1889.] On Selenic Acid and other Selenium Compounds. 21 



room, or much handled, it commences to melt. Like the anhydrous 

 acid, it at once freezes at any temperature below its melting point 

 when a crystal of the same acid is dropped into it. It resembles the 

 anhydrous acid also in having a melting point much higher than 

 sulphuric acid of the same strength, that of glacial sulphuric acid 

 being given by Pierre and Puchot as 7*5°, by Jacquelin as 8°, and by 

 Marignac as 8'5°. It resembles the latter acid in having a melting- 

 point lower than its anhydrous acid, but while the difference is about 

 2'5° in the case of sulphuric acid, the melting points of the two 

 selenic acids differ by 33°. In the following table their melting 

 points are compared : — 



Anhydrous. Monohydrated. 



Sulphuric acid 10-5° 8° 



Selenic acid 58'0 25 



It may be well to state here that 0° is given erroneously in several 

 chemical works as the melting point of anhydrous sulphuric acid. 

 Marignac, the most recent investigator who has studied the subject, 

 assigns the temperature 10'5 o as its true melting point. 



Tbe superfused monohydrated selenic acid has a specific gravity of 

 2'3557 at 15°. That of the solid acid was taken in pure benzene, in 

 which it is insoluble, and on which it is without action at ordinary 

 temperatures, even after standing all night. A portion of the liquid 

 acid was poured into the specific gravity bottle, a crystal dropped into 

 it, and the acid, having become firm and cold, weighed, and the bottle 

 filled with benzene. The specific gravity was found to be 2'6273 at 

 15°. It thus resembles the anhydrous acid in being denser in the 

 solid than in the liquid state ; while melting, the crystals sink rapidly 

 in the liquid portion. 



This acid commences to boil at 205°, the acid vapour given off 

 being at first very weak, but it increases in strength with the rise in 

 temperature. A more dilute acid gives off water only until the tem- 

 perature reaches 205°. Dilute sulphuric acid is stated to behave in a 

 similar manner, giving off nothing but water until the boiling point 

 reaches 205 — 210°, at which temperature it has the strength of the 

 monohydrated acid, H 2 S0 4 ,H 2 0. 



The ease with which an acid of this strength can be obtained and 

 crystallised, supplies a means of separating impurities from selenic 

 acid, all that is necessary to do being to boil a dilute acid until the 

 temperature reaches 205°, cool, and drop in a crystal from an acid 

 already frozen ; the resulting crystals can then be melted and recrys- 

 tallised. 



It may be well here to draw attention to the conflicting statements 

 which are made in various works regarding the crystalline form of 

 giacial sulphuric acid. Watts's 'Dictionary,' vol. 5, and Hichter's 



