506 Dr. W. W. J. Nicol on the Vapour-pressures 



all made with the same thermometer and are thus strictly 

 comparable among themselves, and as the boiling-pressures 

 of pure water were determined at the same time with the same 

 thermometer, we have a natural standard by which the results 

 may be corrected if need be. Nothing, however, would be 

 gained by such a correction ; for, as the figures in Table I. 

 show, the thermometer, which was by Geissler and divided 

 into tenths, was very nearly exact, the results agreeing well 

 with Regnault's, when it is remembered that his figures are for 

 steam and mine for water. With regard to the probable error 

 for any single determination, that does not much exceed + *3 

 millim., and is of course proportionately less for the mean 

 results of four determinations which are given in the Tables. 



Table I. 

 Boiling-pressures of pure Water (Zinc). 



t°. 



70°. 



75°. 



80°. 



85°. 



90°. 



95°. 



Eegnault . . . 

 Found 



233-3 



228-3 



288-8 

 283-2 



354-9 

 349-4 



433-2 

 428-3 



525-5 

 521-4 



633-7 

 631*0 



Difference ... 



50 



5-6 



5-5 



4-9 



41 



, 7 



At present I have examined only four salts, which may be 

 regarded as typical of the class which crystallize without 

 water ; they are NaCl, KOI, NaN0 3 , and KN0 3 . I intend 

 soon to examine the behaviour of some typical hydrated salts. 



The results thus obtained enable us to examine, not only 

 the effect of varying amounts of salts at the same temperature, 

 but also that of fixed amounts of salts at varying temperatures; 

 that is, the Tables given may be read either vertically or 

 horizontally. Each Table consists of two halves. The first 

 contains the value of p—p', where p = the vapour-pressure of 

 pure water at the given temperature as contained in Table I., 

 p' the pressure of water-vapour from the salt-solution of the 

 composition n molecules of salt per 100 molecules of water. 



The second half of the Table contains - — — ; that is, the effect 



on the vapour-pressure of pure water by the presence of one 

 salt-molecule in solutions of the stated composition. A glance 

 at the Tables shows at once that the four salts range them- 

 selves in two classes, when the effect of varying amounts of 



