10 
within a few degrees of the boiling point of mercury without 
producing the slightest decomposition of the salt. On the 
further application of heat, a black mass is formed, and the 
salt finally volatilizes as Thallium-chloride. 
The crystalline form of Thallium-perchlorate is that of a 
right rhombic prism (ppi), in which the faces of the rhombic 
octohedron (rr x ) and the basal faces of the prism (c) generally 
appear ; the crystals being lengthened, as is the case with the 
Alkaline-perchlorates sometimes in the direction of the pris- 
matic, and sometimes in the direction of the octohedral faces. 
A careful measurement of the crystals gave the mean value of 
102° 50' for the angle pp\\ and that of 102° 6' for the angle 
rr x on c. Hence the relation of the axes is 0*7978 : 1 : 0*6449. 
The angles observed by Mitscherlich,* in the case of Potas- 
sium- and Ammonium-perchlorates agree exactly with the 
above; for the first of these salts pp\==- 103° 58', rr x on c= 
101° 19'; and for the second pp^lOS 0 11', and rr Y on c=. 
102° 4', giving the relation of the axes to be (1) 0*7817 : 1 : 
0*6408 and (2) 0*7926 : 1 : 0*6410. The following analysis, 
made in my laboratory by Mr. T. E. Thorpe, shows that the 
formula of the salt is T1 Cl 0 4 . 
(1) Determination of Thallium. The crystals were well 
dried in vacuo over Sulphuric Acid, and precipitated with 
Platinum-tetra-chloride, the passage of the finely divided 
precipitate through the filter being avoided by evaporating to 
* Pogg. Ann. xxv. (1832), p. 301. 
