36 Dr. Prout on the ultimate Composition 



that could be thought of, as likely to succeed, were tried, but 

 without effect, and I was obliged to relinquish the matter in 

 despair, and endeavour to contrive some other mode of ana- 

 lysis that should be free from these difficulties altogether. 

 After a good deal of consideration I was induced to adopt a 

 method which had occurred to me long before, but which I 

 had never put in execution. This method is very simple, and 

 founded on the following well known principles. 



When an organic product containing three elements, hy- 

 drogen, carbon, and oxygen, is burnt in oxygen gas, one "of 

 three things must happen. 1. The original bulk of the oxygen 

 gas may remain the same, in which case the hydrogen and oxy- 

 gen in the substance must exist in it in the same proportions in 

 which they exist in water ; (for it is well known that oxygen 

 gas by being converted into carbonic acid gas is not altered in 

 its bulk) : or, 2. The original bulk of the oxygen may be in- 

 creased, in which case the oxygen must exist in the substance 

 in a greater proportion than it exists in water; or 3. The 

 original bulk of the oxygen gas may be diminished, in which 

 case the hydrogen must predominate. 



Hence it is obvious, that in the first of these cases the com- 

 position of a substance may be determined by simply ascfer- 



200 grs. of the oxide of copper, recently ignited, gained, after ten or fifteen 

 minutes, exposure to the air, a quantity varying from '02 to *05 gr. one 

 half of which, or even more, was acquired before it became cold; that 

 is to say, before it had cooled down to 100°, considerably above which point 

 it began to acquire weight. Of the increase of weight above mentioned, it 

 was found that about j, or £, (for the proportion varied from causes that I 

 could not discover,) was due to the condensation of air, the rest was due to 

 moisture. The oxide I employed was perfectly pure, and prepared by ex- 

 posing metallic copper to heat. Dr. Ure states, " that 1 00 grs. of the oxide 

 prepared from the nitrate of copper exposed to a red heat merely till the 

 vapours of nitric acid were expelled, absorbed in the ordinary state of the 

 atmosphere from '1 to '2 gr. in the space of an hour or two, and about half 

 that quantity in a very few minutes." 



In determining the quantity of water formed by the oxide of copper in the 

 usual manner, there is yet another difficulty to contend with, to which we 

 have alluded above, and which we shall here briefly notice. It has been 

 stated that complete combustion seldom or never takes place during one 

 exposure to heat, and that in many cases the oxide ought to be removed 

 from the tube and retriturated. Now it was found, almost invariably, that 

 during the second exposure to heat, the tube, instead of losing additional 

 weight, actually became heavier, sometimes to the amount of *03 gr. though 

 often much less than this. I was a good deal puzzled to account for this 

 anomaly at first, but believe that it arose chiefly from the reoxydation of 

 the partially reduced oxide, by the air of the atmosphere. 



Since this paper was read before the Royal Society, I have observed one 

 or two other singular facts connected with this subject, which will be no- 

 ticed when we come to speak of the analysis of substances containing 

 azote. 



taining 



