It proceeds probably from the strong attraction of oxygen for hydrogen; for if the one rapidly absorb the other from the animal fibre, a disorganisation of the substance must ensue. EMILY. Caustics are, then, very properly said to burn the flesh, since the combination of oxygen and hydrogen is an actual combustion. CAROLINE. Now, I think, this effect would be more properly termed an oxydation, as there is no disengagement of light and heat. MRS. B. But there really is a sensation of heat produced by the action of caustics. EMILY. If oxygen is so caustic, why does not that which is contained in the atmosphere burn us? MRS. B. Because it is in a gaseous state, and has a greater attraction for its electricity than for the hydrogen of our bodies. Besides, should the air be slightly caustic, we are in a great measure sheltered from its effects by the skin; you know how much a wound, however trifling, smarts on being exposed to it. CAROLINE. It is a curious idea, however, that we should live in a slow fire. But, if the air was caustic, would it not have an acrid taste? MRS. B. It possibly may have such a taste; though in so slight a degree, that custom has rendered it insensible. CAROLINE. And why is not water caustic? When I dip my hand into water, though cold, it ought to burn me from the caustic nature of its oxygen. MRS. B. Your hand does not decompose the water; the oxygen in that state is much better supplied with hydrogen than it would be by animal matter, and if its causticity depend on its affinity for that principle, it will be very far from quitting its state of water to act upon your hand. You must not forget that oxyds are caustic in proportion as the oxygen adheres slightly to them. EMILY. Since the oxyd of arsenic is poisonous, its acid, I suppose, is fully as much so? MRS. B. Yes; it is one of the strongest poisons in nature. EMILY. There is a poison called _verdigris_, which forms on brass and copper when not kept very clean; and this, I have heard, is an objection to these metals being made into kitchen utensils. Is this poison likewise occasioned by oxygen? MRS. B. It is produced by the intervention of oxygen; for verdigris is a compound salt formed by the union of vinegar and copper; it is of a beautiful green colour, and much used in painting. EMILY. But, I believe, verdigris is often formed on copper when no vinegar has been in contact with it. MRS. B. Not real verdigris, but compound salts, somewhat resembling it, may be produced by the action of any acid on copper. The solution of copper in nitric acid, if evaporated, affords a salt which produces an effect on tin that will surprise you, and I have prepared some from the solution we made before, that I might show it to you. I shall first sprinkle some water on this piece of tin-foil, and then some of the salt. --Now observe that I fold it up suddenly, and press it into one lump. CAROLINE.