Hartiey—Wolecular Constitution of Supersaturated Solutions. 589 
erystallization takes place on agitation, or scratching the surface 
of the containing vessel. The phenomena appear to be 
altogether different from those of such hydrates as sodium 
sulphate, which do not crystallize by shaking or other means of 
agitation. Glycerol,! again, is a substance which, by a very low 
temperature and continual vibration, becomes crystallized, but its 
persistence in a liquid state, notwithstanding low temperatures, 
must be due to what is ordinary called superfusion. 
The instances quoted by Nicol are acetanilide, hydroquinol, 
acetamide, malonic acid, mandelic acid, resorcinol, tartaric acid, and 
citric acid. 
In this latter example, water plays an important part; and no 
fewer than four different crystalline forms of citric acid have 
been recognised. 
The following general statement has been formulated by 
Nicol as applicable both to anhydrous and hydrated compounds, 
to supersaturation of solutions, and to superfusion :— 
‘As soon as the conditions of the experiments are such as to 
admit of two allotropic modifications of the dissolved or of the 
molten substance, then the occurrence of supersaturation, or of 
superfusion respectively becomes possible.” 
The word ‘ allotropic ’ is used in a wider sense here than is usual, 
since it is applied to different crystalline modifications of one 
and the same substance, which are conditional on the presence or 
absence of molecules of other substances, as, for example, water of 
crystallization. It is further stated, with respect to substances 
which combine with water :— 
“J. Supersaturation of a solution is occasioned by allotropy, 
which is conditional upon the presence of molecules foreign to the 
salt, particularly water of crystallization.” 
“TI. Supersaturation is occasioned by the existence of allo- 
tropic (whether enantiotropic or monotropic) forms.” 
In the latter case the behaviour of potassium nitrate, ammonium 
nitrate, and silver nitrate is considered, as also the organic sub- 
stances before mentioned. The anhydrous salts are said to form 
supersaturated solutions; but each can, in the anhydrous state, 
exist in two different crystalline forms, one of which is unstable, 
1 Jour. Chem Soc., xxix., p. 384. Gladstone. 
