1889.] On Selenic Acid and other Selenium Compounds. 23 



absorb water ; the acid then weighed 1*8152 grams, indicating absorp- 

 tion of rather more than 25 molecules of water by 1 molecule of 

 anhydrous acid. Having arrived at this stage, it commenced to give 

 oil" a little of the water which it had previously taken up, but the 

 weather became warmer just at this period, so that probably the 

 above amount does not represent the total absorption of which the 

 acid is capable. It is less, however, than the amount taken up by 

 the molecule of sulphuric acid, which is variously stated at from 80 to 

 100 molecules of water. 



The Conditions which affect the Freezing Points of Selenic Acid and 

 Sulphuric Acid. 



It has long been recognised that in order to determine the melting 

 point of a chemical compound with accuracy it is necessary, by crys- 

 tallisation or other means, to obtain it in a pure condition. The 

 necessity for such a precaution is well shown in the case of the oxides 

 and acids of sulphur and selenium. An example taken from the 

 former class of bodies is furnished by sulphuric anhydride. Up to a 

 comparatively recent date great diversity of opinion prevailed 

 regarding the melting point of this substance until Weber showed 

 that, as hitherto examined, it had usually contained a minute quantity 

 of water, which had the effect of altering its melting point, crystal- 

 line form, and other properties. Sulphuric acid supplies another 

 instance of a similar effect. Its melting point in the anhydrous 

 condition is 10'5°, while that of the monohydrated acid is 8° ; yet 

 commercial sulphuric acid has usually been found to remain liquid 

 above a temperature of — 30° or —40°, and it is stated in some 

 chemical works that by addition of a little water to the commercial 

 acid its freezing point has been lowered to — 80° ; but it is not men- 

 tioned whether or not this occurred in closed or open vessels. A still 

 more striking example of the influence of want of purity upon the 

 melting point is afforded by selenic scid. In the anhydrous state 

 it melts at 58°, but a slightly dilute acid, as we have found, was 

 frozen only when a temperature of — 51*5° was reached, showing a 

 fall of 109"5°, and probably further dilution would be attended by a 

 still greater reduction of the freezing point. The depression of the 

 freezing point can be due only partly to superfusion since the super- 

 fused anhydrous acid freezes at about 5°. The monohydrate present 

 in the dilute acid therefore exerts an influence in lowering the 

 freezing point of the anhydrous acid, and also its melting point. An 

 analogous action probably occurs in the case of some metallic salts, 

 which, although without apparent chemical action upon each other, 

 have a lower fusing point when mixed than when heated separately. 

 A consideration of the foregoing facts leads to the conclusion that dilute 

 selenic acid having a strength greater than 88'96 per cent, contains the 



