Besides those which Sir H. Davy has obtained, there are several that have been recently discovered, whose properties are yet but little known, as for instance, titanium, which was discovered by the Rev. Mr. Gregor, in the tin-mines of Cornwall; columbium or tantalium, which has lately been discovered by Mr. Hatchett; and osmium, iridium, palladium, and rhodium, all of which Dr. Wollaston and Mr. Tennant found mixed in minute quantities with crude platina, and the distinct existence of which they proved by curious and delicate experiments. CAROLINE. Arsenic has been mentioned amongst the metals. I had no notion that it belonged to that class of bodies, for I had never seen it but as a powder, and never thought of it but as a most deadly poison. MRS. B. In its pure metallic state, I believe, it is not so poisonous; but it has such a great affinity for oxygen, that it absorbs it from the atmosphere at its natural temperature: you have seen it, therefore, only in its state of oxyd, when, from its combination with oxygen, it has acquired its very poisonous properties. CAROLINE. Is it possible that oxygen can impart poisonous qualities? That valuable substance which produces light and fire, and which all bodies in nature are so eager to obtain? MRS. B. Most of the metallic oxyds are poisonous, and derive this property from their union with oxygen. The white lead, so much used in paint, owes its pernicious effects to oxygen. In general, oxygen, in a concrete state, appears to be particularly destructive in its effects on flesh or any animal matter; and those oxyds are most caustic that have an acrid burning taste, which proceeds from the metal having but a slight affinity for oxygen, and therefore easily yielding it to the flesh, which it corrodes and destroys. EMILY. What is the meaning of the word _caustic_, which you have just used? MRS. B. It expresses that property which some bodies possess, of disorganizing and destroying animal matter, by operating a kind of combustion, or at least a chemical decomposition. You must often have heard of caustic used to burn warts, or other animal excrescences; most of these bodies owe their destructive power to the oxygen with which they are combined. The common caustic, called _lunar caustic_, is a compound formed by the union of nitric acid and silver; and it is supposed to owe its caustic qualities to the oxygen contained in the nitric acid. CAROLINE. But, pray, are not acids still more caustic than oxyds, as they contain a greater proportion of oxygen? MRS. B. Some of the acids are; but the caustic property of a body depends not only upon the quantity of oxygen which it contains, but also upon its slight affinity for that principle, and the consequent facility with which it yields it. EMILY. Is not this destructive property of oxygen accounted for? MRS. B.