478 Proceedings of Royal Society of Edinburgh. [sess. 
cover the held, and these after a time become changed in places ; 
triclinic crystals of the ordinary modification make their appear- 
ance, which slowly eat away the unstable modification. 
(h.) Mandelic acid. Supersaturated solutions can be easily pre- 
pared on the large scale. A hot concentrated solution deposits a 
feathery crystalline modification, succeeded by the rhombic plates 
of the stable form, which grow slowly at the expense of the 
unstable form. 
(i.) Resorcin ; readily forms supersaturated solutions in quan- 
tity. A strong hot solution deposits prismatic crystals of the 
unstable form, which sometimes remain unchanged for a consider- 
able time, but ultimately crystals of the ordinary rhombic variety 
make their appearance, and grow rapidly at the expense of the 
other. 
(j.) Tartaric acid. The limits of supersaturation appear to be 
very narrow, but a very concentrated solution gives a mass of 
crystals of a new modification, followed later on by the ordinary 
monoclinic variety, which slowly eat away the crystals first 
formed. 
(A) Citric acid. This forms supersaturated solutions with great 
case, and, inasmuch as the ordinary crystals contain one molecule 
of water, it ought to have been included in class 1 ; but the super- 
saturation is only partly due to water of crystallisation, for a long 
and careful study of the behaviour of a solution crystallising under 
the microscope has satisfied me of the existence of no less than four 
crystalline citric acids. Of these four forms, one, the ordinary 
rhombic modification, contains of course one molecule of water, 
and is the ultimate stable form ; the others are either anhydrous 
or contain less water than the former, and are possessed of dif- 
ferent degrees of stability. As the solution cools, the first crystals 
to appear are prismatic in form, succeeded or accompanied by 
others consisting of twinned plates arranged about a common 
centre, which slowly eat away the first formed crystals. At the 
same time, the first formed crystals are seen to undergo a change, 
becoming opaque, and where the edges have been rounded by the 
action of the plate variety, fresh prisms shoot out, never, however, 
reaching the latter, but after a time are, in their turn, eaten away 
by it Finally, if the edge of the drop be touched with a crystal 
