§ 431. Thus, then, we have demolished the pretensions of our first classical element, air, to rank amongst the "first or constituent principles of things," and have established three chemical elements in its stead; namely, azote, oxygen, and carbon. The two first have never yet had their particles brought within the verge of cohesive attraction, and have resisted all attempts to liquefy them; while the utmost powers of heat have been applied in vain to destroy the cohesion of the last, or to bring it into the state of vapour.

Amongst the ingredients of the atmosphere, we have also found a second classical element, water, into the pretensions of which to the elementary character we must next proceed to inquire.

§ 432. We must attempt its analysis by the means which we have already applied successfully in similar cases. If we throw a piece of the metal potassium upon water, it will float upon it: intense action will take place; a flame will burst forth from the point of contact, and the metal will disappear. After this operation we shall find that the water has become strongly alkaline, and upon evaporation we shall obtain a white solid, exactly similar to that which was obtained by burning potassium in common air or oxygen. We infer, then, that the metal has taken oxygen from the water, and that the compound has been dissolved as it was formed. But if so, what is the substance with which the oxygen was combined? To determine this, we must operate more carefully, that nothing may escape in the elastic form; and, as the action is violent, we must proceed with caution. For this purpose we may fill a strong narrow glass jar with water, and invert it upon the water-trough, and wrapping a piece of potassium closely in paper, or placing it in a small glass tube closed at one end and carefully covering it with fine sand, slip it dexterously under the edge of the jar. The metal is momentarily protected from the water, which, however, soon reaches it; a smart concussion takes place, and a quantity of gaseous matter rises to the top of the jar. Upon reversing the jar, and presenting a lighted taper to the gas, we shall find that it will burn with flame; that is, that it will enter into combination with the oxygen of the air, with the extrication of light and heat.

§ 433. This decomposition of water may be accomplished

by less costly means than potassium, making use of the elective affinity of more common metals for oxygen.

Iron may be employed for the purpose; but it will be necessary to exalt its affinity by heat. For this purpose, turnings of iron may be put into a gun-barrel placed across a furnace, one end of which is connected with a small water-boiler, and the other with a bent tube passing under the edge of a jar upon the pneumatic trough. Upon heating the iron to redness, and allowing the steam of boiling water to pass over it, abundance of inflammable air may be collected; and the same compound which is formed by the combustion of iron wire in oxygen may be extracted from the barrel (103).

§ 434. The affinity of zinc may also be had recourse to, and that at ordinary temperatures. But for this purpose it is necessary to remove the compound of the zinc and oxygen as forms, which would otherwise incrust the surface, and put an end to the process. This oxide is not soluble in water, like the similar compound of potassium; but by the addition of a little sulphuric acid, or oil of vitriol, it will form a secondary compound, which will instantly be dissolved. Large quantities of inflammable air may thus be readily and economically procured.

The compound of oxygen and zinc, again, may be separated

(103) a represents a glass retort containing a little water, which may be raised in steam by the application of a lamp. The steam then traverses the tube, c c, in which is placed some iron wire heated to redness by the furnace, b b. The iron combines with the oxygen of the water, and the inflammable gas passes into the worm, d d, which uncombined steam, is kept cool by the water in the tub. Here with which it may be mixed, is condensed, and the water drops into the vessel, c, placed for its reception. The gas ultimately passes out of the pipe, f, and may be burned or collected in the usual way.

any

from the acid by elective affinity; for the addition of some of the alkaline solution of the compound of potassium and oxygen will abstract the acid and precipitate the oxide (104).

435. The inflammable air, which has been named hydrogen because it enters into the constitution of water, burns, as we have seen, in common air, but extinguishes flame, as may be ascertained by plunging a lighted taper into it. When a mixture is made of two parts of common air and one of hydrogen, or of two parts of hydrogen with one of oxygen, the combination, which proceeds silently when a moderate jet of the inflamed gas issues quietly into the atmosphere, takes place at once throughout the mass with a strong detonation, owing to the mutual percussion of the particles of the highly-expanded air returning into the vacuum which is produced. When the mixture is merely confined by a film of water in a soap-bubble, the explosion is sharp and deafening (§ 8).

§ 436. If a current of hydrogen be inflamed when issuing from a small aperture, and a tube to be held over the flame, a musical tone will be produced, varying in its pitch with the length of the tube. These sounds depend upon a series of little explosions which succeed one another with sufficient regularity and rapidity to produce a continuous sound (§ 52). By the strong draught which takes place through the axis of

(104) This figure represents a convenient apparatus for the generation of hydrogen gas in small quantities from zinc. a is a funnelshaped glass vessel, the lower extremity of which

dips into the dilute sulphuric acid contained in the bottle, b. A bent tube proceeds from the upper end of the bottle, which is furnished with a stop-cock at d, and terminates in a narrow jet. From the aperture c, which may be closed with a ground stopper, a piece of zinc is suspended in the acid, and the hydrogen which is disengaged rises to the top of the bottle, and forces the liquid into the upper vessel, the pressure of which will drive the inflammable gas through the pipe d, whenever the stop-cock is opened. As the gas passes out, the liquid will descend again into the bottle, and renew the disengagement of the gas; and the process will go on till the acid becomes saturated with the oxide of zinc.

the tube, the air and the hydrogen are made to form portions of explosive mixtures, which are fired by the contiguous burning parts. Sometimes the draught becomes so strong as to blow out the flame.

§ 437. We are so habituated to speak of the process of combustion, with reference to our atmosphere, and to name bodies combustible which enter into combination with its oxygen, that a prejudice is likely to arise from it with regard to the classification of objects. We constantly see a stream of hydrogen burning in common air, and we call the first a combustible; and the second, a supporter of combustion; but by throwing a jet of oxygen into an atmosphere of hydrogen contained in a large bell-jar, through an aperture at its upper end, when the latter is burning, the flame will be carried down into the body of the jar, and the oxygen will continue to burn in the hydrogen as it issues from the jet. In this case the oxygen may be said to be the combustible, and the hydrogen the supporter. The simple statement of the fact in both cases is, that oxygen and hydrogen combine together, and combustion—that is, the disengagement of light and heat-is the consequence.

$438. Hydrogen gas is speedily fatal to animal life when taken pure into the lungs, but may be respired when mixed with common air, without much injury: it is the lightest form of ponderable matter, and 100 cubic inches only weigh 2.15 grains. Hence its application to aërostation. A thin bladder, or soap-bubble, filled with it rises in the air with rapidity, for the same reason that a cork rises through water. A balloon formed of a sphere of ten feet diameter would contain about 32 lbs. of atmospheric air, but the same bulk of hydrogen would only weigh two pounds; hence, with an ascensional force of two pounds and a half, it would rise in the air with a weight of twenty-eight pounds attached to it. Its levity may also be shewn by reversing the process by which we proved the great weight of carbonic acid (§ 426) viz., by displacing the common air from an inverted jar by upward decantation.

Hydrogen must be deemed an element, because it has resisted all attempts to decompose it: no degree of cold or pressure has yet been found sufficient to reduce it to the liquid state. Its combustibility was known in the beginning of the eighteenth century, when it was often exhibited as a curiosity; but its

nature and properties were first properly investigated by Mr. Cavendish, in 1776.

may

§ 439. Synthesis will confirm this analysis of water; for if we carefully collect the product of the combustion of hydrogen and oxygen gases, we shall find the water in weight exactly equal to that of the gases consumed. The experiment requires some caution, on account of the violence of the action, but be easily performed in the following manner.-Provide a very strong glass tube, closed at one end, and fitted at the other with a brass cap and stop-cock, strongly fixed with cement (105); two small holes must be drilled in the upper part of this tube, into into which two wires must be cemented, the points of which nearly touch on the inside. Let a mixture of very pure oxygen and hydrogen gases be very accurately made, in the proportion of one volume of the former to the two of the latter, in a jar fitted with a stop-cock, to which the cock of the tube may be screwed; extract the air from the tube, by means of the air-pump; fill it with the mixture, and again exhaust it; and once more fill it with the mixture, and carefully close the stop- D cock; pass an electric spark between the wires, and the gases in the tube will explode with a bright flash, but silently, on account of the connexion with the surrounding air being cut off. Allow the tube to cool, and, upon opening the cocks, a fresh portion of the gases will rush in which will be equal to the first quantity, provided the mixture has been accurately made, and the common air perfectly extracted. This

(105) A is a strong glass vessel, fitted with a glass cock E, accurately ground. At the upper end it is provided with a glass stopper, firmly screwed down, and secured by the brass collar and screws, c. Two platinum wires DD pass through this glass cock, and approach one another in the interior, without touching. This vessel may be exhausted by the airpump, and screwed upon the bell-glass, B, filled with the proper mixture of oxygen and hydrogen. When the cocks, E E, are opened, the air in the jar will, of course, rush into the vacuum in A, and then, after carefully closing the stop-cocks, an electrical spark may be passed between the wires, and the gases will combine with a vivid flash of light.

E

B