534 PROF. DITTMAR AND MR C. A. FAWSITT ON METHYL-ALCOHOL. 



scale ; the gas volumes corresponding to the several points are determined by 

 gravimetric calibration with mercury. The apparatus when used is fixed within 

 a glass jacket, in which the required temperature is produced and maintained 

 by means of a continuous current of steam or other vapour. 



To make a determination, the apparatus is filled completely with mercury, 

 and all air-bells are carefully removed. A known weight of the liquid to be 

 operated upon is sealed up in a small bulb which must be almost absolutely 

 full, so that, when heated to the respective temperature, it bursts by internal 

 liquid pressure. The bulb is introduced through the neck, held down by the 

 stopper, while some mercury is made to rise over it into the funnel, and the 

 stopper then inserted. A few centimetres of mercury within the funnel make 

 an absolutely gas-tight joint. The mercury reservoir is in general so placed 

 that the pressure within the apparatus is less than that of the atmosphere ; at 

 the yery end, however, it is so adjusted that the two menisci, in the narrow 

 and in the wide tube, are in the same horizontal plane. The pressure of the 

 vapour is then equal to B +p, where B is the height of the barometer, and p the 

 small excess of capillary depression in the narrow as compared with the wide 

 tube. The correction p is easily determined by filling the apparatus partly 

 with mercury, and reading the difference of level between the two menisci 

 when both tubes are open to the atmosphere. It amounts to less than one 

 millimetre. 



The special advantage claimed for the apparatus is that there is no need of 

 any correction for the temperature of the mercury within the apparatus, which, 

 with the ordinary constructions, is so uncertain. 



When the apparatus was used for the special tension determinations 

 reported on on pages 514 and 51 5 of this Memoir, the upper end of the narrow 

 side tube communicated with the air-space of the mercury reservoir, and through 

 it with that of the shorter limb of the barometer, and with the air-pump, as 

 shown by the figure. 



The same artifice might be resorted to for determining vapour densities at 

 low pressures ; but we have had no occasion to make such determinations. 



