14 Prof. Sir C. A. Cameron and Mr. J. Macallan. [May 2, 



Preparation of Anhydrous Selenic Acid, H 3 Se0 4 . 



Selenic acid has hitherto been known only in a dilute form. When 

 heated to about 260° C. it commences to decompose into sele- 

 nium dioxide, oxygen, and water, which prevents any further concen- 

 tration. Berzelius describes it as containing, when of greatest 

 strength, 4 per cent, of water ; but since his time it has been obtained 

 in a more concentrated condition by Fabian, who, by evaporating the 

 acid to a temperature of 265°, found it to have a strength of 94" 9 per 

 cent., and by placing this acid, while still hot, under the receiver of 

 an air-pump, increased its strength to 97'4 per cent. Sulphuric acid, 

 as is well known, has not been obtained in a perfectly anhydrous state 

 by ebullition — although in its case decomposition does not occur ; for 

 when it reaches a strength of about 98' 66 per cent, it boils without 

 further change. When, however, an acid of this strength is sur- 

 rounded with a freezing mixture, the anhydrous acid, H 2 S0 4 , crystal- 

 lises out. An attempt was first made to obtain anhydrous selenic acid 

 by similar means. Great care was taken to obtain an acid of pure 

 quality. An examination was specially made for nitric, sulphuric, 

 hydrochloric, and hydrobromic acids. Selenious acid, when present, 

 was removed by diluting with about 30 parts of water, saturating 

 with hydrogen sulphide in the cold, filtering and concentrating on the 

 water-bath. The acid thus treated was then examined for sulphuric 

 acid and found to contain none. A portion of the acid, when ignited, 

 left a residue equivalent to 0*07 per cent, of the anhydrous acid 

 present in the specimen used in the experiments ; this was ascertained 

 to consist of neutral sodium selenate, and was, of course, derived from 

 an acid salt, of which it was necessary to take account in the suc- 

 ceeding experiments. 



Some of this acid was gradually heated until the temperature rose 

 to 250°. It was next poured into an open dish, and allowed to cool 

 slowly over sulphuric acid, under an exhausted receiver. The strength 

 was then taken with seminormal soda solution, which was specially 

 prepared for those experiments, by making a solution in water of pure 

 caustic hydrate prepared from sodium, and bringing it carefully to 

 the required strength : 11*81 cubic centimetres of soda solution were 

 required for neutralisation by 0'4371 gram of acid, equivalent to 

 97*75 per cent, of selenic acid, being thus a little stronger than that 

 obtained by Fabian in a similar way. 



The acid thus concentrated was then poured into a stout wide glass 

 tube, having one end closed, and the open end fitted with an india- 

 rubber cork, through which passes a glass rod for the purpose of 

 keeping the viscous liquid stirred, and a capillary tube for admis- 

 sion of air, in order to expose the acid to the full atmospheric pressure. 

 The necessity for those precautions in the case of sulphuric acid has 



