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



anhydrous and also the monohydrated acid existing in a super/used state, 

 and exerting a solvent action upon each other. In the case of sulphuric 

 acid a similar action evidently occurs. The effects of agitation and of 

 alteration of pressure upon the freezing points of the above acids have 

 already been referred to. 



The Method used in Freezing Selenic Acid. 



Liquefied sulphur dioxide was poured into a thick glass tumbler, 

 holding about half a litre, fitting into a somewhat wider and deeper 

 cylindrical gas jar, which served to retain any of the dioxide splashed 

 from the interior vessel, the whole being imbedded deeply in a con- 

 siderable quantity of cotton wadding contained in a wooden box. A 

 rapid current of air was driven through the sulphur dioxide, the air 

 being first dried by means of sulphuric acid and then cooled by pass- 

 ing through a leaden worm surrounded by a mixture of salt and 

 pounded ice, or less effectively, sodium sulphate and hydrochloric 

 acid. Any desired degree of cold within limits could easily be main- 

 tained by regulating the current. When the outside air was at 0°, 

 about half a kilogram of sulphur dioxide was found sufficient for 

 more than two hours' use, evaporation taking place but slowly at the 

 low temperature reached, the latter, measured with an alcohol ther- 

 mometer, falling below — 50° C. In warmer weather nearly as low a 

 temperature was obtainable, but consumption of the sulphur dioxide 

 was much more rapid. No arrangement of freezing mixtures produced 

 nearly so low degrees of cold as were attained in the above manner. 

 Doubtless when a supply of dry snow is available, and the weather is 

 very cold, so that the apparatus and materials used can be well 

 cooled down previous to mixing, a very low temperature is obtainable 

 by ordinary freezing mixtures, but those conditions are not often to 

 be met with in these climates. On the other hand, sulphur dioxide 

 is cheap and easily procurable, and convenient when used in the 

 manner described. 



The Specific Gravities of the Higher Strengths of Selenic Acid. 



The specific gravities of selenic acid for the higher strengths, taken 

 in the liquid state at 15°, are given in the following table. The 

 most concentrated acid, of 99 73* per cent, strength, has been 

 referred to under the head of the preparation of the anhydrous acid ; 

 its specific gravity, 2'6083, was taken while in the superfused con- 

 dition. The next acids down to 94" 74 per cent, were obtained by 

 diluting that of 99*73 per cent. The acid having a strength of 

 93" 70 per cent, was obtained by heating dilute acid in a vacuum at 



* 99'73 per cent, of selenic acid, acid sodium selenate, equivalent to 0*07 per 

 cent, of neutral sodium selenate, and O20 per cent, of water (Se being — 78 87). 



