have said as regards ice, holds good with all solids: a certain amount of heat is in all cases rendered latent. The following table shows the latent heat of different bodies: :

When water freezes, the 140° of heat abandon it, and manifest itself in a free or sensible state. If a flask of boiling water be saturated with the sulphate of soda, and the cork withdrawn and a rough body introduced, the soda will become solid, and the sudden conversion of the liquid into a solid causes it to part with its heat; if a thermometer be introduced the mercury in it will rise 30° or 40°; this is precisely what takes place in the freezing of water. Solids in becoming liquids render a large amount of heat latent, and liquids in becoming solids part with the heat previously rendered latent.

When solids are compelled to liquefy rapidly without a free supply of heat, the temperature of surrounding objects is lowered. Salt in water, or nitre, causes the thermometer to fall several degrees. If NaO,SO, and snow be mingled together the temperature falls to zero. CaCl mixed with snow, lowers the temperature so as to freeze mercury. These are called freezing mixtures.

Latent Heat of Gases.-It is not necessary for a liquid to boil in order to produce vaporisation. The drying up of water by its gradual conversion into vapour is a phenomena with which we are all familiar. If water be raised to 212° it is converted into

steam, but the steam is the same temperature as the water. If it were only necessary to raise water to 212° to convert it into vapour, it would, as soon as it attained that temperature, explode into steam, like gunpowder. A cubic inch of water will make a cubic foot of steam. If vapours contained no more heat than the liquids, they would immediately condense into liquids as soon as they came in contact with any body lower in temperature; but it is found necessary to expose them to a great amount of cold. Ice in becoming water, renders a large amount of heat latent; water in becoming steam also renders a large amount of heat: what occurs with water occurs with all liquids.

If a vessel with water, at 32°, be placed over a regular source of heat, and the time observed necessary to raise it to 212°, it will require five times longer before the water is entirely converted into steam. Now the water has been raised through a temperature of 180°, say in one half hour, we multiply 180 × 5, which is the number of hours taken, and we get 900°: yet the steam has only the temperature of 212°. What has become of this enormous amount of heat? It has been rendered latent in the steam, and this can be demonstrated by condensing the steam, and observing the amount of heat evolved. Let the steam be conducted into ice-cold water, and it will part with its latent heat, and raise the water to its boiling point. If 11 cubic inches of water be taken at 32° and raised to 212° by the steam of 2 cubic inches of water, we shall have 13 cubic inches at 212°. 2 cubic inches of water, in the form of steam, have raised 11 cubic inches of water, mercury 180°, and the whole 13 cubic inches have a temperature of 212°. If a cubic foot of steam, at 212°, be contained in a close vessel,

and 5 inches of ice-cold water, at 32°, be injected into this vessel, we shall have 6 cubic inches of water at 212°. Now it is evident that in returning to a state of water the steam has given out sufficient heat to raise 5 cubic inches of water from 32° to 212°. The heat which was latent in the steam has now been made sensible in raising the 5 cubic inches of water through 5 times 180° of heat, viz. 990°.

CRYSTALLISATION.

In order that a body may crystallise it is necessary that the particles be free to move; when this occurs they arrange themselves in most cases in regular crystalline forms.

From this it appears that all crystalline bodies must at some time have been fluids. The regular formation of a crystal may be seen by splitting it, which may be easily done with some crystals. If the subdivision of the crystal be continued in those directions called planes of cleavage, a symmetrical figure is obtained which is the primitive form of the crystal, which may widely differ from the figure at first; should this be the case the latter will be the secondary form of the crystal. By subdividing the primitive form you get a number of minute fractions resembling it in figure; these are regarded as the type of the atoms of which the body is composed.

Among natural crystalline forms we may mention rock crystal, quartz, granite, calcareous spar, fluor spar, copper ore, pyrites, the loadstone, snow, and ice. Artificial crystals, alum, common salt, saltpetre, sulphate of copper, sugar, sulphur, and bismuth, and a variety of others.

In the tinning of iron plates the crystallism of the

tin is evident when acted on by nitric acid, producing the Mora antique, as also in the galvanising of iron. The planes of cleavage are seen in lead ore, Iceland spar.

When Iceland spar is broken it gives perfectly formed rhomboidal crystals.

The primitive form is in most cases taken as the basis of classification of different crystalline bodies. The chief points to be attended to in crystals are the angles and the centres of their edges and sides. The lines joining these points and passing through the middle of the crystal are called the axes. Most crystalline bodies have three such axes at right angles to each other.

These axes are either all equal, or two of them are equal, or all three unequal. That one most dissimilar from the other two is called the principal axis; the others secondary. In the cube either of the three may be regarded as the principal axis. A double pyramid has four.

The term amorphous is applied to bodies destitute of any regular form. Dr. Wollaston, however, proposed another theory; he regarded small spheres as the ultimate particles when placed in the same plane. All metals under favourable circumstances assume a crystalline form.

Crystalline forms are generally arranged into six systems.

1. The regular system. The crystals of this division have three equal axes at right angles to each other; the cube, the regular octohedron, and the rhombic dodecahedron. Many substances, simple and compound, assume this form. The metals, also carbon, the diamond, common salt, iodide of potassium; the alums, fluor spar, bisulphide of iron.

2. The square prismatic system. These axes are all at right angles; two only are of equal length, peroxide of tin, the ferrocyanide of potassium

3. The right prismatic system. Three axes of unequal length; the right rhombic prism, sulphur, nitrate of potash, sulphate of potash, sulphate of baryta.

4. The oblique prismatic system. Crystals belonging to this group, the oblique rectangular prism, the oblique rhombic prism-sulphate of soda, carbonate of soda, biborate of soda.

5. The oblique double prismatic system. Sulphate of copper.

6. The rhombohedral system. Ice, nitrate of soda, quartz, arsenic, antimony.

APPARATUS.

Expertness and neatness in the construction and use of apparatus can only be acquired by practice.

Corks are of constant use, and they should be of the best kind. Before fitting them into apparatus they should be softened by rolling them under the foot on a piece of paper, so as to keep them free from dust.

When glass tubes have to be passed through the cork, holes should be made with a cork-borer rather smaller than the tube to be inserted, and nicely eased to the proper size with a rat-tail file. If the cork should prove porous the exposed parts must be covered with a solution of sealing-wax in spirits of wine.

Glass tubing may be cut into the required lengths by marking round the tube with a three-cornered file; the tube is then easily broken. The depth of the mark made by the file must depend on the thick