diffused-that is to say, if it be the light of direct sunshine the union is accompanied by an explosion. The passage of an electric spark through this mixture of hydrogen and chlorine in equal volumes, causes a similar union to ensue; and may readily be accomplished by means of the eudiometer of Cavendish.

By a modification of this treatment also, the vapours of iodine and of bromine may also be caused to unite with hydrogen; in which case hydriodic or hydrobromic acid results.

In procuring these acids, however, in practice, indirect processes are had recourse to. The materials for generating hydrochloric acid are chloride of sodium (common salt), sulphuric acid, and water, all of which being distilled together, hydrochloric acid results, and sulphate of soda is left. In this operation the chlorine of the acid is yielded by the chloride of sodium, and the hydrogen by the water.

Hydrochlorate of ammonia (sal ammoniac, chloride of ammonium), may also be employed instead of common salt for the generation of hydrochloric acid; sulphuric acid being employed as the decomposing agent.

The usual process, however, of generating hydrochloric acid is the former-namely, by mixing together common salt and commercial sulphuric acid; which is a hydrate of the real acid, or a compound of it with water. Hydrochloric acid in its uncombined state is a colourless, acid, irritating gas, possessing so

remarkable a tendency to unite with water, that it cannot be collected in a trough containing that liquid.

Hydriodic and hydrobromic acids cannot be generated in this manner, a still more indirect process of formation being necessary; but the results when obtained manifest a complete analogy between all three.

Thus hydrogen unites with chlorine, bromine, and iodine, forming three acid bodies, having very striking analogies; the force, however, wherewith it combines is not the same in either case. Thus, for instance, if chlorine be passed into hydriodic or hydrobromic acid gas, decomposition ensues, hydrochloric acid gas is formed, and iodine or bromine set free.

Here

But it is not so much for the purpose of indicating the chemical reactions of these substances that I mention them, as to point out the remarkable significance of their atomic or combining weights. is hydrogen, the lightest known ponderable substance in nature, combining with chlorine in the ratio of one part by weight to 36, with bromine in the ratio of one part by weight to 80, with iodine in the ratio of one part by weight to 125; satisfying the affinity of combinating in either case, generating powerful acids, and presenting an equality of force as measured by electrical decomposition; how great then is the active power of hydrogen in relation to its combining weight! This is one of the great

features-one of the prominent characteristics-the "points," of hydrogen, to which I especially wish to direct your attention.

And now time admonishes me to be brief. I might go on illustrating farther points of interest possessed by this beautiful element; but I must conclude. The chief features, the distinctive points of hydrogen, are, as you have seen, the exteme powers of combination with which it is endowed, associated with so little ponderable matter; the force of its combining agency with oxygen; the stability, the blandness, the extensive distribution, and manifold uses of water, the result; its intimate association with organic structures, and thence the relation it seems to have with every corporeal living form; and, lastly, the tranquil placidity of its waiting condition until, by flame or electricity, or catalysis, it is caused to unite.

HYDROGEN-ITS WEIGHT AND SPECIFIC GRAVITY.

100 C. I. To Air Hydrogen

gr. 2.1318

0.0694

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To me this waiting state of elements is one of the most beautiful topics of philosophic contemplation. When I consider the multitude of associated forces which are diffused through nature-when I think of that calm and tranquil balancing of their energies which enables elements, most powerful in themselves,

most destructive to the world's creatures and economy, to dwell associated together, and be made subservient to the wants of creation,—I rise from the contemplation more than ever impressed with the wisdom, the beneficence, and grandeur, beyond our language to express, of the Great Disposer of all!

INTRODUCTION TO LECTURE IV.

NITROGEN-ITS SYNONYMES AND ETYMOLOGY, HISTORY, NATURAL HISTORY, PREPARATION, AND QUALITIES.

SYNONYMES AND ETYMOLOGY. Nitrogen, νιτρον, Nitre ; γεννάω, I form:-Azote, a, priv., wn, life. (Lavoisier.)

Mephitic air. A term not only applied to nitrogen, but to other gases which are deleterious to life.

Phlogisticated air. (Priestley.)

HISTORY. This element was first recognised as distinct from all others by Dr. Rutherford in 1772. (Thesis de Aëre Mephitico.)

NATURAL HISTORY.-Nitrogen is found both in the inorganic and organic kingdoms. As a constituent of the inorganic kingdom, we have it (1) in the atmosphere, of which it constitutes four-fifths; (2) as a constituent of ammoniacal salts; (3) as a constituent of coal beds; (4) as a constituent of nitrates of soda and of potash-salts which are extensively diffused in some parts of the world. In the organic kingdom.

N