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THE PLATINUM METALS.

2MC1, PtC1, or MC1,,PtCl. These compounds are commonly called chloroplatinates; by means of them the composition and combining weights of many organic compounds have been determined. It is only necessary to ignite a weighed portion of the chloroplatinate, and to weigh the residue of pure platinum which is left after the organic matter has all been driven off, in order to ascertain how much platinum is contained in the compound. This fact having been determined, the quantity of the organic radical, or rather of the chloride of the radical, which was combined with the chloride of platinum in the chloroplatinate, may be readily calculated.

708. With gold and platinum are classed several rare metals which are never found except in association with platinum, and which closely resemble that metal. They are commonly called platinum metals, and the group may be appropriately termed the platinum group. The whole group consists of Rhodium (atomic weight=104), Ruthenium (104), Palladium (106-5), Gold (1967), Platinum (1974), Iridium (198), and Osmium (199). Palladium is used to impart to brass gas-fixtures a peculiar reddish tint, sometimes called salmon-bronze. Iridium is used for the very hard tips of gold pens. Osmium forms, among other oxides, a volatile compound OsO,, whose vapors are intensely poisonous. The metals of this group are noble metals; they withstand the action of the atmosphere; none of them are acted upon by nitric acid, though they dissolve in chlorine and in aqua regia. Their oxides part with all their oxygen when simply heated, leaving the metal behind.

SYMBOLS AND ATOMIC WEIGHTS,

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CHAPTER XXXV.

ATOMIC WEIGHTS OF THE ELEMENTS CLASSIFICATION.

709. An alphabetical list of the sixty-five recognized elements, with their symbols and atomic weights, is here given for convenience of reference. The names of those elements which are so rare as to be at present of little importance are printed in italics:—

710. In the following table the elements are arranged in what are believed to be natural groups. Without accepting any one infallible criterion of classification, or insisting upon any systematic arrangement of the elements in groups with that strenuousness which is apt to make classification rather a hindrance than a help, the student may provisionally use this subdivision of the elements into groups as a help in remembering facts, as a guide to the prompt recognition of general properties and general laws, and as a suggestive compend of his whole chemical knowledge:—

ATOMIC HEATS OF THE ELEMENTS.

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711. Atomic Heats of the Elements.-The power of heat to cause changes of temperature is not the same for any two substances, but varies with the nature of the substance submitted to its action. Each chemical element is peculiarly affected in this respect by heat. The quantity of heat needed to raise the temperature of a certain weight of water from 0° to 1° being called unity, the quantity of heat required to raise the temperature of the same weight of any element by the same amount is the specific heat of that element (§ 31). In the second column of the following table will be found the specific heats of a number of representative elements, selected from each group of elements except the carbon group, and arranged in the order of their atomic weights. (Compare § 710.)

The preceding table contains only solid substances. It has been found that the specific heat of the same body is commonly greater in the liquid than in the solid state, and always less in the gaseous than in the liquid state. Accordingly in instituting any comparison between different bodies, based on their specific heats, it is essential to compare them in the same physical condition, solids with solids, liquids with liquids, gases with gases. The second column of the following table contains the specific heats of the four elements which are gaseous at the ordinary atmospheric temperature and pressure.

On comparing together the numbers in the second and third columns of the preceding tables, it will be noticed that the lower the atomic weight of an element is, the higher is its specific heat, and vice versa. In the fourth column of the tables, under the name of atomic heat, will be found the product of the specific heat by the atomic weight of each element. The atomic heats of the elements represent the quantities of heat which are required to cause equal alterations of temperature in atomic proportions of the several elements. But the tables show that these quantities of heat are nearly the same for each and all of the solid elements compared in the first table, and are again approximately equal for the gaseous elements which are grouped in the second table.

This striking principle is deducible from the foregoing considerations—namely, that while the capacities for heat of the same weights of the various elements are very different, the capacities for heat of the atoms, or atomic proportions, of the elements are nearly identical, provided that the elements compared be in the same physical condition. In other words, those weights of the elements which are assumed to represent the relative weights of their atoms require approximately the same amount of heat to raise them through an equal number of degrees of temperature; while the amounts of heat required to raise equal weights of the elements through an equal number of degrees are expressed by very different numbers (the specific heats). It is essential, however, that the elements compared should be in the same physical condition.

It is true that the numbers representing the atomic heats show considerable discrepancies; but when it is remembered that there are unavoidable errors attaching to the determinations both of the specific heats and of the atomic weights, that many of the elements cannot yet be obtained in a condition of purity, and that the two factors of the product (specific heat and atomic weight) vary in the proportion of 1 to 30, it will be seen that the accordance is distinct enough to indicate the existence of a general