ACCOMPANYING CHEMICAL COMBINATION.

63

gen and oxygen did exclusively combine in the proportion in which the two gases had been originally evolved from water. The hydrogen and oxygen afterwards respectively admitted in excess took no part in the formation of water.

The essential changes which the properties of the elements undergo in associating to form chemical compounds are sufficiently obvious from experiments with which we are already familiar.

It would be impossible to quote a more instructive example

FIG. 59.

of such changes than that furnished by the transformation of a mixture of hydrogen and chlorine into hydrochloric acid. In the mixture of the two gases obtained by the electrolysis of hydrochloric acid, the properties of each gas are easily recognized. The mixture retains the fundamental property of hydrogen; it is still inflammable. The mixture also possesses the yellowish-green colour of chlorine, its odour, and its bleaching powers. On treating the mixture with water, its soluble constituent is absorbed, its colour, odour, and bleaching powers become weaker and weaker, until at last there only remains the colourless, incdorous, tasteless hydrogen. Now let the mixture, by one of the processes with which we have become acquainted,

64

MECHANICAL MIXTURE -CHEMICAL COMBINATION.

be converted into a chemical compound, and instead of the yellowish-green, colour-bleaching, suffocating chlorine, sparingly soluble in water—instead of the inodorous, tasteless, inflammable hydrogen-we have a colourless gas, possessing no longer the slightest bleaching power, absorbed by water with avidity, of pungent odour and taste, and utterly incapable of combustion.

In like manner, even a cursory comparison of the properties of hydrogen and oxygen gases with those of the liquid water which they produce by their union exhibits differences as striking as can well be conceived; but even in water-gas (dry steam), the fundamental property of hydrogen (inflammability), and that of oxygen (power of supporting combustion), are found to be entirely extinct.

Not less striking is the change in the properties of hydrogen and nitrogen, when associated to form ammonia. Two elementary gases, inodorous, insoluble in water, without action on vegetal colours, are converted by chemical combination into a gaseous compound, possessing a most penetrating odour, capable of restoring acid-reddened litmus-paper to its pristine blue colour, and so intensely and rapidly soluble in water, that the fluid rushes into a tube filled with ammonia, as into a

vacuum.

The preceding remarks sufficiently mark the difference between mechanical mixture and chemical combination. In mechanical mixtures the elements are capable of interfusion in any proportion whatever; in chemical compounds they unite in definite immutable proportions, volumetric and ponderal. The mechanical mixture exhibits properties intermediate between those of its ingredients; in the chemical compound the properties.of its constituents become extinct, their individuality being, so to speak, merged in the formation of a new body, with new properties.

The recognition of these marked differences between mere mechanical mixtures and definite chemical compounds very naturally leads us to examine the conditions under which the

CHEMICAL ACTION OF LIGHT AND HEAT.

65

former become converted into the latter. In this case, too, the formation of hydrochloric acid and of water affords us welcome elucidations.

A mechanical mixture of hydrogen and chlorine, as furnished by the electrolysis of hydrochloric acid, may, if due care be taken to protect it from the action of light, be preserved for an indefinite period without undergoing the slightest change. Under the influence of ordinary diffuse daylight, the transformation of the mixture into a compound is accomplished, as we have seen, in the course of a few hours. Direct sunlight, or certain artificial lights of great intensity, effect the transition instantaneously (comp. p. 46). Combination then takes place with violent explosion, sometimes shattering the vessel which contained the mixture. Transformation of the mixture into a compound may in this case also be accomplished by contact with a burning body, or by the passage of an electric spark; both which processes are attended by explosive violence.

The transformation of a mechanical mixture of hydrogen and oxygen into the chemical compound, water, is less easily accomplished. It appears from recent experiments, that they also may be caused to combine by sunlight though only on very protracted exposure to its influence. On the approach of a flame, however, or the passage of the electric spark, the mixture instantaneously explodes, and is, at the same moment, converted into water.

Hence it appears that mechanical mixtures are often transformed into chemical compounds by the action of light, and, more frequently still, by the powerful influence of heat. The experience acquired in the study of hydrochloric acid and water cannot, unfortunately, be extended by the similar examination of ammonia, seeing that no process is known for directly effecting the transformation of a mixture of hydrogen and nitrogen into a compound thereof. But this exception renders it the more necessary to mention that the conditions which determine the formation of hydrochloric acid and water from mixtures of their elements suffice for the accomplishment of

66

CHEMICAL EVOLUTION OF LIGHT AND HEAT.

like results in an endless variety of cases; heat being especially distinguished as the most general promoter of chemical transformation.

When proceeding hereafter with the special study of the individual elements, we shall have frequently to resume the consideration of these conditions of chemical change. Opportunities will then also occur for examining, minutely, certain interesting concomitants of chemical activity; such as, for instance, the remarkable development of light and heat which we have already seen attending the reactions in several of our illustrative experiments.

LECTURE IV.

Chemical symbols- their nature and value — diagrammatic symbols initialed-figured--symbolic equations constructed therewith-information thereby conveyed-formulæ thence derived-summary of information condensed in chemical formula-hydrochloric acid, water, and ammonia, considered as types of chemical combination-hydrobromic and hydriodic acids their construction upon the type of hydrochloric acidponderal analysis and volume-weights of these compounds-volumeweights of bromine and iodine gases-sulphuretted and selenetted hydrogen-their construction upon the type of water-ponderal analysis and volume-weights of these compounds-volume-weights of sulphur and selenium gases.

THE principal facts determined by experiment, as to the composition of hydrochloric acid, water, and ammonia, are susceptible of clear and concise expression in a few happilychosen symbols, which experience has shown to be so powerful as instruments of chemical research, and so invaluable as adjuncts of chemical nomenclature, that they may justly claim precedence over all the subjects pressing for attention at this early stage of our inquiry.

If equal volumes of hydrogen, chlorine, oxygen and nitrogen. (taken, of course, at like temperature and pressure) be represented by equal squares, having the initials of these elements inscribed therein, the composition, by volume, of hydrochloric acid, water, and ammonia, may be thus expressed :

By inscribing within these symbolic squares the volume-weights