Notes on Secondary Batteries. 163 



to be removed by a charging current much more readily than 

 when sulphuric acid alone is employed. 



Mr. Preece did not explain the beneficial action of the 

 sodium sulphate, but in the discussion following the lecture 

 Sir F. Abel threw out the idea that it was " probable that a 

 double salt of sulphate of lead and sodium was formed which 

 was soluble, whereas the sulphate of lead was comparatively 

 insoluble." 



We have made some experiments to test the value of this 

 suggestion. It may be well, however, to note that lead sul- 

 phate is so nearly insoluble in dilute sulphuric acid that com- 

 paratively large errors are easily possible. 



We took advantage of the greater solubility of lead sulphate 

 in strong acid. Measured volumes of this were added to water, 

 the precipitate formed allowed to settle, and the lead salt 

 remaining in solution determined by precipitation as sulphide. 

 The mean results are : — 1 part of lead sulphate dissolves 

 in 105,000 parts of the acid mixed with water, in 90,000 

 parts of a similarly diluted acid containing 5 per cent, sodium 

 sulphate, and in 107,000 parts of Mr. Preece's solution. 



As these numbers deal with such minute quantities we lay no 

 stress on their absolute value. Their relative values are more 

 useful, and evidently the addition of the sodium salt has not 

 materially increased the solubility of lead sulphate. 



This conclusion was confirmed by experiments of another 

 type. To equal quantities of a dilute acid and a similar acid 

 containing sodium sulphate we added a few drops of strong 

 acid saturated with lead sulphate. Precipitates were formed 

 in both cases, but it was possible so to adjust the number of 

 drops that the precipitate did not appear for an appreciable 

 time. We could therefore see in which it appeared first. In 

 many experiments and many variations of procedure, we nearly 

 always obtained earlier precipitation in liquids containing 

 sodium or magnesium sulphates than in those consisting 

 simply of dilute acid. We judge therefore that lead sulphate 

 is not more soluble in these liquids, and that the explanation 

 of the action of sodium sulphate must be sought in some other 

 direction. 



Mr. Swinburne has suggested that, as lead sulphate is dis- 

 solved by sodium hydrate, the action may be due to that 

 hydrate being produced by electrolysis of the sodium sulphate. 

 Experiment showed that in a simple solution of neutral sul- 

 phate of soda, a paste made of equal parts of red lead and 

 sulphate of lead was more readily reduced than in dilute sul- 

 phuric acid. It was also easy to detect sodium hydrate on the 

 cathode by red litmus-paper. But this last resuit was no 



M2 



