other instances, than in the application of tin to the surface of copper or of iron: but, were there a hundred similar instances, they would not lessen the value of this, as affording an illustration of that principle which has been borne in mind throughout this treatise. Consider only the respective degree of abundance of each of the three metals just mentioned, and the difference in some of their qualities with respect to external agents, and we shall have ample reason for being assured that, on this and on every other occasion, we may say of the Creator of material things-" In wisdom hast Thou made them all." And not only is it true that "The world by difference is in order found;" of but the difference is so adjusted in every instance, that, if it were varied, the value of the substances in which the difference is observable would be destroyed. Thus, of the three metals now under consideration, iron and copper, from the degree of their malleability, are easily formed into those various vessels which are of daily use for culinary and other purposes; while tin possesses the property malleability in comparatively a slight degree: and, correspondently with the extent of their use, iron and copper are found in great abundance and in almost every part of the world; while tin is of very rare occurrence. Again, the two former metals are easily rusted; and, from the poisonous quality of the rust of copper, fatal effects on human health and life would be frequently occurring, used so extensively as that metal is for the construction of vessels in which our food is prepared, were it not defended by that superficial coating of tin, which is commonly applied to the inner surface of such vessels; tin being neither easily rusted, nor capable of communicating any poisonous quality to substances brought into contact with it. Let us then suppose that the respective degree of maileability, or of fusibility, were reversed in these metals; and observe the inconvenience that would ensue. Let the tin have that degree of malleability, for instance, which would render it capable of supplying the place of the iron, or the copper, in the construction of various economical vessels and instruments; yet, from the small quantity in which it occurs in the world, the supply of it would soon be either exhausted, or its price would be so enhanced that it could not be purchased except by the rich. And, even if the supply were inexhaustible, yet, from the softness of the metal, the vessels made of it would be comparatively of little use; and from the low temperature at which it melts, it could not be readily used for the generality of those purposes to which copper and iron are commonly applied. On the other hand, let the copper or the iron be as fusible as tin; and let the tin be as refractory under the action of heat as iron and copper are: in that case, how could the tin be applied with any degree of economy to the surface of either of the other two; while they themselves would be unfit, from their easy fusibility, to withstand that degree of heat to which they are necessarily exposed in many of the economical uses to which they are applied? There remains to be considered one property of metals with respect to their fusibility, which is of the highest practical importance; for on this property depends the possibility of uniting together portions of the same, or of different metals, without fusion of the metals themselves. If two metals be melted into one uniform mass, the compound is called an alloy; and in the greater number of instances, if not in all, the alloy is more readily fusible than either of the component metals: and hence it easily becomes a bond of union between the two metals, or different portions of either of them. Such an alloy, when so employed, is called a solder. In considering the present subject, we cannot overlook a remarkable analogy between metallic substances and building stones, with reference to one mode in which they may respectively be united to each other, so as to form one solid mass; mortar being to stones what solder is to metals. Thus, in uniting two metallic surfaces by means of solder, it is requisite that the latter should be in a fluid state, or melted; and, in uniting the surfaces of two bricks or stones by means of mortar, the latter must be, if not in absolutely a fluid, yet in a soft and yielding state and the final hardening of each is the efficient cause of permanent union. The period indeed requisite for the due consolidation of the uniting medium is very different; the solder becoming fixed in a few seconds, the mortar requiring some hours, perhaps days, for its consolidation: but, in the end proposed, there is no essential difference; for the mortar, if originally tempered well, and well applied, as firmly unites the stones, as solder the metals: so that mortar might be called a slowly acting solder; and solder, an extemporaneous or quickly acting mortar. It would appear a paradox, if not an absurdity, to affirm abruptly that a liability to rapid decay is among the most important properties of any substance in general use: and yet this may be truly affirmed of iron. For though, in one sense, its liability to rust diminishes the value of this useful metal, because it is consequently almost impossible to preserve it very long in an entire state; yet, indirectly, this property, though detrimental to individuals, is beneficial to the community: for, in the first place, the presence of iron ore is so general, and its quantity so abundant, that there is no probability of any failure in its supply: and, in the next place, numerous branches of trade are kept in continued employ, both in working the ore, and in meeting the constantly renewed demand for implements made of iron, owing to the rapid corrosion of this metal. Among the metals there is one, the history of which ought not to be overlooked on the present occasion, from the very circumstance that its value in a great measure depends on the absence of most of those properties which render all other metals valuable. Quicksilver is the metal in question: and what an anomaly does it not present in the general history of metals; existing, under all common variations of temperature, in a fluid state, while all other metals, with which we are familiar, are, under the same variations, solid; nor indeed are they capable of becoming fluid, but by an elevation of temperature to which they are hardly liable to be exposed, unless designedly: lastly, in consequence of its fluidity, destitute of malleability and ductility; which are among the most valuable properties of the metals taken collectively? This state of fluidity, however, is the very point on which the value of this metal in a great measure turns: for hence it is successfully employed for many purposes, to which, were it solid, it would be inapplicable. How valuable is its use in the construction of the common thermometer and barometer; the value, in the case of the former instrument, depending entirely on its fluidity, and on the physical characters of the fluid itself-the equable ratio, for instance, of its contraction and expansion under widely varying degrees of temperature; and its property of remaining fluid through a greater range of temperature than any other known substance.* 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 number 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 these 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 * Quicksilver does not become solid till exposed to a temperature of about seventy degrees below the freezing point in the scale of Fahrenheit; nor does it pass readily 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. 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. SECTION VII. Common Salt, &c. Ir 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 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 *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 the water contained in it may be heated over a fire without destroying its substance: and the Caraibs 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.) 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*) conjectured. CHAPTER VIII. ADAPTATION OF VEGETABLES TO THE PHYSICAL CONDITION OF MAN. SECTION 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 amongst 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 rainbles 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 cease 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? It may be said indeed that the pleasure of such recollections is for the most part of a moral and intellectual nature; and, so far, is EPT KAI HM. line 151. * Χαλκῷ δ ̓ ἐργάζοντο, μέλας δ' οὐκ ἔσκε σίδηρος. LUCRET. V. 1285. |