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tained; for, as the result of all the forces brought into play, the potassium acts just as if it had a greater tendency to unite with the oxygen than the hydrogen has, with which the oxygen is previously combined. And if, again, zinc is added to hydrochloric or to sulphuric acid, hydrogen is likewise, and for the same reasons, liberated, with the simultaneous production of chloride or sulphate of zinc, as the case may be. By one of these latter methods the student invariably isolates this element, using the apparatus shown in the accompanying diagram.

The hydrochloric acid and zinc are placed in the generator (A), and some water in the wash-bottle (B) for the purification of the gas; the delivery-tube (C) dips beneath the surface of some water in a basin, while the bubbles of hydrogen are caught as they issue in test-tubes previously filled with water, and inverted, with their mouths just below the level of the water in the basin.

Fig. 1.

A

B

C

The following observations and experiments should be made with this substance :

:

Hydrogen is a transparent colourless gas, scarcely soluble in water, 1 volume of water dissolving only 015 of a volume of hydrogen. When pure it is inodorous, but, as commonly prepared, it has a peculiar odour which is easily recognized. It does not combine with oxygen at common temperatures, but if a small portion only of the gas be highly heated by the application of a burning body, that part then combines with the atmospheric oxygen present, and the heat evolved by this chemical union is sufficient to raise another portion of the gas to the temperature at which this combination takes place; this is repeated until the whole of the hydrogen is converted into its oxide water (H2O). Thus if a light be applied to the gas in a tube or other vessel having its mouth open to the air, a blue flame is seen to move slowly down till it reaches the bottom, marking, in fact, in its passage the surface of the unconsumed hydrogen.

The foregoing experiment may be so modified as to burn all the hydrogen at once; for this purpose it is only necessary to mix the gas thoroughly with half its bulk of oxygen and apply intense heat to any portion of the mixture, as by the application of a burning body; oxygen is then ready at every point to combine with the hydrogen; and so great is the heat evolved that the vapour of the water formed is expanded suddenly and immensely, and the result is an explosion. A less violent result is obtained by mixing air instead of oxygen with the gas; and by lighting either mixture in an open test-tube all unpleasant effects are obviated.

The density of this element, hydrogen, is less than that of any other known body. The density of gaseous bodies is, however, generally referred to that of air as a standard, and air being taken as 1.000, hydrogen is 0692. This low density is well exemplified by taking two tubes which have been filled with the gas, uncovering their mouths, and holding one in its ordinary, and the other in an inverted position; from the former the gas may be shown to have escaped almost immediately, for on the application of a lighted taper a few seconds only after the uncovering, no ignition will take place; the second vessel will, however, retain its contents for a considerable length of time, for it will be found that the gas in it will take fire after the lapse of some minutes, on the approach of the lighted taper.

A peculiarity with regard to this gas is, that if burnt at a jet (formed conveniently of an upright tube with a fine point and small aperture, inserted in the cork of the washing-bottle figured above, instead of the ordinary delivery-tube), and a tube of about 2-inch internal diameter be placed cautiously over the flame so as not to extinguish it, a variety of peculiar notes are produced, some of which are remarkably clear and musical.

All chemical reactions, or changes which take place when bodies act chemically upon each other, are rendered most distinctly intelligible by being placed in the form of an equation, and the student should familiarize himself with the method of so expressing them. Thus in the changes above alluded to as taking place in the production of hydrogen, the decompo

sition of water by potassium is clearly shown in the following

manner:

H0+K=KHO+H

Hydrate of
potassium.

In the same way the decomposition of hydrochloric or sulphuric acid by zinc may be exhibited ::

HCl+Zn=ZnCl+H

Chloride

of zinc.

H, SO1+2Zn=Zn2 SO1+2H

Sulphate
of zinc.

CHAPTER III.

THE ACID ELEMENTS.

UNDER this title are comprehended the remainder of the elementary bodies. The acid elements are also sometimes termed “non-metallic elements" and "salt radicals." Although for the most part possessed of properties powerfully antagonistic to those of the basic elements, they do not all bear to each other the strong resemblance exhibited in the case of the latter class of bodies; they may, however, be arranged in subdivisions, each containing a few members which present among themselves many points of accordance. The state of physical aggregation in which these substances exist, is as varied as that which the basic elements present; the majority of them occur in the solid form, one, bromine, in the liquid, and several in the gaseous condition. The class of acid elements may for convenience sake be thus subdivided:

1. Bodies which, in combining with hydrogen, lose nothing of their acid character:

Chlorine Cl. Bromine Br. Iodine I. Fluorine=F.

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2. Bodies, the acid character of which is masked by combination with hydrogen :

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Oxygen=0. Sulphur S. Selenium - Se. Tellurium-Te. 3. Bodies which lose their acid character completely by combining with hydrogen :

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Carbon C. Boron-Bo. Silicon Si. Tantalum Ta. Niobium=Nb. Pelopium-Pe. Titanium=Ti.

4. Bodies which acquire a basic character by combining with hydrogen :

Nitrogen N. Phosphorus P. Arsenic-As.

SUBDIVISION I.

CHLORINE, BROMINE, IODINE, FLUORINE.

The three former of these substances are known, the fourth has never yet been isolated; they are possessed of the most energetic tendency to combine with the basic elements, and it is this obstacle which has always hitherto prevented the isolation of the last member of the group, fluorine. The intensity of their chemical attraction for the bodies most opposed in chemical character is well exhibited by the fact that many metals, upon simply being brought into contact with these substances, ignite, burning in the act of combination; and as may be readily supposed, the compounds thus formed are as difficult of decomposition as they are easy of production. The first member, chlorine, is a gas; the second, bromine, a liquid; and the third, iodine, a solid: fluorine is believed to be a gas. As will be seen hereafter, the three first members of this group exhibit a most singular family resemblance both in their chemical and physical properties, and it is generally thought that some far more intimate connexion subsists between them than any of which we are at present aware.

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I. CHLORINE Cl. The non-metallic element, or salt-radical chlorine, is a gas at all ordinary temperatures and pressures, but under a considerably increased pressure it liquefies; no combination of cold and pressure has, however, yet proved capable of effecting its solidification.

Chlorine occurs in nature in combination with many of the metals, but its most abundant source is common salt, NaCl, which exists so largely in the sea and in extensive deposits in different parts of the earth.

From common salt, chlorine is frequently prepared by a method which consists essentially of two processes joined into one: -the mode of proceeding is to add sulphuric acid (H, SO,) to a mixture of black oxide of manganese (Mn, O2) and common salt (NaCl), and to apply heat, when chlorine is given off, and may be collected either over hot water or by displacement. For the better understanding of this process, the action may be supposed to be divided into two parts; the first consisting of the addition of sulphuric acid (H, SO,) to chloride of sodium (NaCl): here a simple interchange between the basic and acid elements occurs, the result being the production of sulphate of soda (Na, SO1) and hydrochloric acid (HCl), thus

H2SO,+2NaCl = Na, SO1+2HC1; and the second action being that of the black oxide of manganese, Mn, O,, upon the hydrochloric acid, the final result being represented thus—

2

Mn, 02+4HCl=2MnCl+2H,O+2C1.

A very usual way of preparing chlorine is by performing the latter only of the processes given above, that is, to commence with Mn, O, and HCl, hydrochloric acid being a cheap article of commerce. The student will do well to prepare chlorine by both methods, and to observe the following properties of the gas, collecting it both by displacement and over warm water :—

(a) It has a peculiar yellowish-green colour, whence its name; it has also a peculiar unmistakeable odour: both of these characters it imparts to water when dissolved in that liquid.

(3) It has the property of bleaching almost all animal and vegetable colours; the presence, however, of a trace at least of water is necessary before submitting the coloured substances under examination to the action of the gas.

(y) It exerts a very singular action on some bodies containing carbon and hydrogen, by reason of its tendency to combine

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