46 Prof. B. A. Lehfeldt on the 



sures, taking eight observations at temperatures rising from 

 45 c to 55°, and then falling to 45° again, and taking the 

 refractive index of the residue : the refractive index was 

 throughout used as a means of analysis (as in the previous 

 paper, g. ?\), and it showed that no appreciable change of 

 composition took place during the experiments. 



The instruments used were the following: — (i.) A catheto- 

 meter (by Fuess) to read the manobarometer with. Its scale 

 was taken as correct; and since readings were only taken to 

 one-tenth of a millimetre, no difficulty was encountered. 

 Pressures were in all cases corrected for temperature of the 

 mercury, and reduced to sea-level in lat. 45°. 



(ii.) Five thermometers, of which the most important were 

 "A" of range 40° to 60° in I, used for the boiling-tube, and 

 " B " of range —5° to + 35° in 1, used for the refractometer, 

 both by C. E. Midler. Their errors, which were very small, 

 were determined by comparison with the standards of the 

 Reichsanstalt. nos. 7346 and 7347, belonging to the Davy- 

 Faraday laboratory. 



(iii.) Pulfrich refractometer of the old pattern (by Max 

 Wolz). Measurements of the refractive indices of alcohol 

 (1*36), carbon tetrachloride (1'46)_, and benzene (1*50) were 

 taken by it and a small Schmidt and Haensch spectrometer. 

 The values given by Pulf rich's table were thus found to be 

 too low by 0*00620 for alcohol, 0*00613 for carbon tetra- 

 chloride, and 000615 for benzene. Accordingly 0*0062 was 

 added to the number from the table in each case. Measure- 

 ments of the refractive index of the mixtures were always 

 taken as near to 18° as possible (though unfortunately in the 

 summer the room rose frequently to 21°) : the temperature- 

 coefficients were interpolated from the known coefficients of 

 the pure substances (this involves very little error as the 

 coefficients only vary from 0*00040 for alcohol to 0*00064 

 for benzene), and the readings reduced to the standard 

 temperature. 



An accuracy of the order 1 in 1000 was aimed at in the 

 various measurements ; but of course the final results are 

 hardly reliable to that extent. To take a typical case, the 

 saturation-pressure to be measured might be about 250 mm.: 

 there is of course no difficulty whatever in reading the height 

 of the mercury-column to one thousandth of that (|- mm.). 

 The corresponding temperature needs to be known to about 

 -^ 6 degree, since 1° would make about 10 mm. difference in 

 pressure. The thermometer was read to §*§ by eye with 

 certainty; and when the apparatus was in good working order, 

 it kept constant to that extent. Again, to change the vapour- 



