Several of the elements, the action of which has been examined, occupy abnormal positions, and any reason for this, except allotropy, remains to be explained. It is difficult to offer any mechanical theory to account for the action of the elements, but it may perhaps be well to give a rough indication of what may take place. If five spheres, representing atoms of a certain volume, are arranged so as to touch each other, it will be evident that the addition of an element with a small atomic volume may improve the tenacity by filling up the central space which would otherwise remain void; with such an arrangement of five atoms the addition of an element with the same atomic volume as themselves will tend to drive them slightly further asunder, and should, therefore, act prejudicially in a five-atom group, although it would exactly fill the space between a six-atom group, but in either case the insertion of a larger atomic volume than that of each member of the group, must tend to drive the members of either the five or six-atom group further asunder, and by so doing would diminish the cohesion of the mass. No doubt, in some cases, condensation takes place, and this may explain some of the abnormal results.

The curve representing the extensibility of the gold to which impurities have been added resembles that shown in Fig. 31. Cadmium exhibits marked irregularity in both curves; but the only striking difference between the two curves is caused by tellurium and bismuth, the former of which seems to be more prejudicial to the elongation of gold than to its tenacity. The influence of cadmium in increasing the extensibility is very remarkable. Aluminium, indium, and lithium occupy somewhat abnormal positions on the curve of tenacity, for they possess high atomic volumes, and yet they appear to increase the tenacity of gold, although they reduce its capability of being elongated. The author has some researches in progress, the results of which appear to enable this abnormal behaviour to be explained.

With regard to the applicability of these generalisations to other metals, it may be observed that recent investigations of Hadfield as to the physical properties of aluminium-steel, show that the part played by aluminium is almost identical with that of silicon, and this fact strongly supports the view above stated, for it is remarkable that the two bodies, aluminium and silicon, of which, when in a free state, the physical and mechanical properties are totally different, should nevertheless when they are alloyed with iron affect it in precisely the same way. Silicon and aluminium have almost the same atomic volume.

Questions of great industrial interest present themselves, espe

cially in connection with iron. With regard to this metal, the evidence as to the action of other elements upon it tends in the same direction as in the case of gold, although the question is greatly complicated by the relations of iron to oxygen, and by the presence of occluded gases.

In the case of iron, it is difficult to say what property of the metal would be most affected by the added matter; but the author pointed out, in a lecture delivered at Newcastle in 1889, that the direct connection with the Periodic law will probably be traced by the effect of a given element in retarding or promoting the passage of ordinary iron to the allotropic state, a point of much importance, as the mechanical properties of the metal must depend on the atomic arrangement in the molecules. Osmond* has since satisfied himself of the accuracy of this view. He considers, as will be shown in the following chapter, that there are two modifications of iron; the a, or soft variety, which exists in pure iron at temperatures below 855° C., and in iron containing certain other elements, if it has been cooled slowly. There is also the ẞ, or hard modification, which exists at high temperatures or if certain elements be present, in iron which has been rapidly cooled from temperatures above 855° C., or if the iron has been obtained by electrolysis.

The foreign elements, whose action on the critical points of iron has been studied by Osmond experimentally, are ranged as follows in two columns in the order of their atomic volumes :

The elements in column I., whose atomic volumes are smaller than that of iron (7.2), delay during cooling the change of the ß, or hard allotropic variety of iron, into the a, or soft variety, as well as that of hardening carbon into carbide carbon. For these two reasons they tend to increase, with equal rates of cooling, the proportion of ẞ iron that is present in the cooled iron or steel, and consequently the hardness of the metal. Indeed, their presence is equivalent to a more or less energetic hardening. To these elements hydrogen may be added. As is well known, this element renders electro-deposited iron hard and brittle; perhaps

* Comptes Rendus, vol. cx. (1890), p. 346.

it contains the metal, Graham's hydrogenium, for hydrogen gas does not appear to have a marked influence on the critical temperature.

The elements in column II., whose atomic volumes are greater than that of iron, tend to raise or at least maintain near its normal position, during cooling, the temperature at which the change of ẞ to a iron takes place. Further, they render the inverse change during heating more or less incomplete, and usually hasten the change of "hardening carbon" to "carbide carbon." Thus, they maintain the iron in the a state at high temperatures, and must therefore have the same effect in the cooled metal. In this way, they would act on iron as annealing does, rendering it soft and malleable, did not their individual properties, or those of their compounds, often intervene and partially mask this natural consequence of their presence.

Thus, foreign elements alloyed with iron either hasten or delay the passage of the iron, during cooling, to an allotropic state, and render the change more or less incomplete, according to whether the atomic volume of the added impurity is less or greater than that of iron. In other words, foreign elements of low atomic volume tend to make iron itself assume or retain the particular molecular form that possesses the lowest atomic volume, whilst elements with large atomic volume produce the inverse effect.

While obeying the general law, carbon possesses on its own account the property of undergoing, at a certain critical temperature, a change, the nature of which is still disputable, although its existence is acknowledged. This property gives carbon a place by itself in the metallurgy of iron.

Whatever may ultimately prove to be the true nature of the molecular change which accompanies the thermal treatment of iron and determines its mechanical properties, there is little doubt but that there is a close relation between the action of foreign elements and their atomic volume. Few metallurgical questions are of greater interest at the present time than those which relate to the molecular structure of metals, and Osmond has shown it to be very probable that the presence of a small quantity of a foreign metal may cause a mass of another metal to pass into an allotropic state. In relation to iron and steel the problems are of great industrial importance, and it is fortunate that we appear to be nearing the discovery of a law in accordance with which all metallic masses are influenced by "traces.”

The effect of comparatively large quantities of certain elements on the mechanical properties of iron has already been stated in

the preceding chapter, and the subject will be again referred to in Chapter IV.

Colour of Alloys.—It will now be well to examine some effects of uniting metals by fusing them together, and also to consider the direct influence of a minute quantity of one metal in changing the mass of another in which it is hidden, causing it to behave in a different way in relation to light, and consequently to possess a colour different from that which is natural to it. The added metal may so change the chemical nature of the metallic mass that varied effects of colour may be produced by the action of certain "pickling" solutions. This portion of the subject is so large that reference can only be made to certain prominent facts.* First, with reference to the colour produced by the union of metals. Take, for example, a mass of red copper, and one of grey antimony; the union of the two by fusion produces a beautiful violet alloy when the proportions are so arranged that there is 51 per cent. of copper, and 49 per cent. of antimony in the mixture. This alloy was well-known to the early chemists, but, unfortunately, it is brittle and difficult to work, so that its beautiful colour can hardly be utilised in art. The addition of a small quantity of tin to copper hardens it, and converts it, from a physical and mechanical point of view, into a different metal. The addition of zinc and a certain amount of lead to tin and copper, confers upon the metal copper the property of receiving, when exposed to the atmosphere, varying shades of deep velvety brown, characteristic of the bronze which has from remote antiquity been used for artistic purposes. But by far the most interesting copper-alloys, from the point of view of colour, are those produced by its union with zinc, namely brass. Their preparation demands much care in the selection of the materials.

The colouring power of metals in alloys is very variable. Ledeburt arranges the principal metals in the following order :

Each metal in this series has a greater decolorising action than the metal following it. Thus, the colour of the last members is

* A list of books and papers dealing with the colours of metals and alloys, and with the production of coloured patina is given by Professor Ledebur in his work Die Metallverarbeitung, 1882, p. 285.

+ Die Legierungen, Berlin, 1890, p. 47.

concealed by comparatively small amounts of the first members. A good example is afforded by the alloy used for the continental nickel coinage. This consists of three parts of red copper with only one part of white nickel. The comparatively small quantity of nickel is, however, sufficient to completely hide the red colour of the copper.

Of the very varied series of alloys the Japanese employ for art metal-work, the following may be considered the most important and typical. The first is called shaku-do, it contains, as will be seen from Analyses Nos. I. and II.,* in addition to about 95 per cent. of copper, as much as 4 per cent. of gold. The quantity

of gold is, however, very variable, some specimens which have been analysed containing only 1.5 per cent. of the precious metal. Another important alloy is called shibu-ichi. Analyses of this alloy gave:

Copper
Silver

Gold.
Iron

IV.

III.

There are numerous varieties of it, but in both these alloys, shakudo and shibu-ichi, the point of interest is that the precious metals are, as it were, sacrificed in order to produce definite results, gold and silver, when used pure, being employed very sparingly to heighten the general effect. In the case of the shaku-do, we shall

see presently that the gold appears to enable the metal to receive a beautiful, rich purple coat or patina, as it is called, when treated with certain pickling solutions; while shibu-ichi possesses a peculiar silver-grey tint of its own, which, under ordinary atmospheric influences, becomes very beautiful, and to which the Japanese artists are very partial. These are the principal alloys, but there are several varieties of them, as well as combinations of shaku-do and shibu-ichi in various proportions, as, for instance, in the case

* Analyses Nos. I. and III. are by Mr. Gowland, of the Imperial Japanese Mint at Osaka; Nos. II. and IV., by Prof. Kalischer, Dingl. Polyt. Journ., vol. ccxv. p. 93.