TO THEIR STATE OF IONIZATION—MACGREGOR. 228 
of accuracy. The expansion of water was taken, according to 
his observations, to be 00,878 for the same interval. 
Briickner’s observations of viscosity were made at 15° C.; 
but he gives an interpolation formula, applicable between 15° 
and 20°, by means of which at least approximate values for 18° 
were obtained. His values for water at 15° and 20° do not 
agree well with those given by Landolt and Bornstein. I have 
therefore taken 0010613 as the viscosity at 18° of the water 
used by him, a value which has to his value at 15° the same 
ratio as Landolt and Bornstein’s for the same temperatures. 
The actual concentrations of Briickner’s solutions differed from 
those given in the tables below by about 0:1 per cent. ; but so 
small a difference could produce no appreciable error in the 
result. He gives as his “mean probable error of observation,” 
+2'4 in the fifth place of decimals for sodium-chloride solutions, 
and +1°8 for those of potassium chloride. 
Rother’s observations of surface-tension were made at 15° 
and are therefore not precisely comparable with calculated 
values based on the values of ionization-coefficients for 18°. 
From Kohlrausch’s data,* however, it would appear that 
between 15° and 18° in the case of potassium-chloride soiutions 
containing 0°5 and 3 gramme-molecules per litre, the ionization- 
coefficient changes only by about 0°13 and 1:3 per cent. respec- 
tively ; and in the ease of sodium-chloride solutions of the same 
concentrations only by about 0-4 and 0°6 per cent. respectively. 
For the more dilute solutions, therefore, my calculations will be 
practically comparable with Rother’s observations. He seems to 
regard his determinations as possibly in error by +5 to 8 in the 
the third place of decimals. The surface-tension of the water 
he used he found to be 7:°357. 
Bender’s observations of refractive index were made at 
15° C., but were reduced to 18° by means of data provided in 
his paper, based on observations made by Fouquét. The 
refractive index of the water he used he found to be 1°33310 
*Wied. Ann. xxvi. (1885) p. 223. 
+ Compt. Rend. \xiv. (1867) p. 121. 
