In illustration of these remarks, take the following instances:—Acetic Acid contains Carbon, Hydrogen, and Oxygen; by the action of Chlorine the Hydrogen may be removed in the form of Hydrochloric Acid, and an equal number of atoms of Chlorine be substituted. In this way a new compound is formed, termed Chloracetic Acid, resembling in many important particulars the Acetic Acid itself. Notice particularly that the peculiar properties characteristic of Chlorine are completely masked in the substitution body, and no indication of its presence is obtained by the usual tests! A soluble Chloride gives with Nitrate of Silver a white precipitate of Chloride of Silver, unaffected by Acids, but the Chloracetic Acid does not; hence it is plain that the Chlorine exists in a peculiar and Ultimate state of combination different from what is usual.
The substance we have been previously considering, viz. Pyroxyline, affords another illustration of the Law of Substitution. Omitting, for the sake of simplicity, the number of atoms concerned in the change, the action of concentrated Nitric Acid upon ligneous fibre may be thus explained:—
| Cotton or |
| Carbon Hydrogen Hydrogen Oxygen | + Nitric Acid |
equals
| Pyroxyline or |
| Carbon Hydrogen Peroxide Nitrogen Oxygen | + Water |
Or in symbols:—
CH11O + NO5 = C (Hn-1NO4) O + HO
By a reference to the formula, it is seen that the fifth atom of Oxygen contained in the Nitric Acid takes one of Hydrogen, and forms an atom of Water; the NO4 then steps in, to fill the gap which the atom of Hydrogen has left. All this is done with so little disturbance that even the fibrous structure of the cotton remains as before.
b. Chemical Composition of the varieties of Pyroxyline.—Mr. Hadow has succeeded in establishing four different substitution compounds, which, as no distinctive nomenclature has been at present proposed, may be termed compounds A, B, C, and D.
Compound A is the most explosive Gun-Cotton, and contains the largest amount of Peroxide of Nitrogen. It dissolves only in Acetic Ether, and is left on evaporation as a white powder. It is produced by the strongest Nitro-Sulphuric Acid which can be made.