408 On the Definite Proportions in which the [Dec. 



3. Examine if the substance can combine with the oxide of lead, 

 or with some other binary oxide, in a greater proportion. In that 

 case the number of times that the binary oxide is multiplied in the 

 new combination points out whether the number of volumes of 

 oxygen found be true or not; for example, if the number of 

 volumes of oxygen in acetic acid be three, the subacetate ought to 

 contain, combined with the same quantity of acid, either three or six 

 times as much base as the neutral acetate, and not two or four times 

 as much, as is obvious from the laws of definite proportions. Unfor- 

 tunately this check can be applied only in a few cases. \A hen it is 

 wanting we must rest satisfied with the two preceding ones. Sup- 

 pose, for example, that we have a vegetable salt to examine with a 

 base of lead, and that this salt contains combined water, of which 

 it cannot be deprived, and of the existence of which in consequence 

 we cannot be absolutely sure ; when we analyse it we find the 

 elements of the water mixed with those of the acid ; but we do not 

 perceive this, because the hydrogen of the water constitutes a certain 

 number of entire volumes, and because the oxygen is always a 

 multiple of the oxygen of the base. In that case the third check 

 proves the existence of water ; but unless we can have recourse to 

 it we are deceived by the analytical result, unless we can find an 

 anhydrous combination of the acid examined, and expose it to an 

 analytical experiment. 



If by these reflections the reader perceives on one side how 

 necessary it is to be circumspect in these experiments, and to seize 

 everv circumstance that can throw light upon them, in order to 

 avoid being deceived by false resemblances, I hope he will sec 

 equally on the other side that by careful experiments and due cir- 

 cumspection we shall at last arrive at results respecting organic 

 bodies almost mathematically exact, at least as far as the relations 

 between their elements are concerned. 



I shall now say a few words on the method of obtaining and 

 analysing combinations of the ternary oxides with oxide of lead. 

 We prepare a combination of the substance to be examined and 

 pure ammonia, and there ought to be no excess of alkali. A 

 neutral solution of nitrate of lead is poured into this solution drop 

 by drop, taking care not to add an excess of it, for in that case the 

 precipitate often contains traces of nitrate, which cannot be removed 

 by the most careful washing. We obtain the subsalts by digesting 

 this precipitate, well washed, and dried in caustic and concentrated 

 ammonia. The washed precipitates are dried in the temperature of 

 boiling water; or if exposure to the air be capable of altering their 

 composition, we dry them in a vacuum, introducing along with 

 them a quantity of sulphuric acid, having as large a surface as 

 possible. To deprive these vegetable substances, or their combina- 

 tions, of all water chemically combined or mechanically adhering, 

 I employed the following method : — I introduced the dried sub- 

 stance inio a flagon, the mouth of which 1 covered with paper. 

 Ahis flagon I plunged two-thirds into a sand-bath, which I had 



