558 Mr. Carey Lea on a New Method of Determining 



of dilution, otherwise the results would not be strictly com- 

 parative. Having obtained normal acid by titration with 



N 

 pure sodium carbonate, this was further diluted to -rr, and 50 



to 100 cub. centim. were found a convenient quantity to employ. 

 The salt to be tested was finely powdered and thoroughly 

 dried at 100°, or at whatever higher temperature it could 

 support. It was then placed in a weighing-bottle and cooled 

 in a desiccator and kept there except for a few moments at a 

 time. By using the dry salt, the dilution of the acid was kept 

 constant. When the point of extinguishment seemed to be 

 reached, at least four final crystallizations were made. Great 

 care is necessary to seize the exact point of extinguishment. 

 The quantity of the salt found is then reduced to correspond 

 with one gram-molecule of sulphuric acid. It is next divided 

 by its own molecular weight : this gives the number of mole- 

 cules of the salt needed to extinguish the reaction in one 

 molecule of sulphuric acid. In order to make it possible to 

 compare acids of different basicities, the figures thus obtained 

 must next be modified to correspond with the basicity of the 

 acid used. If the acid is bibasic, no change will be needed. 

 If monobasic, the figures obtained must be divided by 2. If 

 tribasic, they must be multiplied by § &c. Finally, as the 

 quantity characteristic of the acid is the excess of the quantity 

 found over the amount equivalent to one molecule of sulphuric 

 acid, unity is deducted from the amount obtained, and the 

 residue thus found represents the comparative affinity of the 

 acid, and may be called its index. 



This may be rendered more clear by one or two instances. 



In the case of hydrochloric acid, there was needed as a 

 mean of many determinations 29*37 gram-molecules of sodium 

 chloride to extinguish the reaction in one gram-molecule of 

 sulphuric acid. At this point the solution necessarily contained 



Na 2 S0 4 + 2 HC1 + 27*37 NaCl. 



This is proved beyond question by the fact that the solution 

 no longer gives a trace of reaction of free sulphuric acid. 

 The quantity of 27*37 gram-molecules of sodium chloride is 

 the proportion of undecomposed sodium chloride that must 

 remain in the solution in order that the sulphuric acid may 

 be completely converted into sodium sulphate, and may remain 

 as such in the solution in a condition of equilibrium. 



This number 27*37 therefore represents the strength of 

 the affinity of hydrochloric acid for sodium. But in order to 

 compare acids of different basicities it is convenient to refer 



