I should not omit to mention, that a very splendid, and, at the same time, most powerful battery, was, a few years ago, constructed under the direction of Sir H. Davy, which he repeatedly exhibited in his course of electro-chemical lectures. It consists of two thousand double plates of zinc and copper, of six square inches in dimensions, arranged in troughs of Wedgwood-ware, each of which contains twenty of these plates. The troughs are furnished with a contrivance for lifting the plates out of them in a very convenient and expeditious manner.* [Footnote *: A model of this mode of construction is exhibited in PLATE XIII. Fig. 1.] CAROLINE. Well, now that we understand the nature of the action of the Voltaic battery, I long to hear an account of the discoveries to which it has given rise. MRS. B. You must restrain your impatience, my dear, for I cannot with any propriety introduce the subject of these discoveries till we come to them in the regular course of our studies. But, as almost every substance in nature has already been exposed to the influence of the Voltaic battery, we shall very soon have occasion to notice its effects. CONVERSATION VI. ON OXYGEN AND NITROGEN. MRS. B. To-day we shall examine the chemical properties of the ATMOSPHERE. CAROLINE. I thought that we were first to learn the nature of OXYGEN, which come next in our table of simple bodies? MRS. B. And so you shall; the atmosphere being composed of two principles, OXYGEN and NITROGEN, we shall proceed to analyse it, and consider its component parts separately. EMILY. I always thought that the atmosphere had been a very complicated fluid, composed of all the variety of exhalations from the earth. MRS. B. Such substances may be considered rather as heterogeneous and accidental, than as forming any of its component parts; and the proportion they bear to the whole mass is quite inconsiderable. ATMOSPHERICAL AIR is composed of two gasses, known by the names of OXYGEN GAS and NITROGEN or AZOTIC GAS. EMILY. Pray what is a gas? MRS. B. The name of gas is given to any fluid capable of existing constantly in an aeriform state, under the pressure and at the temperature of the atmosphere. CAROLINE. Is not water, or any other substance, when evaporated by heat, called gas? MRS. B. No, my dear; vapour is, indeed, an elastic fluid, and bears a strong resemblance to a gas; there are, however, several points in which they essentially differ, and by which you may always distinguish them. Steam, or vapour, owes its elasticity merely to a high temperature, which is equal to that of boiling water. And it differs from boiling water only by being united with more caloric, which, as we before explained, is in a latent state. When steam is cooled, it instantly returns to the form of water; but air, or gas, has never yet been rendered liquid or solid by any degree of cold. EMILY. But does not gas, as well as vapour, owe its elasticity to caloric? MRS. B.