1877.] 



Supersaturated Saline Solutions. 



elusions of this paper have been called in question by some of the French 

 authorities on this subject, yet I venture to submit that the foregoing 

 details support our main conclusion, which may be so far modified as to 

 present itself in this form, namely, that during certain states of the 

 atmosphere, if not always, such a connexion really exists. 



On the 1st of July rain set in, and the weather during some days was 

 cloudy and unsettled. The effect of this change on some of the solutions 

 was remarkable ; but I saw reason to direct my attention specially to the 

 behaviour of the sodic acetate solution. During these wet and un- 

 settled days the solution exposed in drops remained permanently liquid. 

 I found that even when crystals. of the salt from a drop that had previously 

 become solid w r ere brought into contact with the liquid drops, they no 

 longer produced crystallization. This led me to suspect that the salt is 

 deliquescent as well as efflorescent in varying states of the atmosphere. 

 Accordingly on placing a known weight (30 grains) of the dry salt on 

 filtering-paper in the scale-pan of a balance I found that it increased 

 in weight from day to day in w T et weather, and lost weight in dry. 

 The same experiment w^as tried repeatedly with the supersaturated 

 solution, of which 30 grains placed in the glass scale-pan became 33 in 

 the course of some hours, and went on increasing from day to day during 

 damp weather ; but when dry weather set in it lost in weight, and when 

 the solution crystallized there was a further loss of 6 or 8 grains from 

 evaporation consequent on the heat liberated during solidification. 



Here, then, is the explanation of the fact which has excited the surprise 

 of several observers, namely, that supersaturated solutions of sodic ace- 

 tate may remain liquid for days, and even for weeks together, exposed all 

 the while to the dust of the room. Such a fact has been cited in dis- 

 proof of the theory of aerial nuclei, and was appealed to by the French 

 chemists as a complete answer to the theory of surface-tension ; and it 

 had so powerful an effect on the mind of Professor Van der Mensbrugghe 

 as to lead him to withdraw from all further connexion with that 

 theory. 



"With a westerly wind, a bright sun, a tolerably high temperature, and 

 a moist air, I have known the drops of sodic acetate to remain liquid 

 during eleven or twelve days, and drops that had previously become solid 

 liquefied again. The point of a needle drawn across the drops, which had 

 thus absorbed water, had no action in inducing crystallization, though a 

 considerable amount of friction was used. This was on glass ; on writing- 

 paper each drop, after remaining liquid for some days, had a wet halo 

 round it of considerable width ; but it seems that as the paper absorbed 

 water from the drop, the drop was absorbing moisture from the air. On 

 passing the point of a pencil through the drops on paper, so as to draw 

 out a tail, there was no action, and a crystal of the salt placed in the 

 drop was equally inert. In fact the solution was no longer super- 

 saturated. Sodic carbonate remained for days together on paper without 



