Another compound of mercury is the well-known medicine called calomel, which is formed by the union of the metal with a gas named chlorine. The compound is heavy, white, tasteless, crystalline, and nearly insoluble in water. It may be taken in doses of several grains without any effect but that of a purgative. It is composed of 200 parts of mercury, and 36 of chlorine. If, however, the mercury be made to combine with 72 parts of chlorine, it produces corrosive sublimate, a semitransparent white mass, of an acrid, nauseous, metallic taste, soluble in water and alcohol, and highly poisonous. In these examples, it will be seen that substances comparatively inert, may produce by their union compounds of highly active properties; and that a compound of two bodies, which, in one proportion, if taken internally, will have but a slight effect upon the animal frame, if united in a different proportion will prove destructive to life. On the other hand, highly active bodies of opposite properties will also produce, by their union, substances of a mild character, and they are then said to neutralise each other. Sulphuric acid, or oil of vitriol, is highly corrosive, and intensely sour.

If brought into contact with any substance stained with vegetable blue colours, as those of violets or litmus, it instantly changes them to a bright red, a property which belongs to all the acids. Barilla, or carbonate of soda, is a solid that is soluble in water, of a hot, acrid, bitter taste. It changes the blue colour of vegetables to green, which property is common to all alkalies. If into a solution of this substance we carefully drop a portion of the former, a brisk effervescence will ensue, and carbonic acid will be given off. If the experiment be performed with care, and the affusion of the acid stopped the moment the effervescence ceases, the solution will be found to be warm, and possessed of properties totally different from those of the acid or alkali from which they have been formed; it will no longer be sour, corrosive, acrid, or hot; it will have no action on vegetable colours, and all the active properties of the original bodies are said to be neutralised. The compound is slightly bitter, saline, and cooling. If part of the water be evaporated by heat, a solid will be deposited in regular crystalline forms, called sulphate of soda, or Glauber's salt.

The operation of affinity, which produces chemical composition, exists in a body in different degrees towards other bodies; and if to a compound of two bodies a third be presented which has a stronger attraction for either of the component parts than they have for each other, a decomposition will take place, and a new compound will result Carbonate of soda is composed of carbonic acid and soda; and in the experiment above referred to, it is decomposed, the sulphuric acid uniting with the soda, and the carbonic acid escaping in the gaseous state. In like manner, if a solution of carbonate of soda be boiled with caustic lime, the carbonate of soda will be decomposed, the carbonic acid will quit the soda and unite with the lime, forming carbonate of lime or chalk, and caustic soda will remain in the solution. These affinities are conveniently represented in the following diagrams: -

In these diagrams the substance at bottom is the new compound, which is precipitated, or thrown down, and that at the top either escapes or remains in solution; this action is called single elective affinity. If two compounds are brought together in solution, it not unfrequently happens that a process of double decomposition and composition occurs; that is, the two original bodies are decomposed, and two new compounds produced, by a mutual interchange of ingredients: this compound action has been named double elective affinity. Decomposition, which cannot be effected by single, may by double elective affinity. Sugar of lead, or acetate of lead, is a compound of acetic acid and lead. White vitriol, or sulphate of zinc, is compounded of sulphuric acid and zinc. If in a solution of acetate of lead we suspend a piece of metallic zinc, we shall have an example of single elective affinity; the acetic acid will act upon the zinc, and the lead will resume its metallic state, and be precipitated on the zinc in a beautiful arborescent form.

But if a solution of acetate of lead be mixed with a solution of sulphate of zinc, the acetic acid will unite with the zinc, and, at the same instant, the sulphuric acid will combine with the lead, forming an insoluble precipitate, which will fall to the bottom of the vessel while the acetate of zinc remains in solution. The latter is an instance of double elective affinity, and may be represented by the following diagram: -

Here the original substances are placed on the right and left of the diagram, while the new compounds occupy the top and bottom.

Having given a general view of the nature of chemical attraction, we shall next proceed to sketch the most important facts as connected with this subject relative to light, heat, and electricity. The phenomena resulting from the motion of light will be found in the article Optics; we shall, therefore, in the present sketch, refer to the mechanical properties of light only so far as is necessary to render its chemical effects intelligible. When a pencil of light is admitted through a hole in a window shutter into a darkened room, and made to fall on a triangular glass prism, it is turned out of its natural or straight-lined direction, and prevented from falling on the floor, where it would produce a spot of white light, instead of which it forms a spectrum of splendid colours on the wall, or on a screen placed to receive it. From this circumstance it appears that light is compound and separable by means of an inequality in the refrangibility. In the annexed diagram a b represents a ray of light, which falling on the prism e, is diverted from its course, and dispersed into colours occupying the space c d.