Page:EB1911 - Volume 18.djvu/217

 symptoms in the rabbit to those which characterize the disease in man.

Excluding the peculiar changes in the joints which occur in rheumatoid arthritis and in Charcot’s disease, and which are almost certainly primary affections of the nervous system, it is found that a large number of individuals suffer from pain in the joints, in the muscles, and in the fibrous tissues, chiefly on exposure to cold and damp or after indiscretions of diet. This so-called chronic rheumatism appears to be a totally distinct condition from rheumatic fever; and although its pathology is not determined, it looks as if it were due either to a diminished elimination or an increased production of some toxic substance or substances, but so far we have no evidence as to their nature.

Rickets is undoubtedly a manifestation of a profound alteration of the metabolism in childhood, but how far it is an idiopathic condition and how far a result of the action of toxins introduced from Without is not yet definitely known. Kassowitz long ago showed that the bone changes are similar to those which can be produced in animals by chronic phosphorus poisoning, and that they are really irritative in nature. Spillmann, in his work Le Rachitisme, discusses the evidence as regards the action of various conditions, and comes to the conclusion that there is no evidence that it is due to a mere primary disturbance of the metabolism, or to excessive production of lactic acid, or to any specific micro-organismal poisoning. But he adduces evidence, perhaps not very convincing, that in the disease there is a specific intoxication derived from the alimentary canal and provoking inflammatory lesions in the bones.

METABOLISM (from Gr. , change), the biological term for the process of chemical change in a living cell (see ).

METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. , in the same sense, probably connected with  , to search after, explore,  , after,  , other). Originally applied to gold, silver, copper, iron, tin, lead and bronze, i.e. substances having high specific gravity, malleability, opacity, and especially a peculiar lustre, the term “metal” became generic for all substances with these properties. In modern chemistry, however, the metals are a division of the elements, the members of which may or may not possess all these characters. The progress of science has, in fact, been accompanied by the discovery of some 70 elements, which may be arranged in order of their “metallic” properties as above indicated, and it is found that while the end members of the scale are most distinctly metallic (or non-metallic), certain central members, e.g. arsenic, may be placed in either division, their properties approximating to both metallic and non-metallic. One chemical differentia utilizes the fact that metals always form at least one basic oxide which yields salts with acids, While non-metals usually form acidic oxides, i.e. oxides which yield acids with water. This definition, however, is highly artificial—and objectionable on principle, because when we speak of metals we think, not of their chemical relations, but of a certain sum of mechanical and physical properties which unites them all into one natural family.

All metals, when exposed in an inert atmosphere to a sufficient temperature, assume the form of liquids, which all present the following characteristic properties. They are (at least practically) non-transparent; they reflect light in a peculiar manner, producing what is called “metallic lustre.” When kept in non-metallic vessels they take the shape of a convex meniscus. These liquids, when exposed to higher temperatures, some sooner than others, pass into vapours. What these vapours are like is not known in many cases, since, as a rule, they can be produced only at very high temperatures, precluding the use of transparent vessels. Silver vapour is blue, potassium vapour is green, many others (mercury vapour, for instance) are colourless. The liquid metals, when cooled down sufficiently, some at lower, others at higher, temperatures freeze into compact solids, endowed with the (relative) non-transparency and the lustre of their liquids. These frozen metals in general form compact masses consisting of aggregates of crystals belonging to the regular or rhombic or (more rarely) the quadratic system. Compared with non-metallic solids, they in general are good conductors of heat and of electricity. But their most characteristic, though not perhaps their most general, property is that they combine in themselves the apparently incompatible properties of elasticity and rigidity on the one hand and plasticity on the other. To this remarkable combination of properties more than to anything else the ordinary metals owe their wide application in the mechanical arts. In former times a high specific gravity used to be quoted as one of the characters of the genus; but this no longer holds, since we now know a series of metals lighter than water.

Non-Transparency.—This, in the case of even the solid metals, is perhaps only a very low degree of transparency. In regard to gold this has been proved to be so; gold leaf, or thin films of gold produced chemically on glass plates, transmit light with a green colour. On the other hand, infinitely thin films of silver which can be produced chemically on glass surfaces are absolutely opaque. Very thin films of liquid mercury, according to Melsens, transmit light with a violet-blue colour; also thin films of copper are said to be translucent.

Colour.—Gold is yellow; copper is red; silver, tin, and some others are pure white; the majority are greyish.

Reflection of Light.—Polished metallic surfaces, like those of other solids, divide any incident ray into two parts, of which one is refracted while the other is reflected—with this difference, however, that the former is completely absorbed, and that the latter, in regard to polarization, is quite differently affected; The following values are due to Rubens and Hagen (Ann. der Phys., 1900, p. 352); they express the percentage of incident light reflected. The superiority of silver is obvious. Crystalline Form and Structure.—Most (perhaps all) metals are capable of crystallization. The crystals belong to the following systems: regular system—silver, gold, palladium, mercury, copper, iron, lead; quadratic system—tin, potassium; rhombic system—antimony, bismuth, tellurium, zinc, magnesium. Perhaps all metals are crystalline, only the degree of visibility of the crystalline arrangement is very different in different metals, and even in the same metal varies according to the slowness of solidification and other circumstances.