the 

 whole tube, whereby nitric acid is driven off and passes into the water of 

 the bulb-apparatus. 



SALTS OF SELENIOUS ACID. 



The salts here to be treated of have been obtained in the following 

 manner. The method, that seems to have been employed by Muspratt to 

 procure a neutral alkali-salt, viz. that of treating the carbonate with 

 selenious acid, as long as carbonic acid is given off, being found attended 

 with serious difficulties, and the reaction of the neutral salts on litmus 

 being moreover no criterion of the saturation of the acid since the neutral 

 salt reacts as if alkaline, neutral salts of alkali were prepared as follows. 

 To a given quantity of carbonate was added the requisite quantity of sele- 

 nious acid, weighed in form of anhydride, such as it was obtained by the 

 process of sublimation just described. The strongly sour reacting di- and 

 tetraselenites of the alkaline metals were procured in the same manner. 

 With such a mode of preparation, it was easy to remove the difficulty of 

 which Muspratt complains. 



Among the neutral salts of selenious acid only a very few are 

 soluble in water, viz. those of the monatomic metals. The others are 

 only with difficulty or not at all to be dissolved; they are easily obtained 

 from selenite of sodium, by means of double decomposition with some 

 other soluble salt, forming thereby a very often crystalline precipitate. 



