256 Proceedings of Boyal Society of Edinlurgh. [sess. 
proportions with the corresponding hydroxides in aqueous solution, 
form solutions of the alkaline sulphides ; hut occasionally this state- 
ment is denied on more or less plausible grounds. Mendelejeff, for 
example, in his Vrinci-ples of Chemistry^ writes as follows (vol. ii. 
p. 207) : — “From this [the non-evolution of heat on mixing hydro- 
sulphide with hydroxide] it must not he concluded, as Thomsen 
concludes, that hydrogen sulphide is a monobasic acid. The impro- 
priety of this conclusion is not only seen from the decomposing 
action of water on potassium sulphide, hut also from the fact that 
for the formation of this salt there must react KHS-l-KHO = 
K 2 S- 1 -H 20 , substances which closely resemble each other, and in 
such cases hardly any heat is developed, — for example, in double 
decomposition between salts. The resultant substances, potassium 
sulphide and water, undoubtedly act on each other, and thus would 
cause the absorption of heat did not the reaction of potassium hydro- 
sulphide with potassium hydroxide also develop heat.” The author 
here seems to be of somewhat divided mind as to the actual state 
of an aqueous potassium sulphide solution ; hut further on, p. 213, 
he indicates the real explanation : — “ Potassium hydroxide, water, 
and sulphuretted hydrogen form a system whose complex equili- 
brium is subject to the laws of dissociation, and depends on the 
relative mass of each substance, on the temperature, and on the 
dissociation pressure of the component elements.” It was with a 
view to determine this equilibrium in solutions of sodium sulphide 
and sodium hydrosulphide that the following experiments were 
made. 
Sodium hydrosulphide solution smells strongly of sulphuretted 
hydrogen at the atmospheric temperature, and, on boiling, easily 
loses half its sulphur as sulphuretted hydrogen, the sodium sulphide 
solution thus formed losing sulphuretted hydrogen much more slowly 
when the boiling is prolonged. At the ordinary temperature a solu- 
tion of sodium sulphide does not smell of sulphuretted hydrogen, 
nor do the crystals 9 H 2 O in perfectly acid-free air, and no 
indication of the presence of sulphuretted hydrogen is given by 
lead paper. 
To ascertain the effect of temperature, a 20 per cent, solution of 
the crystalline hydrate, ISTagS, 9 H 2 O, was placed in a flask, and 
through it there was passed a current of hydrogen previously washed 
