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Lavoisier carried on the work begun by Black. He gave the true interpretation of very many chemical changes, on the superficial qualitative examination of which the structure of alchemy had been raised.

That water had been repeatedly changed into earth was granted by all the alchemists. Water was boiled for a long time in a glass vessel; the water disappeared, and a considerable quantity of a white earth-like solid remained in the vessel. Lavoisier placed some water in a weighed glass vessel ; he closed the vessel and weighed it with its contents; he kept the water hot for 101 days, and then poured out the water into another vessel and boiled it until the whole of it had disappeared ; there remained 201 grains of solid earthy matter; he then dried and weighed the glass vessel in which the water had been heated, it weighed 173 grains less than it had weighed before the water was heated in it. Lavoisier concluded that the earthy matter was produced by the action of the water on the glass; that is to say, that the alleged transmutation of water into earth did not occur, but that the earth was a part of the material of the vessel in which the water was heated. The small difference between 201 and 171 grains was due, according to Lavoisier, to experimental errors : this conclusion was fully confirmed when more accurate methods of weighing became possible. From quantitative experiments such as these, Lavoisier drew the all-important conclusion, that the total quantity of matter which is concerned in any chemical change is the same at the end of the change as at the beginning.

Every accurate investigation conducted since the time of Lavoisier has confirmed this generalisation. Under the name of the principle of the conservation of matter, or sometimes conservation of mass, it is now one of the foundations of all modern science. Experimental proofs of this generalisation have been given in preceding paragraphs.

However we may change the form of matter, whatever transmutation we may succeed in accomplishing, there is one thing we cannot change, and that is the quantity, or mass, of matter taking part in each of these transmutations.

The statement of this principle, or law, sometimes takes such a form as this; we cannot create or destroy a single particle of matter, we can only change its form. It is important to notice that the test of creation, or destruction, is here, increase, or decrease, of the total mass of matter.

In place of the indefinite and indefinable elementary

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principles of the alchemists we have the 70, or so, elements of chemistry. Each element is a definite kind of matter characterised by its own properties which can be accurately stated frequently in terms having a quantitative signification. By bringing these elements into contact with each other under various conditions, we can accomplish stranger changes than those which alchemists dreamt of; but we know that the new kinds of matter thus produced are formed by the combinations of the elements; we have learned that no particle of any of the interacting elements is destroyed, but that the quantity of matter in the products is always exactly equal to the quantity of matter in the interacting elements.

CHAPTER V.

LAWS OF CHEMICAL COMBINATION.

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We have seen that a mixture may be made of two elements or compounds in different proportions, that the properties of the resulting mixture are the sum, or nearly the sum, of the properties of its constituents, and that the greater the proportion of one of the constituents the more nearly do the properties of the mixture resemble those of that constituent. A chemical compound, on the other hand, is wholly unlike the elements or simpler compounds from which it is formed; its properties are perfectly definite and fixed, and are different from those of any of its constituents. Does the compound differ also from the mixture in having a fixed composition ? Do the constituents of the compound combine in definite quantities? It is evident that we must quantitatively examine the composition of compounds if we desire to discover the laws of their formation.

Let us return to the first experiment by which we gained a rough notion of the difference between chemical and physical change. Let us again burn the element magnesium in air; but let the magnesium be weighed before it is burnt, and let the magnesia which is produced be collected and weighed. The result of this experiment is ;

1 gram of magnesium when completely burnt in air, or in oxygen, produces 1.66 grams of magnesium oxide or magnesia : we already know that the substance produced is a compound of magnesium and oxygen.

This result may also be stated thus ;

100 parts by weight of magnesium oxide are formed by the combination of

60 parts by weight of magnesium, and
40 parts by weight of oxygen.

There are other ways of preparing magnesia, but 100 parts by weight of this compound, however it has been prepared, can always be resolved into 60 parts of magnesium and 40 parts of oxygen.

If the compound of iron and oxygen produced by burning iron in oxygen is analysed it is found that its composition per 100 parts is ;

iron = 72:41

oxygen = 27.59. By composition per 100 parts is meant a statement of the mass of each of the elements which by their combination produce 100 parts by weight of the compound (s. par. 46).

An experiment was already described by which the substance potassium chlorate was proved to be a compound of the element oxygen and the less complex compound potassium chloride. 100 parts by weight of potassium chlorate are resolved by heating into 39.13 parts by weight of oxygen and 60:87 parts by weight of potassium chloride; if 200 parts of the chlorate are used, 78.26 parts of oxygen and 121.74 parts of potassium chloride are obtained. Potassium chloride is itself produced by the combination of the two elements potassium and chlorine in the ratio 52:41 to 47.59 ; i.e. 100 parts of the compound are composed of 52:41 parts of potassium and 47.59 parts of chlorine. The composition of either potassium chlorate or chloride is definite and unchangeable. By whatever method either of these compounds is prepared, it is always composed of the same elements combined in the same proportions.

The composition per 100 parts of the iron sulphide produced 56 by heating together iron and sulphur is ;

iron = 63.63

sulphur= 36-37. In other words the ratio of sulphur to iron is 1:1:75.

Now if a mixture is made of very finely divided sulphur and iron in the ratio 1:2, and this mixture is heated, a black solid will be formed characterised by the properties of iron sulphide; but it can be experimentally proved that the substance thus produced is not iron sulphide only, but is a mixture of iron sulphide and iron ; and further it can be proved that 2:75 parts by weight of iron sulphide have been formed and that .25 parts of iron remain uncombined with - sulphur. Again, if a mixture of 1.25 parts of sulphur with

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1:75 parts of iron is heated, 2.75 parts of iron sulphide are formed and .25 parts by weight of sulphur remain uncombined with iron. The compound known as iron sulphide is thus shewn to have a, definite and fixed composition: a certain mass of sulphur combines with a fixed mass of iron; if there is more iron than this fixed mass, the iron over and above the fixed mass-generally called the excess of iron-does not combine with the sulphur; if there is an excess of sulphur, some of the sulphur does not combine with the iron.

Experiments have been described by which water has been shewn to be a compound of the elements hydrogen and oxygen. If water is a compound, the composition of water must be definite and unchangeable.

A tube of stout glass is divided into a number of equal parts, preferably into cubic centimetres; the divisions are marked on the outside; the tube is closed at one end; two platinum wires pass through the walls of the tube near the closed end, and are bent so that the ends of the wires nearly, but not quite, touch inside the tube (s. fig. 12). The tube is filled with mercury, with proper precautions, and is inverted in a trough of mercury. A small quantity of oxygen is passed into the tube, and the volume of the oxygen is determined ; let it be 10 c.c. 20 c.c. of hydrogen are now passed into the tube. The tube is then pressed down on a pad of caoutchouc, and firmly clamped (s. fig. 13). An electric spark from an induction-coil is passed from one platinum wire to the other; combination of the hydrogen and Fig. 12. oxygen occurs instantly, and the inside of the tube is slightly dimmed by the minute quantity of water produced. The tube is now raised slightly from the caoutchouc pad ; mercury rushes in and practically fills the tube. It may be proved conclusively that water, and nothing but water, is formed in this experiment.

The result of this experiment shews that 2 volumes of hydrogen combine with 1 volume of oxygen to produce water.

Let the experiment be repeated, but with different volumes of hydrogen and oxygen.

(1) Let there be 20 c.c. hydrogen and 20 c.c. oxygen : when the mercury is allowed to rush into the tube 10 c.c. of gas will remain; this gas may be proved to be oxygen.

(2) Let there be 30 c.c. hydrogen and 10 c.c. oxygen :

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