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about solidification. The solidification is more easily produced by contact with the minutest fraction of the solid itself. At the moment of solidification the temperature rises to that of the melting point but no further: this rise in temperature is produced by the liberation of the latent heat. This acceleration in the rate of motion of the particles corresponds to considerable increase in volume, which, as a rule, appears to take place suddenly on fusion or in part during the softening, this increase amounting in some cases to 12 or more per cent. of the volume of the solid. Yet in the case of some substances, especially water, cast iron, bismuth, and some of its compounds and alloys, and perhaps also in the case of other metals, contraction is known to attend the fusion: a fact which can perhaps be explained as arising from an altered arrangement of the atoms in the molecules. In water this contraction amounts to nearly 10 per cent. of the volume. The change in the state of aggregation produced by pressure depends upon whether fusion be attended by an expansion or contraction. A sufficiently great pressure produces that condition in which the material fills the smallest space. Ice can be liquefied by pressure, but most other solid substances can, by the application of high pressure, be retained in the solid state at temperatures much above their melting points.

§ 66. Melting Points of the Elements. The temperatures at which different substances melt are specific and characteristic for each, and serve, therefore, as important aids for their identification. In § 36 it has already been mentioned that the fusibility of the elements is a periodic function of their atomic weights. This relationship, so far as it has been in any way ascertained, is exhibited in the following table. The melting points of many elements are still unknown, because the temperature at which they melt is either too high or too low to be accurately determined; in some other cases the rarity of the element or the difficulties surrounding its isolation have prevented the exact determination. In the following table the abbreviations used are: a = approximation, b = above, c = very low, d = very high, e = not melted, rh red heat, drh dull red heat, brh = bright red heat,

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The elements are arranged in the horizontal lines in the order of their atomic weights. With these the melting point rises suddenly and falls suddenly; the minima of the melting points are printed in italics, the maxima in block print.

The periods of fusibility do not coincide with those of other physical properties-in fact, are less regular than these, but are nearly related, as has already been shown in § 36, to those of the atomic volumes.

It is remarkable that in every family the members of one group are difficult to fuse, whilst those of the other are easily fusible; e.g. lithium, sodium, potassium, rubidium, cæsium are easily fusible, whilst copper, silver, and gold melt at high temperatures, and similar relationships are found to exist in other families. In separate groups the melting point changes with the atomic weight, but not in the same manner. In some families the melting point falls with increase in atomic weight, thus:

Li 180°, Na 96°, K 63°, Rb 39°, Cs 26°,

Zn 419°, Cd 321°, Hg — 39°;

'The melting points for magnesium, aluminium, copper, zinc, silver, tin, antimony, and gold are taken from the paper of Heycock and Neville, Chem. Soc. Journ. lxvii. 160-199.

in others, again, it rises with increase in atomic weight; for example:Ga 30°, In 176°, TI 294°,

Cl- 105°, Br-7°, I + 114°;

whilst in some families it rises at first to fall again, or falls first and then rises.

§ 67. Melting Points of Compounds. In the melting points of compounds we have similar differences to those exhibited by the elements. By the introduction into a compound of certain elements the fusibility is in some cases raised, in other cases lowered. The oxides of metals, e.g. melt at much higher temperatures than the metals themselves; the majority of the oxides of the non-metals melt more easily than the elements; in one and the same group of elements these changes, as a rule, are found to be of the same character, but even in this case also there are exceptions. Whilst, for example, the infusible element carbon yields an oxide (CO2) which melts at 60°, the corresponding oxide (SiO2) of the infusible silicon has almost as high a melting point as the element itself. Fluorides, chlorides, bromides, iodides, melt, as a rule, much more easily than the oxides, and usually the iodide of an element is more easily fusible than the bromide, and this than the chloride, whilst the fluoride has the highest melting point. Thus, for example, the melting points of halogen compounds of the alkali metals are, according to Carnelley, as follows:1

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The numbers in brackets in this table are taken from the results of Victor Meyer, W. Riddle, and Th. Lamb (Ber. der deut. chem. Ges., 27, 3,129). Somewhat similar results have been obtained by McCrae (Wiedem. Annal. 55 45).

The melting point falls, therefore, with increased atomic weight of the halogen, and similar relationships are to be found in other families of the elements.

Many similar regularities are to be found amongst organic compounds; still our knowledge of the general laws in this province is much less extensive than might be imagined from the thousands of melting-point determinations which have been made.

It is, however, to be observed that in many cases the repeated introduction of a given atom or a group of atoms in an organic compound is accompanied by alternate raising and lowering of the melting point. This, as was first shown by Baeyer, is the case in the normal primary fatty acids of the general formula CH2O2. In these compounds the atom ́ linkage is represented as follows:

2n'

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in accordance with which the several members of the series differ from one another only in the number of CH, groups introduced between the carboxyl group, COOH, and hydrogen. The relationships are shown in the following table:

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From the above it is seen that the first introduction of the group CH, between the carboxyl and hydrogen raises the melting point, whereas the introduction of the second group lowers the melting point; consequently those members in this series of acids which contain an uneven number of carbon atoms melt at a lower temperature than either of their neighbours containing an even number of carbon atoms. As the molecular weight increases, this difference gradually disappears. The melting point of the dibasic acids of the formula,

CH_2O = HO—CO—(CH,), CO—0H,

-2n-2 4

consisting of oxalic, malonic, and succinic acids, &c., exhibit similar relationships.

The melting point of many hydro-carbons, e.g. of benzene, as shown by Jungfleisch, is alternately raised and lowered by the replacement of the hydrogen by chlorine.

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Still, it is only when the chief products of the action of chlorine upon benzene are compared with one another that such regularities are observed. In addition to these, several isomeric compounds are formed, but in much smaller quantities, and these again have different melting points. In fact, it is found that the melting point of a compound is influenced by the positions which the chlorine atoms occupy relatively to one another.

As a general rule, it may be stated that of the three isomeric di-substitution products 1 which may be obtained by replacing two atoms of hydrogen in benzene by two other atoms or radicals, the para- compound has a melting point much higher than the ortho- and the meta-. Which of the latter has the higher melting point depends upon the nature 1 Cf. § 51.

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