On the Dew Point. ■ 63 



clitional facts, that its existence must be iaierred. Iodide of potas- 

 sium, even wiien added in great excess, does not precipitate the 

 whole of ^the arsenite of potassa, nor is it capable of diminishing its 

 alkaline reaction. On the contrary, when the arsenite of potassa is 

 so far neutralized by free acetic or arsenious acid as to be incapable 

 of giving a red stain to turmeric paper, this property is immediately 

 restored upon the addition of the iodide of potassium ; apparently 

 in consequence of an union between the latter substance and the ex- 

 cess of arsenious acid, which, while dissolved, had the power of coun- 

 teracting the alkaline effect. Iodine, alone, also occasions a precip- 

 itate from the arsenical salt, when there is an excess of acid present ; 

 and here, although we may suppose the conversion of iodine into hy- 

 driodic acid, the presence of some free alkali seems necessary to the 

 formation of a double salt. Other considerations lead us to the same 

 result. 



It is well knovra that the arsenites of soda and potassa have the 

 power of discharging the blue color from a mixture of starch and 

 iodine. This, indeed, is a part of the process proposed by Brugna- 

 lelli for the purpose of distinguishing between corrosive sublimate and 

 arsenic. The effect, however, may be shown by experiment, to de- 

 pend more upon the affinity which exists between arsenious acid and 

 iodide of potassium than upon the facility with which iodine is iicidi- 

 fied by exposure to alkaline solutions. Thus, for instance, carbonate 

 of potassa may be mixed with this blue compound of iodine and starch 

 for a long time, at the ordinary temperature, before there appeiu's to 

 be any diminution of color, and arsenious acid alone even heightens 

 the tint, but upon adding the acid subsequently and some time after 

 the potassa, the color at once flies. In this case, the presence of ar- 

 senious acid leads to the rapid formation of hydriodate of potasisa, at 

 the expense of the iodine, upon which the color depended. 



If subsequent experiments should establish the existence of such a 

 compound, it will be a solitary but striking example of what mity be 

 considered as a chemical hybrid. 



Art. X. — On the Daw Point ; by A. A. Hayes. 



To assist those v/ho are not intimately acquainted with hygrome- 

 try, the following illustrations of the facts, on which the experiments 

 with the hygrometer are based, — with the tables, which for conven- 



