402 On the Definite Proportions in which the [Dec. 



having borrowed from them the use of the hyper-oxymuriate of 

 potash, which I substituted for the brown oxide of lead that I was 

 in the habit of employing before I was acquainted with the experi- 

 ments of the French chemists. 



1. The apparatus of Thehara and Gay-Lussac has a stop-cock, 

 through the openiog of which the balls must pass, in order to be 

 received into a metallic tube, the lower extremity of which is to be 

 heated red-hot. The stop-cock requires to be well greased in order 

 to answer the purpose. Now as the balls are obliged to make half 

 a turn in this stop-cock before they fall down, it is scarcely possible 

 to prevent them from taking up some grease, which will be decom- 

 posed along with them, and render the result to a certain degree 

 inaccurate. 



2. The necessity of moistening the substance under examination 

 in order to form it into balls with the hyper-oxymuriate of potash, 

 prevents the possibility of reducing it to the same degree of absolute 

 dryness as before the operation, and may sometimes occasion evert 

 an alteration in the mass during the drying. It is only by some 

 such circumstance that I am able to explain the differences which 

 sometimes exist between the result of the analysis of the French 

 chemists and mine. 



3. But the. most important objection is, that in their method the 

 quantity of hydrogen is determined by the loss which in some cases 

 may be owing to other unforeseen circumstances, and which in all 

 cases ought to be a little more than the quantity of water produced. 

 Now we shall see hereafter that it is a very essential point to be able 

 to determine with the most rigorous exactness the quantity of 

 hydrogen in these substances, because, as its volume is very light, 

 a small error in the experiment may occasion several volumes too 

 much of hydrogen ; while it would require a very considerable 

 error to occasion a mistake in the number of volumes of oxygen or 

 carbon. 



4. Another observation respecting the experiment of the French 

 chemists, which however does not affect their method, is that Gay- 

 Lussac and Thenard paid no attention to the water of combination 

 in several organic bodies. They satisfied themselves with drying 

 them at the temperature of boiling water, and did not examine 

 whether the substances which they considered as dry contained 

 water or not. This circumstance is by no means indifferent, as we 

 shall see hereafter. They made some of their analyses on vegetable 

 acids combined with lime and barytes, without attending to the 

 water contained in these salts. Thus considering the mixture of 

 acid and water as pure acid, their result differs of course very con- 

 siderably from the truth. When we correct it, by subtracting this 

 quantity of water, it agrees in general with mine. 



Thenard and Gay-Lussac paid no attention to chemical propor- 

 tions. Tliis is not suprising ; because, when they wrote, these pro- 

 portions were unknown, though we possessed very good indications 

 of them. Chemistry is indebted to the genius of Gay-Lussac for 



