SPONTANEOUS CRYSTALLISATION OF MONOCHLOKACETIC ACID, ETC. 343 



the freezing-point of ft in that solution. If the tubes are allowed to stand, the 

 y-crystals become transformed into with a similar rise of temperature. Sometimes 

 they appear to be transformed directly into a, with a rise of temperature to that of 

 the freezing-point of a in the given solution. If y-crystals which have been formed 

 on a glass slide be inoculated at their margin with a minute fragment of /3, the 

 transformation takes place in a very remarkable manner. Crystals of the latter 

 modification grow rapidly across the slide from the point of inoculation. They grow 

 as single crystals and preserve their orientation quite independently of the orientation 

 of the y-crystals, and as they grow they possess a perfectly definite outline showing 

 sharp edges just as though they were growing in a liquid. If y-crystals on a slide be 

 inoculated at their margin with a fragment of a, a similar transformation takes place 

 into , the only difference being that the crystals grow at a much greater speed, so 

 that it is more difficult to follow the growth and distinguish the outline of the 

 individual crystals. 



Transformation of ft. When ft is crystallised from solution in sealed tubes, it 

 is almost impossible to transform it immediately into a, but it is easy to obtain 

 ^-crystals on a slide either from a drop of solution by friction or from y-crystals by 

 merely touching them. The /3-crystals may then be transformed into a by inoculating 

 with a small fragment of . Here again the a-crystals grow in the solid ft from the 

 point of inoculation in the manner just described, advancing with definite outlines and 

 sharp edges. The rate of growth is much slower than when y is directly transformed 

 into a. 



C. Aqueous Solutions of Monochloracetic Acid. 



PICKERING (' Journ. Chem. Soc.,' 1895, 67, p. 664) has traced the solubility curves 

 for solutions of the three modifications which he calls a, ft, and 8 of monochloracetic 

 acid in water ; he gives one point only on the solubility curve of an aqueous solution 

 of what he terms y. 



We have therefore investigated aqueous solutions of monochloracetic acid in order 

 to determine, if possible, the supersolubility curves for the various modifications of 

 the acid described by PICKERING, i.e. the curves which separate the metastable region 

 (in which crystals will grow in a supersaturated solution if the solution be inoculated) 

 from the labile region (in which crystals will form spontaneously in the solution). 



The aqueous solutions of monochloracetic acid were treated by the methods already 

 employed for the various solutions and mixtures for which the supersolubility curves 

 have been determined by us, such as sodium nitrate, sodium chlorate, &c. 



Weighed quantities of monochloracetic acid and water were heated together to 

 between 80 and 90 in a loosely, stoppered flask, and the heated solution was then 

 placed in the trough of the inverted goniometer, described in 'Phil. Trans.,' 1903, A, 

 vol. 202, p. 459. A glass prism of known angle and refractive index was immersed in 

 the hot solution, and as the solution cooled, the changes in the refractive index of the 



