On Super saturation of Salt-Solutions. 295 



more than 003 microfarad, better less than ^oo microfarad ; 

 for the inspector, Test the system for safety. The test is very 

 easy. Place a secondary generator of greatest capacity at one 

 end of the line and connect its secondary circuit to earth 

 through any instrument suitable for measuring alternate cur- 

 rents under one ampere ; put the other end of the primary to 

 earth ; the reading of the current-measuring instrument 

 should not exceed such a current as it may be demonstrated a 

 man can endure with safety. 



XXXI. On Supersaturation of Salt- Solutions. By W. W. J. 

 Nicol, M.A., D.Sc, F.R.S., Lecturer on Chemistry, Mason 

 College, Birmingham* ', 



IN my previous paper f on this subject I described experi- 

 ments which proved that a so-called supersaturated 

 solution was able to dissolve more of the salt it already con- 

 tained, provided only that the salt was added in the dehydrated 

 state, and under conditions which precluded the possibility of 

 access of crystals of the hydrated salt. I also expressed the 

 opinion that a supersaturated solution is merely a solution of 

 the anhydrous salt which may, or may not, be saturated; and 

 that it differs in no way from an ordinary solution so long 

 as no disturbing cause operates to bring about the formation 

 of a hydrate which exists only in the solid state. 



Until I made the experiments described in the following 

 paper, I was of the opinion that although a hydrated salt 

 existed in solution in what may be termed an anhydrous state, 

 still, when a dehydrated salt is dissolved in pure water, 

 solution always took place in two stages — the first consisting 

 of the hydration of the salt to form the solid hydrate, and that 

 this was followed by solution accompanied by the decomposi- 

 tion of the hydrate thus formed ; but the experiments I am 

 about to describe are, I think, conclusive, and show clearly 

 that solution of the dehydrated salt in pure water is unattended 

 by the formation of any hydrate, solid or liquid. 



Two similar wide-mouthed bottles, capable of holding some 

 60 cubic centim. of water, were taken ; in each were placed 

 25 cubic centim. of the same sample of distilled water, and in 

 each a glass bulb containing 5 grins, of dehydrated Na 2 S0 4 . 

 One of the bottles (No. 1) was placed at once in the constant- 

 temperature bath at 20° C. ; the other (No. 2) was placed in 

 a water-bath which was raised to the boiling-point ; it was 

 then, after cooling, placed alongside the first bottle. After 



* Communicated by the Author, 

 t Phil. Mag. June 1885. 



