268 Proceedings of Royal Society of Edinhurgli. [apeil 2, 
About the same time Vobl described a very large number of 
“ double-double” sulphates of the general formula 
(M"S04 . M'2S04 . 6H2O) -f (M”S04 • . 6H2O) , 
obtained by mixing the component sulphates in equivalent propor- 
tions, and allowing the solutions to evaporate spontaneously. 
Von Hauer, who followed a similar line of investigation to 
Schauffele, drew special attention to a series of definite compounds 
amongst the vitriols (‘‘Uber eine Eeihe von Verbindungen der 
Vitriole in bestimmten Aequivalent verbal tnissen,” Pogg. Ann., 
cxxv. 635, 1865). 
J. M. Thomson Assoc. Rep., 1877) endeavoured to prepare a 
nickel-cobalt- potassium sulphate of the type described by Vohl, but 
his efforts were unsuccessful. 
Still more recently, Aston and Pickering (Chem. Soc. Jour. Trans., 
1886) have made another unsuccessful attempt to obtain salts of 
Vohl’s type. They, however, arrive at the conclusion that not only 
are salts of the type described by Vohl not formed when the con- 
stituent sulphates are mixed together according to his directions, 
but further that there is not the slightest tendency among the so- 
called vitriolic sulphates to form definite molecular compounds with 
one another. 
These latter investigations certainly seemed to throw very grave 
doubts on the trustworthiness of Vohl’s early research, but seemed to 
me insufficient to justify the sweeping statement of Aston and Picker- 
ing. As an attempt to settle the question, I took up this investigation. 
General RemarTcs on Methods of Preparation and Analysis. 
In all the preparations which I am about to describe, the con- 
stituent sulphates, after being weighed out, were dissolved together 
with the calculated quantity of an alkaline sulphate in cold water 
by trituration in a mortar, and a little free sulphuric acid added. 
In every case the amount of water was more than what was exactly 
necessary to effect solution j in other words, the solution was never 
allowed to be saturated, although it was always nearly so. In order 
to obtain results strictly comparable with One another, the con- 
stituent sulphates were usually mixed together in equivalent or 
very nearly equivalent proportions. The solutions thus obtained 
were allowed to evaporate spontaneously in flat-bottomed crys- 
