1889.] On Selenic Acid and other Selenium Compounds. 



19 



It instantly solidifies at any temperature below 58° if a crystal of the 

 solid acid be dropped into it, and it freezes sometimes at ordinary 

 temperatures when rubbed with a sharp piece of glass or with the 

 point of a pipette. It thus exhibits the property of superfusion in a 

 remarkable degree, and to a greater extent than anhydrous sulphuric 

 acid, which, according to Marignac, possesses eminently the pi^operty 

 of superfusion (' Annales de Chimie,' vol. 39, 1853, p. 184). Its 

 melting point, 58°, is higher than that of anhydrous sulphuric acid, 

 10*5°, but lower than that of telluric acid, which may be heated 

 nearly to redness without melting. 



Anhydrous selenic acid crystallises in long interlacing hexagonal 

 prisms. In an impure condition from the presence of selenium dioxide 

 and . other substances, its melting point is lowered, and under those 

 circumstances it is sometimes deposited slowly and spontaneously in 

 the form of double pyramids, many of them intersecting in pairs. 



Although much has been written upon the freezing-point of sul- 

 phuric acid, but little information appears to be published regarding 

 its crystalline form. It is stated, however, in Graham's ' Chemistry,' 

 vol. 1, that the most concentrated acid, when frozen, often yields 

 regular six-sided prisms of a tabular form. Chaptal describes the 

 crystals as being six-sided prisms terminating in pyramids with six 

 faces. Both accounts agree in placing them in the hexagonal system. 

 In order to see if the appearance of the crystals agreed with either 

 of the above descriptions, some sulphuric acid was strengthened by 

 boiling for some time, and then cooled down until it froze. The 

 crystals obtained were found to be long six-sided prisms ending in 

 pyramids, as described by Chaptal, and no prisms of a tabular form 

 were observed. It is thus interesting to find that both anhydrous 

 sulphuric and selenic acid crystallise in prisms in the hexagonal 

 system, but it remains doubtful whether or not they are strictly iso- 

 morphous. 



Selenic acid in the anhydrous condition possesses a powerful affinity 

 for water, absorbing it quickly from the atmosphere. Their combina- 

 tion is attended with contraction and considerable evolution of heat, 

 but less so than in the case of water and sulphuric acid. Like the 

 latter, it disintegrates and blackens many organic substances, such as 

 cork, india-rubber, &c. From others it withdraws the elements of 

 water ; thus, alcohol heated with it yields ethylene, and glycerine, 

 acrolein. On cellulose it has an action similar to that of strong 

 sulphuric acid, paper being converted by it into a tough parchment- 

 like substance. For this reason it should not be filtered through 

 filtering paper, except when cold and very dilute. Iodine dissolves 

 in the superfused acid when heated, forming a brown-coloured solu- 

 tion. It is acted on violently by pentachloride of phosphorus in the 

 cold — a reaction which we are at present examining. Oxychloride of 



c 2 



