temperature than any other known substanced. And, in the case of the barometer, what fluid is there which could supply the place of quicksilver, with any degree of convenience? since, from the great specific gravity of this metal, a column of the perpendicular height of about thirty inches, sufficiently answers the intended purpose; which column in the case of almost every other fluid, would amount to as many feet. And as, in such a case, the column must necessarily be contained in a glass tube, in order to make the alterations in its height visible, how would it be possible to render such an instrument portable? and yet, if not portable, it would often be of no use when most wanted. In those numerous philosophical experiments in which it is requisite to insulate portions of various gaseous substances, for the purpose of examining their properties, how could the experimentalist proceed without the use of the metal now under consideration; which by its fluidity readily yields its place to the various kinds of gas which are to be transferred to vessels previously filled with the quicksilver; and, having no chemical affinity for the greater num d Quicksilver does not become solid till exposed to a temperature about seventy degrees below the freezing point in the scale of Fahrenheit; nor does it pass rapidly into a state of vapour till exposed to a temperature equal to nearly three hundred and seventy degrees above the boiling point of water, on the same scale. ber of gaseous substances, is calculated to retain them in an insulated and unaltered state for an indefinite length of time? nor let us forget to observe, how the properties of the metal, which is necessarily in contact with the gaseous substances in question, conspire with the properties of the glass vessels containing those gaseous substances, to facilitate the observations of the philosopher: for, if the glass were not both a transparent body, and equally devoid as the quicksilver of any chemical affinity for the gas contained in it, the metal itself would be of little use for the purpose intended; since we are not acquainted with any other substance that could supply the place of glass-with the exception perhaps of rock crystal; which however could only be procured in small quantity any where, and could not be worked into a convenient form but at a most enormous expense. SECT. VII. Common Salt, &c. It does not appear that the mineral kingdom contains a single species capable of being employed as food: but there is one mineral species, which indirectly contributes to the nourishment of many other animals as well as man; and that is common salt; the flavour of which, to a certain extent, is not only grateful to the palate, but, practically speaking, mankind could not exist, or at least never have existed, without the constant use of it. Thus, though employed in very small quantities at a time by any individual, and almost exclusively for the purpose either of preserving or of rendering his food more palatable, this substance may fairly be classed among the principal necessaries of life: and, correspondently with this statement, we find that nature has supplied it in abundance, indeed in profusion often, in various parts of the globe for, to say nothing of those apparently inexhaustible masses which occur among the solid strata of the earth, and which have been constantly quarried through successive ages from the earliest records of history, the ocean itself is a never-failing source of this valuable substance. In other instances salt springs afford the means of a ready supply: and, throughout a considerable part of the sandy districts of Africa and Asia, the soil itself abounds with it. The abundant supply of common salt e It does not belong to our present purpose to describe the common processes by which the salt is obtained either from the sea, or from any other liquid that may hold it in solution: but the following account of a particular process, for this purpose, so well illustrates the ingenuity of the human mind in taking advantage of natural hints, if the expression may be permitted, that no excuse can be necessary for its introduction. In Guiana there is a very common species of palm, the flowers of which are enveloped by a sheath capable of holding many pints of water; and the density and general nature of the sheath is such, that coincides with its extensive utility. It is every where indispensable to the comforts of man; and it is every where found, or easily obtained by him. And, though not to the same extent, the same observation holds with reference to many other natural saline compounds. Thus carbonate of potash, and natron or carbonate of soda, alum, borax, sal ammoniac, and sulphate of iron, or green vitriol, which are most extensively useful salts in many processes of the arts, are either found abundantly in various parts of the world, or may be obtained by very easy means: while a thousand other saline compounds, which are rarely of any practical importance, are scarcely known to exist in a native state. And it is probable that that useful metal, copper, in consequence of its frequent occurrence in a native state, was employed long before the mode of reducing iron from its ores had been discovered; as Werner (and Hesiod, and Lucretius, ages before him f) conjectured. the water contained in it may be heated over a fire without destroying its substance: and the Caraïbs actually employ these sheaths, in evaporating the sea-water for the purpose of obtaining a quick supply of salt. (Diction. des Sciences Nat. tom. xxxvii. p. 283, 4.) f [ Χαλκῷ δ ̓ ἐργάζοντο, μέλας δ ̓ οὐκ ἔσκε σίδηρος. EPT. KAI HM. line 151. Posterius Ferri vis est, Ærisque reperta. LUCRET. V. 1285. CHAP. VIII. Adaptation of Vegetables to the Physical SECT. I. General Observations on the Vegetable Kingdom. THE vegetable kingdom has this distinction with reference to the subject of the present treatise, that its productions are among the first objects that forcibly attract the attention of young children; becoming to them the source of gratifications, which are among the purest of which our nature is capable; and of which even the indistinct recollection imparts often a fleeting pleasure to the most cheerless moments of after-life. Who does not look back with feelings, which he would in vain attempt to describe, to the delightful rambles which his native fields and meadows afforded to his earliest years? Who does not remember, or at least fancy that he remembers, the eager activity with which he was used to strip nature's carpet of its embroidery, nor ceased to cull the scattered blossoms till his infant hands were incapable of retaining the accumulated heap? Who, on even seeing the first violet of returning spring, much more on inhaling its sweetness; or in catching the breeze that has passed over the blossom of the bean or of the woodbine, does not again enjoy the very delights of his early childhood? |