Chemical Observations. 357 



the other phial was scarcely altered during the first four 

 months ; but during the last two months, which were colder 

 and damper, it became opaque on the surface, and the crys- 

 tallization thereafter was propagated inwards to a considerable 

 depth. 



The effect of a small quantity of moisture in enabling solid 

 amorphous matter to crystallize was observed very distinctly 

 in the case of another substance. A quantity of sulphate of 

 soda was rendered anhydrous by heat, and became a heavy 

 powder. Placed in a confined atmosphere, kept purposely 

 humid, the powder slaked like lime, swelling to several times 

 its original bulk. It regains in two weeks its usual combined 

 water (twelve atoms), and was then dry, and not in the slight- 

 est degree crystalline. Two days afterwards the powder was 

 found a mass of crystals of the usual form of sulphate of soda, 

 so dry as not to adhere to the blade of a knife ; and it was not 

 till after weighing that I satisfied myself of the presence of 

 uncombined moisture among the crystals nearly to the extent 

 of an additional atom. Here a small quantity of water 

 allowed the powdery particles to right themselves, and adopt a 

 crystalline arrangement, which they were incapable of assuming 

 without it. 



3. — Detection of Arsenic. 



Perhaps no greater degree of certainty is desirable in the re- 

 cognition of arsenious poisons than is attained by the reduction 

 to the metallic state with subsequent oxidation ; and certainly 

 the addition to the usual suite of operations, which is the 

 object of this notice, did appear, when first suggested to me by 

 Mr. Clark of Glasgow, as a thing worth trying, an over-refine- 

 ment, should it be found to be practicable. But, on ascer- 

 taining the extreme facility of the proposed process by actual 

 trial, a view of the absolute certainty of the demonstration 

 which it clenches completely altered my first opinion. 



We usually stop on recovering the arsenious acid in minute 

 crystals in the upper part of the tube. Scratch the glass tube 

 with a file below the crystalline crust, and break it off. The 

 upper part of the tube containing the supposed arsenious acid, 

 and nothing else, may be boiled for a few seconds with a drachm 

 or two of distilled water, to which a drop of caustic ammonia 

 has been added. A solution is obtained, which, after being 



