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



temperature ; tlie acid does not distil in the anhydrous condition. 

 At about 200° it begins to decompose slowly, and at higher tempera- 

 tures rapidly, into selenium dioxide, oxygen, and water. The latter 

 serves to dilute a portion of the remaining acid, which then at once 

 distils. In order to examine the effect of distilling the anhydrous 

 acid destructively in a vacuum, a portion was heated in a flask con- 

 nected with a condensing arrangement until rapid decomposition 

 took place. The residue always consisted of a mixture of anhydrous 

 selenic acid with selenium dioxide, the proportion of the latter 

 increasing with the rise of temperature and length of time of heat- 

 ing. The former was proved to be present by dropping in a crystal 

 of anhydrous acid, when the liquid froze, and it also gave a coloration 

 in the cold with the selenium test for the same acid which will be 

 described hereafter. A portion which had been distilled for some 

 time left a residue of which O7107 gram neutralised 18'50 c.c. of 

 seminormal soda solution, using litmus as indicator, equivalent to 

 94"27 per cent, of acid calculated as selenic — a result which might be 

 expected, owing to the influence of the selenium dioxide in diminish- 

 ing the acidity, as has been already pointed out. The distillate 

 consisted of selenium dioxide mixed with selenic acid. The latter 

 was in a dilute state, since it would not solidify on addition of a 

 crystal of anhydrous acid, nor would it respond to the selenium test : 

 0'4703 gram neutralised ll'll c.c. of seminormal soda solution, 

 equivalent to 85'46 per cent, of acid calculated as selenic. Taking 

 into account the diminution of acidity caused by the selenium dioxide, 

 it is evident that a very concentrated, although not anhydrous, acid 

 distils over. 



Action of Heat under ordinary Pressures. — When dilute selenic acid 

 is boiled at ordinary pressures nothing but water is evolved until 

 205° is reached, at which temperature it has the composition of the 

 monohydrated acid. In these respects it behaves like dilute sulphuric 

 acid. After passing 205° the distillate contains at first mere traces 

 of selenic acid, but its strength gradually increases. A portion of 

 acid distilled between 205° and 227° yielded a distillate of which 

 0*6826 gram neutralised 0*10 c.c. of seminormal soda solution, equi- 

 valent to 0'53 per cent, of selenic acid. On further heating from 

 227° to 260° a distillate was obtained, of which 0'3818 gram neutra- 

 lised 0'32 c.c. of seminormal soda solution, equivalent to 3'03 per 

 cent, of selenic acid. At the latter temperature the acid commenced 

 to distil over in white fumes. 0'6878 gram of the residue left at 

 260° neutralised 17" 76 c.c. of seminormal soda solution, equivalent to 

 93'41 per cent, of selenic acid. At higher temperatures a portion of 

 the acid is decomposed, the distillate being kept weak by the water 

 continuously set free in the decomposition ; and at still higher tem- 

 peratures much selenium dioxide also distils over. A portion of acid 



