All these curious effects are owing to the very great facility with which this acid yields oxygen to such bodies as are strongly disposed to combine with it. It appears extraordinary indeed to see bodies, and metals in particular, melted down and inflamed, by a gas without any increase of temperature, either of the gas, or of the combustible. The phenomenon, however, is, you see, well accounted for. EMILY. Why did you burn a piece of Dutch gold leaf rather than a piece of any other metal? MRS. B. Because, in the first place, it is a composition of metals (consisting chiefly of copper) which burns readily; and I use a thin metallic leaf in preference to a lump of metal, because it offers to the action of the gas but a small quantity of matter under a large surface. Filings, or shavings, would answer the purpose nearly as well; but a lump of metal, though the surface would oxydate with great rapidity, would not take fire. Pure gold is not inflamed by oxy-muriatic acid gas, but it is rapidly oxydated, and dissolved by it; indeed, this acid is the only one that will dissolve gold. EMILY. This, I suppose, is what is commonly called _aqua regia_, which you know is the only thing that will act upon gold. MRS. B. That is not exactly the case either; for aqua regia is composed of a mixture of muriatic acid and nitric acid. --But, in fact, the result of this mixture is the formation of oxy-muriatic acid, as the muriatic acid oxygenates itself at the expence of the nitric; this mixture, therefore, though it bears the name of _nitro-muriatic acid_, acts on gold merely in virtue of the oxy-muriatic acid which it contains. Sulphur, volatile oils, and many other substances, will burn in the same manner in oxy-muriatic acid gas; but I have not prepared a sufficient quantity of it, to show you the combustion of all these bodies. CAROLINE. There are several jars of the gas yet remaining. MRS. B. We must reserve these for future experiments. The oxy-muriatic acid does not, like other acids, redden the blue vegetable colours; but it totally destroys any colour, and turns all vegetables perfectly white. Let us collect some vegetable substances to put into this glass, which is full of gas. EMILY. Here is a sprig of myrtle-- CAROLINE. And here some coloured paper-- MRS. B. We shall also put in this piece of scarlet riband, and a rose-- EMILY. Their colours begin to fade immediately! But how does the gas produce this effect? MRS. B. The oxygen combines with the colouring matter of these substances, and destroys it; that is to say, destroys the property which these colours had of reflecting only one kind of rays, and renders them capable of reflecting them all, which, you know, will make them appear white. Old prints may be cleaned by this acid, for the paper will be whitened without injury to the impression, as printer’s ink is made of materials (oil and lamp black) which are not acted upon by acids.