1890.] the Validity of Person' 's Absolute Zero. 15 



Substances Investigated. 



The substances investigated were sulphuric acid, the monohydrate 

 of sulphuric acid, the tetrahydrate of calcium nitrate, benzene, and 

 naphthalene, the last mentioned being the only one in which C, c, and 

 I had been determined by previous investigators. The sulphuric acid 

 was the same as that used in my determinations of the freezing points 

 of this substance (' Chem. Soc. Trans.,' 1890, p. 337), the stock acid 

 having been diluted by the addition of ice so as to contain exactly 

 100, and, for the monohydrate, 84 - 488, per cent. H 2 SOi. 



The calcium nitrate was prepared by repeated crystallisation ; the 

 percentage of anhydrous salt in the fused crystals having been de- 

 termined by evaporation and heating at 250, the excess of water 

 which the fused salt was found to contain was driven off by a gentle 

 heat. The melted salt will remain liquid at ordinary temperatures 

 for many days, although its solidifying point is 42'4. 



The naphthalene and benzene were special preparations made by 

 Messrs. Kahlbaum : repeated crystallisation was not found to alter 

 the melting point of either to any appreciable extent. 



Method Employed. 



The substance was placed in a cylindrical platinum bottle measur- 

 ing 9x2 cm., and holding about 30 c.c. Its mouth was closed by a. 

 caoutchouc stopper, through which passed a thermometer with a very 

 narrow bulb, long enough to extend from the top to the bottom of 

 the bottle, thereby giving the mean temperature of the contents 

 more accurately than an instrument with a short bulb would 

 have done. The bottle was placed in a double test tube, and the 

 latter in a double bath containing warm water or a freezing 

 mixture, as the case might be. After the bottle had attained the 

 required temperature, and this had remained constant for some 

 time, it was removed from the test tubes and plunged into the calori- 

 meter, an operation which occupied only two or three seconds. To 

 prevent the deposition of hoar-frost on the bottle while it was being 

 cooled, the inner test tube in which it was placed had a bulb blown at 

 the bottom, in which was kept some sulphuric acid. The calorimeter 

 contained either 600 or 1800 c.c. of water, the quantity being adjusted 

 so that the rise or fall of the temperature in it was about 1, the 

 smallness of the change being favourable to the accuracy of the de- 

 termination by rendering the loss of cooling, or gain by heating, very 

 small. This loss or gain was estimated by determining the rate of 

 cooling at the initial and final temperatures, both thermometers being 

 read at intervals of one minute, and having been compared with each 

 other before the determinations. The rate of cooling during the time 

 when the temperature was rising or falling was taken to be the mean of 



