1889.] On Selenic Acid and other Selenium Compounds. 27 



tables, an acid of 95*9 per cent, will be found to have a specific 

 gravity of 2'5366, while the specific gravities of 94*9 per cent, and 

 97"4 per cent, acids will be respectively 2 5141 and 2"5680, being less 

 than those assigned above. Furthermore, the difference between the 

 two specific gravities given by Fabian is much, less than we find 

 between two acids differing by 2'5 per cent. The most probable 

 explanation of the discrepancy is that the acids which gave the above 

 results contained sufficient selenium dioxide to raise their specific 

 gravities appreciably. It may be shown that the effect of the 

 development of selenium dioxide in selenic acid is to increase the 

 specific gravity relatively to the acidity. In the acids above-men- 

 tioned, selenium dioxide would exist as such, and not as selenious 

 acid, owing to dissociation of the latter at a temperature below those 

 at which the acids were formed, and the weak affinity for water 

 possessed by the resulting dioxide. Clausnizer (' Liebig's Annalen,' 

 vol. 196, 1879, p. 265) gives the specific gravity of selenium dioxide 

 at 15'3° as 3*9538. We have also recently taken its specific gravity, 

 and are in a position to confirm his result. It is thus more than one 

 and a half times as dense as the strongest selenic acid. C. Blarez 

 (' Comptes Rendus,' vol. 103, 1886, pp. 804 — 806) has examined the 

 saturating power of selenious acid. He finds that it is monobasic 

 with cochineal or methyl-orange. With litmus, it is monobasic to 

 ammonia, lime, strontia, and baryta, but with soda or potash the 

 litmus only becomes blue-violet when about 1*5 equivalent of alkali 

 is added. We have obtained a like result with soda or potash and 

 litmus. When one equivalent of acid is saturated, there is a distinct 

 change in the colour of the litmus, so that in the absence of other 

 acids it might be used as an indicator for selenious acid. Taking 1*5 

 equivalent of alkali as the limit, the molecule of selenium dioxide in 

 solution will have a less saturating power than that of selenic acid in 

 the ratio of 2 : 1*5. This will be partly counterbalanced by the 

 higher molecular weight of the latter, but the final effect of the sub- 

 stitution of selenium dioxide for selenic acid will be to reduce the 

 acidity. A gravimetric method, by which selenium dioxide would be 

 oxidised and estimated as selenic acid, would also show a less acidity 

 compared with the specific gravity than if the pure acid were used, 

 but not to the same extent as when a volumetric process is employed 

 for estimating the strength. 



The Action of Heat upon Selenic Acid. 



Action of Heat in a Vacuum. — The effect of heating dilute selenic 

 acid in a vacuum up to 180° has already been described — dilute acid 

 distils until that temperature is reached, when the anhydrous acid 

 remains. 



The result of further heating is merely for a time to raise the 



