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



decimals altogether reliable. All the tables referred to above are quoted in full in 

 COMET'S ' Dictionary of Solubilities.' 



The measurements of PICKERING (' Phil. Mag.,' vol. 37, 359, 1894) must be placed 

 in a different category from those described above. As in previous cases, the stock 

 solution was standardised by titration, but it had been prepared from metallic sodium 

 and in all probability was substantially pure. Unusual care appears also to have 

 been taken with the density measurements, which were made at no less than 144 

 different concentrations between per cent, and 50 per cent. NaOH. No details are 

 given with regard to the titrations, nor is it stated what acid or alkali was used as 

 standard ; the author considers, however, that the value for the strength of the stock 

 solution was correct within 0*05 per cent, of the total alkali present. Had this been 

 the case, the density of the 50-per cent, solution should have been correct within 

 0'0003 ; actually, however, the value is 0'0013 higher than that which we have 

 found for solutions prepared quantitatively from metallic sodium. The difference 

 in density corresponds to an error of 0'25 per cent, in the determination of the total 

 alkali in the solution, and our own experience has shown us that errors of this 

 magnitude may easily be introduced in standardising an alkali by titration against a 

 standard acid. Although, therefore, PICKERING'S relative values for the density at 

 different concentrations must be regarded as substantially accurate, the absolute values 

 (which are given to six places of decimals, and are quoted in this form in WATTS'.S 

 'Dictionary of Chemistry') appear to be unreliable to the extent of O'OOl at 50 per 

 cent. NaOH, and roughly to a proportionate extent at the lower concentrations. It 

 is to be regretted that PICKERING did not apply in this case the method, which 

 proved so successful in his determinations of the density of sulphuric acid (' Trans. 

 Chem. Soc.,' 1890, vol. 57, p. 64), of freezing out a hydrate of definite composition 

 and using this to prepare the solutions ; an application of this method might 

 have given results as accurate as any that can be obtained by weighing out metallic 

 sodium, and a comparison of the two sets of data would have been of great 

 value. 



In weighing out either a hydrate of sodium hydroxide or metallic sodium, it is 

 necessary to introduce an analytical factor based upon atomic weight determinations. 

 As the atomic weights are only known to two places of decimals, it would be useless 

 to pretend to any higher approach to accuracy in the concentration of the solutions 

 used in the density determinations. In the case of sodium hydroxide, this factor 

 may be taken as involving a possible error of about 1 in 5000 in the concentration, 

 corresponding to an error of O'OOOl in the density of a 50-per cent, solution. At 

 the higher concentrations therefore, the densities can only be relied upon to four 

 places of decimals, and in view of the unstable character of the material it is 

 doubtful whether the purity of the solutions would justify a closer approximation, 

 even if the accuracy of the atomic weights should appear to permit it. In a 5 -per 

 cent, solution an error of 1 part in 5000 in the concentration would produce the 



