172 Mr. Colquhoun on the Life and Writings [March, 



to be composed of the black and red oxides only was simple and 

 decisive. The red oxide of iron has a less affinity for acids than 

 the black. Take any oxide of iron supposed to be intermediate 

 between these two, take any pretended third oxide, and to a 

 warm solution of it in muriatic acid add potash in small quanti- 

 ties at a time. At first, the precipitate obtained is of a red 

 colour, and consists of the pure red oxide : by and bye a change 

 almost instantaneous lakes place in the colour of the precipi- 

 tate, which now becomes green, and is neither more nor less 

 than a hydrated black oxide. This latter oxide continues to be 

 the precipitate obtained by this process, so long as an atom of 

 iron remains in solution. Every pretended new oxide subjected 

 to this treatment is thus resolved into two, the black and the 

 red, nor is it possible to detect during the slowest process of 

 • precipitation a single vestige of any intermediate separated 

 oxide. Of course the fair conclusion is, that no such third 

 oxide existed in the solution, and thus the proof that there is 

 no such third oxide is nearly as conclusive as any proof of a 

 negative position ever can be. 



Perhaps the true secret of the question being hung up in sus- 

 pense so long as BerthoUet argued on the other side, is to be 

 found in the fact that Proust had to establish a negative position. 

 If BerthoUet could discover any one substance supporting his 

 doctrines, bis case was made out; while Proust, to give even 

 feasibility to the views he adopted, was obliged to solve every 

 appearance quoted by his antagonist. Under such circumstances 

 it is plain that a man of the extensive knowledge and penetrating 

 ingenuity of BerthoUet could not fail to force an opponent to a 

 very wide range of investigation ere he could hope to establish 

 his theory. Accordingly, besides the oxidized masses of iron, it 

 was necessary for Proust to examine the imperfect oxides of lead, 

 copper, arsenic, tin, &.c. and to resolve a/t of them into those 

 well-known perfect oxides which he alleged to be their invaria- 

 ble state of combination. In this difficult task he was eminently 

 successful, and as an interesting example of his mode of pro- 

 ceeding, we shall quote his experiments on the calcination of 

 tin, and the results which so fairly flowed from them. Take a 

 mass of tin oxidized by calcination, and wash it in water : it 

 immediately discovers itself to be a mixture of unchanged metal 

 and oxidized tin ; for the former, being much heavier than the 

 latter, is disengaged from it in minute particles during this 

 operation, and in fact, in the original mass, this pure metal was 

 merely enveloped in an external coating of the oxide of tin. 

 Take the oxide of tin thus obtained in a state comparatively 

 genuine, and introduce it into cold muriatic acid : a large por- 

 tion of it passes into solution, and an additional residue of 

 metaUic particles is still obtained, which, on account of their 

 tenuity, had been carried along with the oxide during the wash- 



