as clay, or crystallized, as Epsom salt and common salt. Sometimes in crystals one of the elements may be replaced by another element without changing the form of the crystal, as with phosphorus and arsenic compounds. The atoms of such elements are supposed to have the same size and shape, and are called ise- or isomorphous (on and μopon, similar shape). When two compounds have different properties, but the elements present in the same proportion, they are said to be isomeric (1σov, μepos, similar part).

CHAPTER I.

MODE OP DETERMINING THE PHYSICAL OR EXTERNAL CHARACTERS OF BODIES.

IN examining the chemical properties of substances, there are several characters which may be investigated without much comparative manipulation. The principal of these are, 1. The colour of the body. 2. Its hardness, compared with other substances. 3. Its taste. 4. Its smell. 5. The effects of heat upon it ascertained by exposing it on a spatula, or a knife, to the flame of a lamp, if in the solid form; and in a tube, if the body should exist in the fluid state. 6. Its solubility in water, mineral acids, alcohol, and ether. 7. The action of the solution upon litmus paper. 8. Its crystalline form. 9. Its peculiar weight, compared to an equal bulk of water, usually termed its specific gravity.

1. The colour of bodies is often an important index to their nature, especially in the study of the metals. The salts of soda, potash, the other alkalies and earths, are generally colourless or white, for example; while those of iron are green, black, or red; those of manganese, flesh red; of nickel, green; of copper, blue, and green under particular circumstances; of chromium, and protoxide of uranium, of iron, of nickel, green.

2. The hardness of a body is of most importance in reference to minerals. The following is the comparative scale employed for determining this property : —

1. Talc.

2. Rock salt, gypsum.

3. Calcareous spar.

4. Fluor spar.

5. Phosphate of lime.

6. Felspar.

7. Rock crystal.

8. Topaz.

9. Corundum.

10. Diamond.

The first substance in the table is the softest, and the last the hardest. If we find that the substance we are examining scratches talc, but does not affect rock-salt, we say its hardness is 1·5 (1), or some intermediate fraction, of which we judge by the comparative hardness. Sometimes the finger-nail affords an approximation to the hardness of a substance. Thus, sulphate of lime (gypsum), or plaster of Paris, is scratched easily with the nail, while only a trifling effect is produced on calcareous spar.

3. The taste enables us to distinguish sour or acid bodies, as oxalic acid, cream of tartar; sweet, as salts of glucina; bitter, as Epsom salts; saline, or salt, as common salt. By taste we can thus distinguish between oxalic acid and Epsom salt- two bodies

EXTERNAL CHARACTERS.

which are sometimes confounded.

Chemists should therefore

familiarize themselves with the taste of bodies.

4. The organ of smell is an important auxiliary to the chemist. By means of the sense of smell we can often detect most minute quantities of ammonia, nitric acid, &c., when chemical tests would fail to afford any result to be depended on.

5. By applying heat, we judge of the fixed or volatile nature of a body; by the presence of nitrogen in it, when it emits an odour of feathers or burned animal matter-that peculiar smell from which we judge that a woollen rag has been burned. For example, if we expose sugar or starch to the action of a flame, this peculiar odour is not perceptible; but if we burn horn or hair, it is at once obvious to the sense of smell.

6. The solubility of salts is an important character. The insolubility of stony minerals in water is a characteristic property of these products of the mineral kingdom. Almost all the alkaline oxygen salts are soluble in water, and many of the metallic salts are in a similar condition. When salts are insoluble, we take advantage of this property to effect their separation from other bodies in solution. When a substance dissolves in a mineral acid (sulphuric, nitric, or muriatic) with effervescence, that is, the disengagement of carbonic acid, we infer that it is a carbonate. A certain number of stony minerals, containing from five to ten per cent. and upwards of water, are decomposed by muriatic and nitric acids, but the greater bulk of minerals, having silica as their acid, are comparatively unaffected by these agents.

By the use of alcohol we can often separate bodies when both are soluble in water; for example, flour contains sugar and gum, both of which bodies dissolve in water; but by adding boiling alcohol to their solution, the gum is precipitated, while the sugar remains dissolved in the alcoholic fluid.

Some substances are soluble, while others are insoluble, in ether. Thus, the cholesterin of the nerves and brain is soluble in ether, while the acid of the same organs (cerebric acid) is scarcely soluble in that fluid. We are thus enabled to separate them.

7. The crystalline form, in opposition to an amorphous mass, is a valuable index in many cases. The taste and crystalline form enable us to distinguish oxalic acid (a poison) from sulphate of magnesia, or Epsom salt, an important medicine. The following are their characters:

The crystalline forms which substances assume are all reducible to six classes and their modifications. I. The first class is the regular or octahedral system, in

which there are three axes or lines joining the angles, which are all equal. To this belong the octahedron and cube. As illustrations, we have alum

1

1

2

1

as an octahedron, common salt as a cube, garnet as a dodecahedron. II. Pyramidal, square prismatic, or 2-and-1-axed system. The usual form is two pyramids, with square bases applied base to base, each of the faces forming an isosceles or scalene triangle, while, in the regular octahedron, the faces are equal-sided triangles. The angles at the base are replaced by planes in harmotome, and form a 4-sided square prism. The edges of the prism are replaced by planes in calomel, peroxide of tin, and form 4-sided and 8-sided prisms. III. Rhombohedral, or hexagonal, 3-and-1-axed system, three axes being equal and one unequal. A common form of this is the 6-sided prism terminated by the 6-sided pyramid, as rock crystal. IV. Prismatic, or 1-and-1-axed system. The forms are principally three; first, when the faces are inclined towards all the three axes, as in the rhombic octahedron of sulphur; second, when the faces are inclined towards two of the axes, as in the topaz and sulphate of barytes; third, when the faces are inclined towards one axis, as in desmin, chrysolite. V. Oblique prismatic sys

4

3

tem, of three unequal axes, 2-and-1-membered system, as in the oblique rhombic octahedron of gypsum. The oblique rhombic prism is a quadrangular prism, whose bases are equal rhombs, and whose lateral faces are equal oblique-angled parallelograms. The examples of this class are felspar, sulphate and carbonate of soda, sulphate of iron, borax. VI. Class. Doublyoblique prismatic system, 1-and-1-membered system, of three axes, all unequal, cutting each other at acute angles. The

most familiar examples of this class are sulphate of copper, sulphate of manganese and cyanite.

The measurement of the angles in crystals is important, and is