310 Proceedings of Royal Society of Edinburgh. [sess. 
finely crystalline deposits of double sulphates separated. One of 
these on analysis gave for the ratio Tl* : Tl"* the value 2, corre- 
sponding to the formula T1 4 T1 2 (S0 4 ) 5 . As succeeding crops gave a 
varying ratio, it is probable that these substances were really mix* 
tures of two double salts, T1 3 T1(S0 4 ) 3 and T1T1(S0 4 ) 2 , corresponding 
to the anhydrous ammonium double sulphates. 
By mixing thallous sulphate and thallic sulphate solutions in 
molecular proportions, and allowing to crystallise, small anhydrous- 
crystals of the salt T1T1(S0 4 ) 2 were obtained. This salt was pre- 
pared some time ago by Lepsius, who describes the crystals as 
exhibiting faces of the cube and the octahedron, and calls them an 
anhydrous alum.* Those obtained by me showed strong double 
refraction when examined in polarised light, and appeared to be 
rhombic. 
From the results outlined above, especially in connection with 
the potassium compounds, it is evident that these thallic salts 
exhibit several interesting peculiarities. Apparently a £ thallic 
sulphate ’ solution contains a large proportion of basic salt, even 
when the acid concentration is very great, and it would seem to be 
largely a matter of solubility whether normal or basic salts are 
obtained from it. Possibly the difficulty of obtaining the normal 
salt by itself is due to the fact that it unites with sulphuric acid to 
form the complex acid HT1(S0 4 ) 2 : , so that in the solution thero 
exists the balancing system — 
2T10HS0, + 2H 2 S0 4 ^ HT1. 2 (S0 4 ) 3 + 2H 2 0 + H 2 S0 4 ^ 
2HT1(S0 4 ) 2 + 2H 2 0. 
Under some conditions this may practically become— 
T10HS0 4 + H 2 S0 4 === HT1(S0 4 ) 2 + H 2 0 . 
In a similar way the decomposition of the normal potassium 
double sulphate when treated with dilute sulphuric acid may most 
simply be regarded as that represented by the equation — 
2KT1(S0 4 ) 2 + II 2 0 ^ K 2 T10H(S0 4 )o + HT1(S0 4 ) 2 . 
* Central -Blatt, 1891, i. 694. This is probably the source of the statement 
that thallic sulphate forms alums. 
