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of the experiment, and as we know (i) that the symbol of an elementary gas (with a few exceptions) represents the mass of

Fig. 16.

it which occupies 1 volume (i.e. the volume occupied by unit weight of hydrogen), and (ii) that the formula of a compound

106

107

gas represents the mass of it which occupies 2 volumes (v. ante, par. 88), the foregoing equation tells that 2 volumes of watergas react with 2 volumes of chlorine gas to produce 4 volumes of hydrogen chloride gas and 1 vol. of oxygen gas.

H2O+2C1 = 2HCl + O.

vols. 2 + 2 give 4 + 1.

=

The volumes on each side of the sign are not the same; the masses on each side of the sign = are, and in chemical equations always are, the same.

Our study of the properties of water has served to illustrate the nature of chemical change; to emphasise the distinctions between mixtures, elements, and compounds; to shew the importance of the laws of chemical combination; to familiarise us with the use of chemical formulae and equations; and to illustrate the meanings of the terms analysis and synthesis. This study has kept before us the notion of each element and compound interacting chemically with other elements and compounds in certain definite masses which are all simple multiples of one and the same mass. It seems as if a quantity of water, for instance, were composed of a vast number of little particles of water the masses of all of which are the same, and as if chemical interaction occurred between 1, 2, 3,......n of these little particles and a definite number of little particles of the element or compound with which the water interacts. The chemical conception of every element or compound having its own reacting weight leads to some such physical conception as this of small definite particles. Finally, the slight examination we have given to the chemical properties of water has shewn very clearly how closely interwoven chemical changes are with physical changes, and how impossible it is to arrive at any trustworthy conclusions regarding either otherwise than by quantitative experiments and accurate reasoning.

Air. When magnesium is burnt in air magnesia is produced; but magnesia is a compound of magnesium and oxygen, therefore, the chemical change which occurs during the burning of magnesium in air consists in the combination of magnesium and oxygen. Therefore, in all probability, oxygen is a constituent of air. When mercury is heated in a measured quantity of air, mercury oxide is produced, and some of the air disappears; when the oxide is collected and strongly heated, oxygen and mercury are formed, and the quantity of oxygen is equal to the quantity of air which disappeared.

Therefore the heated mercury combined with a part of the air in which it was heated, and this part was oxygen (v. ante, par. 18). From these experimental results we may conclude that if magnesium or mercury is burnt in air, the air which remains after burning will almost certainly differ in properties and composition from the air which was present before burning began. And if this is so we may further conclude that when any element which is known to combine with oxygen is burnt in an enclosed volume of air, the whole or a part of the oxygen in the air will combine with the element, and the air which remains will most probably differ from the original air.

Phosphorus is an element which is easily burnt, and which 108 very readily combines with oxygen.

Let an apparatus be arranged as shewn in fig. 17. glass jar; the space from the cork to within about inches of the open end is divided into 5 equal

parts. The jar is placed, open end down

A is a

3 or 4

wards, in such a quantity of water that the
level of the water stands at the point where
the graduation of the jar begins. B is an
iron cup supported on an iron pillar with
a broad foot. Let a piece of dry phosphorus
be placed on B; let the jar be put over the
phosphorus and iron stand; let the end of
the brass chain be highly heated, and then
let the chain be brought quickly into the jar,
as shewn in the figure, so that the heated
part of the chain touches the phosphorus.
The phosphorus begins to burn, white clouds
of phosphorus oxide fill the jar, and the
water slowly rises in the jar. When the
burning is finished and the clouds have dis-
appeared the phosphorus oxide produced
dissolves in the water-let water be poured
into the outer vessel until the level of the
water inside and outside A is the same. It is seen that
the air has disappeared. Withdraw the cork, and plunge a
lighted taper into A; the flame is instantly extinguished.
Therefore the air in A after the burning of phosphorus is
not the same as the air before the phosphorus was burnt.

A

Fig. 17.

of

By a little careful manipulation, portions of the air which remain after the phosphorus is burnt may be transferred from A to glass tubes or bottles, and the properties of this air may

be examined. It is found to be a colourless, odourless, gas, a very little lighter, bulk for bulk, than ordinary air; it does not support combustion, nor is it combustible; it reacts chemically with but few elements and compounds. Every attempt to separate a specified mass of this gas into unlike parts has failed. But very many compounds are known of each of which this gas is a constituent. The gas is an element; it is called nitrogen.

We know that oxygen is also an element. Hence we have obtained from air two elements nitrogen and oxygen. 109 Is air a compound or a mixture of these gases?

110

111

If it is a

compound, the properties of air must differ considerably from the properties of either oxygen or nitrogen; and these elements must be united in air in a ratio expressed by the formula NO, where x is 1, 2, 3, 4...n times the combining weight of nitrogen (14), and y is 1, 2, 3, 4...n times the combining weight of oxygen (16). If air is a mixture of nitrogen and oxygen, it must be possible to recognise both of these elements in air by making use of the properties which each possesses when unmixed with other kinds of matter.

Whichever hypothesis is adopted as a guide in experimental inquiry, we must begin by determining the properties of air, the properties of oxygen, and the properties of nitrogen. We already know some of the properties of oxygen and nitrogen. Both are colourless, odourless, gases; nitrogen is 14 times, and oxygen is 16 times, heavier than hydrogen. Combustible bodies burn rapidly and brilliantly in oxygen, but they cease to burn in nitrogen. The ratios of diffusion of both are nearly equal; but oxygen passes through a thin sheet of india-rubber about 2 times more rapidly than nitrogen. Oxygen is slightly soluble, nitrogen is less soluble, in water; 1 vol. of water at 16° dissolves 0295 vols. of oxygen, and 0145 vols. of nitrogen. The combining weight of oxygen is 16, and the combining weight of nitrogen is 14.

The prominent physical properties of air are known to all. Accurate analyses of air have shewn that 100 parts by weight of dry air freed from carbon dioxide (v. infra, par. 113) are composed of 23 parts of oxygen and 77 parts of nitrogen, by weight. The simplest formula which will fairly accurately represent this composition, assuming air to be a compound, is N,,O,,; this compound, if it existed, would be composed of 22.5 parts of oxygen by weight, and 77.5 parts of nitrogen, per 100 parts. Five definite compounds of nitrogen and oxygen are known;

51 13

51 13

their compositions are represented by the formulae NO, NO, NO, NO, NO,. It is improbable that a sixth compound of these elements should exist having as complex a composition as N,O, But this is the simplest formula which can be given to air if air is a compound of nitrogen and oxygen. Hence the argument based on analogy of composition leads to the conclusion that air is probably a mixture and not a compound.

Assuming air to be a mixture of nitrogen and oxygen, we next inquire, what volume of air ought to be dissolved by 1 vol. of water, say at 16°? The solution of a mixture of gases by a liquid between which and the gases there is no chemical interaction follows the same course as if each gas were dissolved separately in the liquid. The solution of a gaseous compound, on the other hand, in a liquid which does not interact chemically with the compound follows a course of its own; the vol. dissolved is independent of the vols. of the gaseous constituents of the compound dissolved under the same conditions.

1 vol. of water at 16° dissolves 0295 vols. of oxygen, and 0145 vols. of nitrogen; now 1 vol. of dry air freed from carbon dioxide (v. infra, par. 113) is composed of 2096 vols. of oxygen and 7904 vols. of nitrogen; therefore, if air is a mixture, 1 vol. of water will dissolve (·0295 × ·2096) + (·0145 × ·7904) 01765 vols. of air, at 16o.

=

Experiment proves that 1 vol. of water dissolves '0177 vols. of air at 16o.

1 vol. of water at 16° dissolves 7535 vols. of nitrous oxide (NO); but if this gas were a mixture of nitrogen and oxygen in the ratio in which these gases unite to form 1 vol. of nitrous oxide, water would dissolve 02925 vols. of the nitrous oxide.

These calculations and experiments shew that air is dissolved by water exactly as if the air were a mixture of oxygen and nitrogen and not a compound of these elements. In other words: one of the properties of oxygen is to dissolve in water to a certain definite extent, and one of the properties of nitrogen is to dissolve in water to a certain definite extent; but both oxygen and nitrogen retain this property when they are present in air; therefore air is a mixture, and not a compound, of oxygen and nitrogen.

If

We may carry the inquiry further on the same lines. air is a mixture of oxygen and nitrogen, and if oxygen passes through a thin sheet of india-rubber about 2 times quicke

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