2 Dr. E. J. Mills's Researches on Melting-point. 



A. Benzol Derivatives. 



1. Dinitrobenzol. — The benzol from which specimens A and 

 C were prepared had been purified by myself, first by frac- 

 tional distillation, then by fractional distillation after treatment 

 with bromine, and, lastly, by freezing and pressure. The 

 preparation of the dinitro-compound was effected with the aid 

 of hydric sulphate. The crude product was washed with water, 

 alkaline water, and water successively, and then submitted to 

 a process of purification repeatedly adopted in these experi- 

 ments. 



[The process in question consists in crystallization from 

 distinct successive solvents, followed each time by pressure. It is 

 well known that small quantities of impurity are prone to cling 

 to substances with much tenacity; but the observation has most 

 frequently been made in connexion with some single solvent. 

 One can readily conceive that the tenacity with which a given 

 trace of a foreign body is held may, under such circumstances, 

 be in effect constant. If, however, we now transfer the mix- 

 ture to a new solvent, then, we may fairly presume, the trace 

 will be in a condition of altered adhesiveness, and may be 

 much more readily separable. It is of course not easy to 

 decide whether this result is always attained, or whether the 

 later solvent makes any impression at all; but it will probably 

 be granted that the method of multiple successive solvents is 

 in general expedient to adopt. 



After each crystallization the crystals were in all cases sub- 

 mitted to powerful pressure between folds of carefully cleansed 

 linen or, occasionally, of silk tissue. The surface-tension of 

 the dissolved impurities ajjpears to be for the most part consi- 

 derable; and this, aided by mechanical compression, greatly 

 economizes the time required in preparing a pure substance. 



The pressed crystals were next reduced to a very fine powder, 

 and dried for eleven days over oil of vitriol in the dark. Care 

 was taken never to make determinations with substances pre- 

 viously melted; for it not unfrequently happens that a second 

 melting takes place at an appreciably different temperature.] 



Specimens A and C were crystallized twice from naphtha, 

 and thrice &c. (A 3 &c, C 3 &c.) from purified alcohol. Spe- 

 cimen B was given me by Mr. C. E. Groves, who had prepared 

 it from thrice frozen and pressed benzol. It was crystallized 

 once from water, once from naphtha, once &c. (B } &c.) from 

 pure alcohol. The results are contained in Table I. 



Dinitrobenzol melts with some sharpness after a decided 

 pasty stage. It strongly resists pulverization — a character 

 more especially observable in specimen B. 



