238 Proceedings of Royal Society of Edinburgh. [sess. 
ionisation theory of salt solutions. They are easily explicable on 
the basis of that theory, but not very easily otherwise. They bring 
into striking prominence the contrast between ionisation and ordi- 
nary dissociation, and a case such as this illustrates the advantage 
of using the former term in place of the expression ‘ electrolytic 
dissociation,’ which is still frequently employed. 
It will be observed that thio-urea is to a certain extent analogous 
to a metal ; a molecule of it corresponds to a half atom of a dyad 
metal. Like a metal it unites directly with the halogens to form 
salts. It can c reduce ’ metallic salts from a higher to a lower 
stage; thus Rathke * found that it acts upon cupric sulphate to 
form cuprous sulphate — momentarily — and the sulphate correspond- 
ing to the di-iodide 
2CuS 0 4 + 2CSX 2 H 4 Cu 2 S0 4 + (CSX 2 H 4 ) 2 S0 4 . 
The cuprous sulphate unites with more thio-urea to form a complex 
compound. Other cupric salts behave in a similar manner. 
When thio-urea is added to a concentrated solution of ammonium 
persulphate there is a considerable evolution of heat, and, on cool- 
ing, the above sulphate separates out, while ammonium sulphate 
remains in solution : — 
2CSX 2 H 4 + (NH 4 ) 2 S 2 0 8 =£= (CSX 2 H 4 ) 2 S0 4 + (NH 4 ) 2 S0 4 , 
corresponding to such an action as 
Zn + (XH 4 ) 2 S 2 0 8 == Zn S0 4 + (XH 4 ) 2 S0 4 . 
The exact constitution of the class of salts here dealt with does 
not appear to have been very fully investigated, and in what pre- 
cedes I have simply adopted the formulae generally employed. 
Adopting the imide formula for thio-urea, the most plausible 
assumption is that the salts may be represented by the graphic 
formula — 
XILX XELX 
i i 
H-N =0-S-S-C= N-H . 
The difficulty of satisfactorily investigating this and other points 
Berichte, xvii. i. 297 (1884). 
