20 



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



phosphorus also acts strongly upon it when warmed slightly, the 

 reaction being attended with copious evolution of gas and reduction 

 apparently to lower compounds. Selenium dioxide dissolves in it 

 when heated, but the greater part crystallises out again in the cold. 

 There is no evidence of formation in this way of an acid analogous to 

 hyposulphuric acid, H 2 S 3 6 . The crystals of the solid acid dissolve 

 in strong sulphuric acid, and also in ISTordhausen acid. 



The specific gravity of the superfased acid, taken with a Sprengel 

 tube at 15°, was found to be 2*6083. The specific gravity of the solid 

 acid was taken in pure benzene of specific gravity 08851, which is 

 not acted upon by it in the cold, and in which it is insoluble. As 

 might be expected, it at once blackens commercial benzene. Its 

 specific gravity, taken in this way, proved to be 2*9508 at 15°. It 

 thus resembles anhydrous sulphuric acid in being denser in the solid 

 than in the liquid state. The sj^ecific gravity of the liquid acid is 

 much greater than that of anhydrous sulphuric acid, 1'8384; and on 

 the other hand the specific gravity of the solid acid is less than that 

 of anhydrous telluric acid, which is stated by F. W. Clark to be 3*425 

 at 18*8° ('American Journal,' vol. 14, 1877, p. 281 ; vol. 16, 1878, 

 p. 401). 



Monohydrated Sele?iic Acid: its Preparation and Properties. 



Some selenic acid, which had been concentrated on the water-bath, 

 was heated for some time in a vacuum at 100°, and its strength de- 

 termined: 0*7858 gram neutralised 20*00 c.c. of seminormal soda 

 solution, equivalent to 92*08 per cent, of selenic acid. The acid so 

 prepared was diluted with sufficient water to reduce its strength to 

 88*96 per cent., corresponding to a monohydrated acid, H 3 Se0 4 ,H 2 0. 

 It was then poured into a wide tube and its temperature gradually 

 lowered, the same precautions being taken as to stirring and admission 

 of air as were adopted previously in freezing out the anhydrous acid. 

 Its viscosity increased with the fall in temperature, until at — 32° it 

 froze into a mass of crystals. These were melted and re-crystallised 

 several times, and the resulting product examined. 



A few of the crystals obtained were long needles, but most of them 

 were large and broad, having a general aspect to which the term 

 " glacial " might be applied appropriately, but differing in appearance 

 under the microscope from those of glacial sulphuric acid. 



Its melting point was found to be 25°. Like glacial sulphuric 

 acid, and also like anhydrous sulphuric and anhydrous selenic acid, 

 when once melted it exhibits the property of superfusion, and to as 

 great an extent as the last- mentioned acid, since it may be cooled to 

 more than 50° below its melting point, with constant stirring, before 

 it again freezes. When frozen it remains quite solid at ordinary 

 temperatures j but if the bottle containing it be removed to a warm 



