140 Dr. Thomson's Experiments to determine the [Aug. 



liquid was drawn off and tested. When mixed with a solution 

 of sulphate of soda, no precipitate appeared, so as to indicate the 

 presence of lead. Into another portion of it some nitrate of lead 

 was dropped, and the whole was evaporated to dryness. Dis- 

 tilled water being digested on the dry residue, a complete solu- 

 tion was effected. This indicates the absence of citric acid. 

 Thus we see that the citric acid in 9-5 grs. of the crystals is just 

 capable of saturating all the protoxide of lead which exists in 

 2075 grains of nitrate of lead. This we know amounts to 

 exactly 14 grs. 



The citrate of lead formed in the preceding experiment was 

 dried in a temperature not exceeding 100°. Its weight was 

 exactly 2T25 grs. Now it contained 14 grs. of protoxide of 

 lead ; consequently the other 7*25 grs. must have been citric 

 acid. Thus we see that an atom of citric acid weighs exactly 

 7-25, and an atom of the crystals 9*5. Citrate of lead, when 

 dried at 100°, contains no water of crystallization ; conse- 

 quently, when we wish to analyze a citrate, we must dissolve a 

 given weight of it in water, and throw down the acid by nitrate 

 of lead and evaporation. When the precipitate is washed and 



7 "25 



dried at the temperature of 100°, its weight multiplied by ■ 

 or by 341, gives us the quantity of citric acid in the salt. 



10. Benzoic Acid. 



From the way in which the benzoic acid is produced, there is 

 reason to infer that it contains no water of crystallization. From 

 the analysis of it by Berzelius, it seems to weigh exactly 15. 

 The only insoluble salt which this acid forms, as far as I know, 

 is the perbenzoate of iron. I, therefore, endeavoured to verify 

 the supposed weight of this acid in the following way : 



15 grs. of benzoic acid were dissolved in distilled water by 

 means of ammonia, and the solution was concentrated till the 

 excess of ammonia was driven off. From the experiments on 

 the protosulphate of iron related in a former paper, it follows 

 that 17-375 grs. of the crystals of protosulphate of iron are com- 

 posed of 



Sulphuric acid 5-0 or 1 atom 



Protoxide of iron 4*5 1 



Water 7-875 7 



17-375 



17*375 grs. of these crystals were dissolved in dilute nitric 

 acid, and the solution was heated till the iron was peroxidized. 

 The liquid was then concentrated as much as possible so as to 

 avoid precipitating the iron, in order to get rid of part of the 

 excess of nitric acid. Ammonia was very cautiously added till 



