396 Scientifi.c Proceedings^ Royal Dublin Society. 



readings of vapour-pressure are taken with the liquid and vapour in the 

 experimental tube occupying a series of different volumes. The best results 

 are obtained by first diminishing the volume by means of the steel plunger 

 until the vapour is completely condensed, then drawing out the plunger to 

 reduce the pressure until ebullition takes place,^ and taking the first reading 

 wlien the volume of vapour is small, and subsequent readings with increasing 

 volumes of vapour. The reasons for adopting this procedure have been 

 discussed in previous papers.^ 



T 



The pressure, calculated by the formula ^; = c . — , must be corrected 



(1) for the difference in level of the mercury in the experimental tube and 

 manometer, (2) for the pressure of the column of unvaporized liquid, (3) for 

 the expansion of the heated column of mercury, (4) if necessary, for capil- 

 larity, (6) for the deviation of air (in the manometer) from Boyle's law, as 

 determined by Amagat.^ 



No correction for the vapour-pressure of mercury should be applied, 

 because evaporation through a long column of liquid is an exceedingly slow 

 process. 



If, when the volume is reduced, the last bubble of vapour cannot be con- 

 densed without noticeable rise of pressure, it may safely be concluded that 

 air has not been completely removed. In that case the results must be 

 rejected, and the experimental tube taken down, emptied, cleaned, dried, and 

 refilled, and the determinations repeated. 



For substances, such as carbon tetrachloride, which act on mercury at 

 high temperatures, the experimental tube may be modified in such a manner 

 that the mercury is not heated.'' The method of calculation is more trouble- 

 some ; but accurate results may be obtained. 



Specific GrRAvniKS and Specific Volumes of Liquids. 



Tiie specific gravities of the liquids investigated, with a very few excep- 

 tions, were determined at 0° and at some higher temperature by means 

 of a Sprengel tube of the very convenient form recommended by Perkin'^ 



' When the liquid in the experimental tube is free from dissolved aii', it is impossible to get it to 

 boil again after the vapour has once been condensed, unless the temperature is at least 40 or 50 

 degrees above the boiling-point under normal pressure. At these lower temperatures the volume of 

 vapour should be diminished until only a minute bubble is left, but complete condensation should be 

 avoided . 



- I'roe. Roy. Dubl. Soc, xi., (N.S.), p. 89, 1906 ; Journ. de Chimie physique, iv., p. 425, 1906. 



'■> For talile of correction, see Tr.ivers, "Study of Gases." p. 167 (Maiiiiillan, 1901). 



* Trans. Chem. Soc., lix., p. 911, 1891. 



= Ibid., xlv., p. 444, 1884. 



