AT THE MOMENT OF CHEMICAL CHANGE. 
779 
stances) which I have mentioned, the entire agreement of the experiments proves 
that the action is that which I have here given, yet this threw a certain doubt upon 
it, and it is only in some of my first experiments that I have used this plan of deter- 
mining the oxygen uncontrolled by the following. 
The determinations with platinum were made in a little apparatus of a very simple 
construction, which, as I shall elsewhere have occasion to mention it, I will call a 
bulb apparatus. 
A is a small flask in which the weighed substance, to- 
gether with a small portion of finely-divided platinum 
(prepared from platinum black by ignition) or of animal 
charcoal, which is equally effective, is placed. B is a 
glass bulb holding about two fluid ounces, which, before 
the experiment, is filled with dilute hydrochloric or acetic 
acid, and is closed by a well-ground stopper. C is a dry- 
ing tube of potash. After weighing the apparatus, the 
stopper is loosened and the acid allowed to flow into the 
interior. The evolution of gas takes place immediately, 
but goes much slower than in the experiments with chro- 
mic acid. After eighteen or twenty hours the apparatus 
is again weighed, when the action may be considered 
terminated. It is, however, of course, to be weighed again 
until it ceases to lose weight. During the experiment a 
second potash tube, or a small tube dipping into a vessel of lime, is to be attached to 
the first, to prevent any increase of weight by absorption of moisture from the atmo- 
sphere. The following experiments were made in this manner : — 
I. 2*389 grms. of the preparation A. gave a loss of 0*162 grm. 
II. 2*437 grms. of the same gave a loss of 0*162 grm. 
These experiments correspond to a loss per cent, of 
I. II. 
6*78 6*64 
III. 2*615 grms. of preparation B. gave a loss of 0*175 grm. 
IV. 2*785 grms. of the same gave a loss of 0*184 grm. 
These experiments correspond to a loss per cent, of 
III. IV. 
6*63 6*60 
These determinations entirely agree with each other ; and the per-centage of oxygen 
in the substance is probably in fact (as is given by this experiment) a little less than 
that calculated from the amount of oxygen taken up, as in pulverizing it it is not pos- 
sible to prevent the absorption of a little water by the baryta present, which must 
diminish the per-centage of oxygen. There is no violent rush of gas during the 
5 G 2 
