By the intervention of sulphuric acid, which combines with the potash, and sets the nitric acid at liberty. This I can easily show you, by mixing some nitrat of potash and sulphuric acid in this retort, and heating it over a lamp; the nitric acid will come over in the form of vapour, which we shall collect in a glass bell. This acid, diluted in water, is commonly called _aqua fortis_, if Caroline will allow me to mention that name. CAROLINE. I have often heard that aqua fortis will dissolve almost all metals; it is no doubt because it yields its oxygen so easily. MRS. B. Yes; and from this powerful solvent property, it derived the name of aqua fortis, or strong water. Do you not recollect that we oxydated, and afterwards dissolved, some copper in this acid? EMILY. If I remember right, the nitrat of copper was the first instance you gave us of a compound salt. CAROLINE. Can the nitric acid be completely decomposed and converted into nitrogen and oxygen? EMILY. That cannot be the case, Caroline; since the acid can be decomposed only by the combination of its constituents with other bodies. MRS. B. True; but caloric is sufficient for this purpose. By making the acid pass through a red hot porcelain tube, it is decomposed; the nitrogen and oxygen regain the caloric which they had lost in combining, and are thus both restored to their gaseous state. The nitric acid may also be partly decomposed, and is by this means converted into NITROUS ACID. CAROLINE. This conversion must be easily effected, as the oxygen is so slightly combined with the nitrogen. MRS. B. The partial decomposition of nitric acid is readily effected by most metals; but it is sufficient to expose the nitric acid to a very strong light to make it give out oxygen gas, and thus be converted into nitrous acid. Of this acid there are various degrees, according to the proportions of oxygen which it contains; the strongest, and that into which the nitric is first converted, is of a yellow colour, as you see in this bottle. CAROLINE. How it fumes when the stopper is taken out! MRS. B. The acid exists naturally in a gaseous state, and is here so strongly concentrated in water, that it is constantly escaping. Here is another bottle of nitrous acid, which, you see, is of an orange red; this acid is weaker, the nitrogen being combined with a smaller quantity of oxygen; and with a still less proportion of oxygen it is an olive-green colour, as it appears in this third bottle. In short, the weaker the acid, the deeper is its colour. Nitrous acid acts still more powerfully on some inflammable substances than the nitric. EMILY. I am surprised at that, as it contains less oxygen. MRS. B. But, on the other hand, it parts with its oxygen much more readily: you may recollect that we once inflamed oil with this acid.