182 OUTLINES OF PHYSICAL CHEMISTRY
��III. THE SOLID STATE.
The more important physical properties of solids are:
(1) The melting point ;
(2) The density or specific gravity ;
(3) The specific heat ;
(4) The solubility in different solvents
(5) The crystalline form.
This last-mentioned property is the subject of a special science— crystallography ; the fourth belongs to the domain of descriptive chemistry ; and the third has already been treated of in connection with Dulong and Petit 9 s law. As regards the second it suffices here to recall that the density of the elements, or rather their atomic volume, is the basis of Lothar Meyer's periodic classification.
The melting point of a substance is determined in the following manner (if it melts at a reasonably low temperature, say up to 250 or 800°) :
A very small quantity of the substance finely powdered is introduced into a narrow glass tube with very thin walls and closed at the bottom. By means of a small rubber ring * the tube is fixed on to a thermometer in such a position that the substance is situated close to the mercury bulb. The thermometer and tube are then heated in a suitable bath (water, sulphuric acid, or paraffin) until the substance just begins to fuse. The temperature at which this takes place is noted. 2 The bath should be provided with a stirrer in order to keep the temperature regular.
In special cases the method has to be considerably modified.
1 The rubber ring may be dispensed with, and the tube fixed to the thermometer by capillary attraction.
2 The temperature read off must be corrected by the formula given in the description of the determination of boiling points.
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