306 EESEAECHES OK EVAPOBATION AND DISSOCIATION. 



benzene, and on the spociBc heats of the solid and liquid. 

 Our results have recently been the subject of a critique by 

 E. V. Helmholtz {Wied. Ann., N.P. 30, p. 401), which we 

 regard as justified. In our critique on Fischer's results wo 

 pointed out that the numbers given as theoretical must be 

 regarded as ordy approximate. 



(.S) Do the statical and dynamical methods of measuring 

 vapour-pressure give identical results with dissociable 

 bodies? {PhU. Trans., 188G, part i., p. 71.) To answer this 

 question, experiments were made with chloral hydrate, 

 butyl-chloral hydrate, chloral methyl- and ethyl-alcoholates, 

 ammonium carbamate, ammonium chloride, phthalic and 

 succinic acids, aldehyde ammonia, metaldehydo, nitrogen 

 peroxide, and acetic acid ; besides chlorine hydrate and 

 ethylamino hydrochloride, from which no results were ob- 

 tained, and paraldehyde, which was found to be stable. As 

 a rule, the methods already described, were employed, but 

 with nitrogen peroxide and ammonium chloride special 

 methods were devised. It may be mentioned however that 

 with ammonium carbamate and ammonium chloride, cylin- 

 drical blocks were cut out of large blocks of the salt, and 

 wore drilled with holes to fit the thermometer. Phthalic 

 acid was dissolved in water, and the thermometer-bulb, 

 covered with cotton-wool, was dipped repeatedly in the 

 boiling aqueous solution, and then hung in a boll-jar over 

 sulphuric acid for several days untfl dry. With nitrogen 

 peroxide, asbestos was substituted for cotton-wool. 



As the temperatures at which it was necessary to measure 

 the vapour-pressures of ammonium cldorido wore so liigh 

 that tlie sum of its vapour-pressure and that of mercury 

 would have exceeded the limits to which the ordinary 

 method is applicable, a piece of ammonium chloride was 

 placed in the closed end of a U-tube, and kept in position 



