280 MR. W. R. BOUSFIELD AND DR. T. M. LOWRY ON THE ELECTRICAL 



General Results of the Investigation. Reference may now be made to the more 

 striking results that have followed from the investigation of the influence of 

 temperature on the density of aqueous solutions of caustic soda. The most marked 

 characteristic, which was noticed very early in the investigation, is the great 

 simplification observed in the behaviour of the solutions as the concentration and the 

 temperature are increased. This is indicated by the gradual alteration in the 

 coefficients, but is also shown by the density-temperature curves of Diagram III. 

 The maximum density falls to C. at about 0'6 per cent. NaOH, and is given by 

 the formula as 10 C. at 2 per cent. NaOH. The curves become steadily flatter as 

 the concentration increases, and at 42 '8 per cent, the density decreases in a linear 

 manner with the temperature, a behaviour which is in marked contrast to the complex 

 character of the curve that represents the influence of temperature on the density of 

 water. The gradual reduction in curvature, indicated by the decrease with increasing 

 concentration of the coefficient /3, was noticed by BREMER (' Rec. Trav. China.,' 1887, 

 vol. 7, 268-309) in the case of aqueous solutions of calcium chloride, but our 

 observations show further that ft may actually become zero. At still higher concen- 

 trations /8 becomes positive in sign and the curvature is reversed. 



Interesting results would doubtless accrue from a comparison of the influence of 

 increased concentration and of increased temperature in straightening out the density- 

 temperature curves. In general terms it may be stated that the curve connecting 

 density and temperature for water between 100 C. and 200 C. would probably be very 

 similar to that for a solution containing 10 per cent. NaOH between C. and 100 C., 

 and that if a solution containing 10 per cent. NaOH were cooled down from 0C. to 

 100 C., its behaviour would probably be very similar to that of water between 

 1 00 C. and C. The cause of the simplification in behaviour of the concentrated 

 solutions is discussed in connection with the changes of molecular volume described in 

 the following Part IV. of the paper. 



PART IV. THE MOLECULAR VOLUME OF SODIUM HYDROXIDE IN AQUEOUS 



SOLUTION. 



The molecular volumes of dissolved salts are usually much less than those of the 

 solid salts. This contraction cannot be attributed to the mere liquefaction of the salt, 

 as this would probably give rise to an increase of volume, but it may well be 

 attributed to the combination of solvent and solute. In the case of a few compounds, 

 of which the hydroxides of sodium and lithium are by far the most striking examples, 

 the addition of the substance to water actually causes a decrease in the total volume 

 of liquid, and the molecular volume is negative for dilute solutions. In the case of 

 sodium hydroxide the effect of this contraction is remarkable ; it is possible to take a 

 litre of water and dissolve in it no less than 100 grammes of sodium hydroxide without 

 causing any increase in volume, whilst if 50 grammes are added there is actually a 

 contraction of 3 cub. centims. 



