SPONTANEOUS CRYSTALLISATION OF MONOCHLORACETIC ACID, ETC. 339 



curve and melting-point curve. Preliminary experiments tended to confirm the 

 former, but we were unable to verify the latter. Further, the microscopic examin- 

 ation of the mixtures containing excess of naphthalene above the eutectic compo- 

 sition showed that all the crystals which separated in the early stages of consolidation 

 possessed the form and the optical properties of pure naphthalene. This led us to 

 doubt whether any mixed crystals whatever are formed in these solutions, and to 

 suspect that monochloracetic acid and naphthalene yield in reality only another 

 example of the crystallisation of the two pure substances from their mixtures, like 

 the mixtures of salol and betol which we have already investigated. On the one 

 side of the curve, representing excess of naphthalene above the eutectic composition, 

 we were able to trace a perfectly definite supersolubility curve separated by about 

 2'5 from the solubility curve ; this supersolubility curve belongs to crystals possessing 

 the form and optical properties of naphthalene. On the side of the curve corre- 

 sponding to mixtures containing excess of monochloracetic acid above the eutectic 

 composition, we again obtained evidence of a definite supersolubility curve, separated 

 in this instance by about 10 from the solubility curve, but for these mixtures 

 spontaneous crystallisation appeared also to take place at several other very well 

 defined temperatures. Experiments made upon drops of these mixtures viewed under 

 the microscope, with the object of determining the optical properties of the mixed 

 crystals which we then expected to find, proved very difficult to interpret, and led us 

 to suspect that crystals of two or three distinct sorts were making their appearance in 

 addition to crystals of naphthalene. We also obtained no less than three different 

 temperatures of spontaneous crystallisation for pure monochloracetic acid. 



In a paper on the properties of acetic acid, and its chloro- and bromo- derivatives, 

 published in the ' Journ. Chem. Soc.,' 1895, 67, p. 664, PICKERING traced the freezing- 

 point curves of three distinct modifications of monochloracetic acid, which he 

 distinguished as a, ft, and 8, together with indications of a fourth modification which 

 he called y. It therefore seemed probable to us that the multiple freezing-points of 

 the mixtures containing excess of monochloracetic acid may really belong to the 

 different modifications of this substance, and that none of these mixtures really yield 

 homogenous mixed crystals of monochloracetic acid and naphthalene. If so, the study 

 of these two substances would not yield any information concerning the spontaneous 

 crystallisation of mixed crystals possessing a minimum freezing-point, but they might 

 provide material for an even more interesting study that of the spontaneous crystal- 

 lisation of the different modifications of a substance dissolved in another substance 

 which is not polymorphous. We were led thus to begin with an investigation of 

 monochloracetic acid and its solution in water with the object of ascertaining the 

 solubility and supersolubility curves of the different modifications from aqueous 

 solutions. There is at present no evidence which proves that each modification of a 

 polymorphous substance possesses a definite and different temperature of spontaneous 

 crystallisation. 



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