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



lating on the acid with the sodium selenate deducted, the three results 

 become respectively — 



I. IT. III. 



99-80 9971 99-77 



The difference between the above results and 100 per cent, must be 

 ascribed partly to the very hygroscopic character of the acid in the 

 anhydrous condition, and consequent slight absorption of moisture 

 during the process of weighing. It is necessary to observe, however, 

 that in making the above calculations, and all through in this paper, 

 78'87 has been adopted as the atomic weight of selenium, the number 

 given by Meyer and Seubert in their ' Recalculations of the Atomic 

 Weights.' If 78'80, the number given in Clarke's ' Recalculations,' 

 be taken, the three results become— 



I. II. III. 



99-75 99-66 99-72 



Petersson and Ekman state that the results of a great many 

 analyses show that the most probable atomic weight of selenium is 

 79-08 ('Berichte Deutsch. Chem. Gesell.,' vol. 9, p. 1210). If this 

 number be taken, the above percentages become respectively — ■ 



I. II. ILL 



99-94 99-85 99*91 



The results arrived at from the foregoing experiments lead to the 

 conclusion that at 180° in a vacuum, selenic acid parts with all com- 

 bined water, and remains as the anhydrous acid, H 3 Se0 4 . 



It may be well here to summarise the precautions necessary to be 

 taken in preparing the anhydrous acid, so far as we have ascertained 

 them. They are briefly as follows : — To use for the purpose an acid 

 as pure as possible, to have a thoroughly good vacuum, to avoid too 

 high or prolonged heating, and to keep a sufficient quantity of solid 

 potash in close proximity to the acid all through — which may be 

 arranged by using a small flask, and having the tube leading from it 

 short and wide. . 



Properties of Anhydrous Selenic Acid. 



Anhydrous selenic acid is a white crystalline solid melting at 58° to 

 a colourless oily-looking liquid. When thoroughly melted it remains 

 in a superfused state, and usually requires to be cooled to about 5° 

 with constant stirring before it again freezes. The temperature then 

 rises rapidly to 58°, and remains stationary until complete solidifi- 

 cation of the acid has taken place. When at rest it can be cooled 

 lower than 5° without freezing, and it will remain in a stoppered 

 bottle for months, and during frosty weather, in a liquid condition i 



