CHEMICAL THEORIES

5

sequently the constants we arrive at, in a certain group of phenomena, need not of necessity form the limits of our knowledge, but may in turn form the subject of research, if we investigate the conditions under which they vary, and in this way arrive at constants of a higher order. But in spite of all. the progress we have made the determination of the constants still remains the problem for investigation. We are content when we succeed in predicting the phenomena which result as a natural consequence from certain constants, and the varying relations which these constants bear to each other.

§ 4. Development of Chemical Theories.-The inductive method was first applied in chemistry at a comparatively late stage in its history. It was only at the end of the seventeenth, and more particularly during the eighteenth century that all the then known facts were systematically arranged and a logical classification of bodies into combustible and incombustible, burnt and unburnt, was made. The hypothesis which was employed to account for the difference between the two large classes of bodies proved incorrect. This hypothesis assumed the existence of a peculiar combustible principle, the so-called 'phlogiston,' in all combustible substances. Combustion consisted in the evolution of phlogiston. In recent times it has been shown that the phlogiston theory is not altogether devoid of truth. For what was formerly termed phlogiston is almost identical with our present notion of potential energy. It was during the two hundred years when the phlogiston theory prevailed that the application of inductive methods revealed the general truth that matter can neither be created nor destroyed. This discovery led to conclusions rendering the doctrine of phlogiston untenable, and resulting in its replacement by Lavoisier's theory of combustion. According to this theory the process of combustion is not due to an evolution of phlogiston, but to 'oxidation'—that is, to the combination of the combustible body with oxygen, one of the constituents of atmospheric air.

During the period of quantitative analysis, which begins with this theory, great stress was laid on the investigation of the proportions by weight in which different substances unite together, and thus a new field of research was opened up, which rapidly acquired unexpected dimensions.

The most important result of this new development was to strengthen our knowledge of the fact that nothing is lost and nothing is gained when substances undergo chemical change. When substances unite together, the weight of the compound is exactly equal to the sum of the weights of the constituents. When several bodies act on each other, it was formerly a difficult matter to decide which were compounds and which were constituents; but by the light of this new law the question can easily be answered. When red-hot iron is hammered it yields forgescales, and on exposure to damp air it rusts. In either case it gains in weight; consequently it has combined with something, and not lost anything as was formerly supposed. It has combined with oxygen, and the increase in weight is equal to the weight of oxygen the metal has united with in its conversion into oxide (rust or forge scales). Consequently the oxide is the compound and the metal is a constituent; but in the last century the reverse was held to be the case. In this way ' quantitative chemistry' effected an accurate distinction between elementary bodies and their compounds, and imparted a degree of exactness to the methods of investigation, of which in previous centuries there had been no conception.

We are acquainted with about seventy bodies which have up to the present time resisted all attempts to decompose them. We therefore consider these substances as invariable in composition until the contrary is proved, and consequently regard them as the fundamental constants of chemistry. The aim of the science of chemistry is to investigate the laws which govern the combination of these elements, and to determine in what way the character and properties of the compounds are affected by the nature of the constituent elements.

1

§ 5. Stachiometric Laws.-The further investigation of the quantitative composition of chemical compounds led to the foundation of the science of stœchiometry by Jeremias Benjamin Richter. The most important facts of stœchiometry were discovered almost simultaneously by Proust. The fact pointed out by Proust, that definite chemical compounds always contain their constituents in fixed and invariable proportions, was strongly disputed by no less an authority than C. L. Berthollet.

1 Tà σTOIXEîα, the constituents. μéтpov, the measure.

LAW OF MULTIPLE PROPORTIONS

7

Richter's views on the laws which govern the combination of acids with bases to form salts remained for a long time neglected and almost unnoticed. The credit of establishing the value of these laws (so far as they were correct) belongs to J. J. Berzelius, who obtained important aid from an hypothesis propounded by John Dalton.

The fundamental law of stochiometry, discovered by Richter and confirmed and developed by Berzelius, states that all true chemical changes (i.e. changes of composition) take place between definite volumes or weights of the substances. This is equally true whether a substance decomposes into its constituents or is formed from its constituents, or when different compounds exchange one of their constituents.

When water is formed from its constituents 7.98 parts by weight of oxygen unite with one part by weight of hydrogen, never more or less, and the two constituents are produced in exactly these proportions when water is decomposed.

All other substances, whether elements or compounds, behave in the same way; that is to say, they only enter into combination or undergo decomposition in definite and fixed proportions by weight.

It often happens that the bodies unite together in several distinct proportions, but these different proportions always bear a simple relation to each other.

This empirical law is known as the law of multiple proportions. For example, there is another compound of hydrogen and oxygen, hydrogen peroxide, which contains 15.96 parts by weight of oxygen to 1 part by weight of hydrogen-that is, twice as much oxygen as unites with 1 part by weight of hydrogen in water. By mixing these two oxides of hydrogen a liquid is obtained in which the quantity of oxygen lies between that contained in water and in hydrogen peroxide. The resulting liquid is not a chemical compound, but merely a mechanical mixture, for its properties are those of its constituents, and the act of admixture is not followed by those changes in the material nature of the substances, which are characteristic of chemical combination.

Nitrogen forms a larger number of oxides, in which one part by weight of nitrogen is combined respectively with 0.5696, 1.1392, 1.7088, 2.2784, and 2.8480 parts by weight of oxygen.

The relation between these quantities is expressed by the whole numbers 1, 2, 3, 4, 5.

The numbers indicating the proportions in which substances unite together are called 'combining weights,' or stœchiometric quantities. It is remarkable that they apply not merely to two given elements but to all elements without exception. For example, one part by weight of copper is combined with 0.1263 part by weight of oxygen in cuprous oxide, and with 0.2526 part by weight of oxygen (i.e. exactly double) in cupric oxide. The quantities of sulphur combined with one part by weight of copper in the sulphides are also in the proportion of 1 to 2, cupric sulphide containing 0.5062 and cuprous sulphide 0.2531 part by weight of sulphur; 0.2531 part by weight of sulphur on combustion unites with 0.2526 part by weight of oxygen. This is exactly the quantity of oxygen which unites with one part by weight of copper to form cupric oxide.

The combining weights for copper and sulphur and for copper and oxygen are also valid for the compounds of sulphur and oxygen. This rule is true of all elements. It may be generally expressed in the following words :

If we know the proportions by weight in which a series of elements unite with a certain given element, then these elements either unite with each other in the quantities represented by these proportions or in some simple multiple of them. If A, B, C, D represent the proportions by weight in which the different elements unite with a definite quantity of another element, then any compound of these elements can be represented by the formula

n. A+ n1.B+ng. C + nz. D+...

when n, m, n,, ng represent whole (generally small) numbers. The values A, B, C, &c. are the fundamental constants of stœchiometry.

§ 6. Atomic Hypothesis.-The stochiometric laws are purely empirical, and were discovered by induction. They have been confirmed by thousands of experiments, and their validity is independent of any hypothesis. But the human mind suspects a cause for every law, and is disinclined to acknowledge the existence of a law unless it can account for the cause of it.

Consequently the stœchiometric laws, which are now regarded as the most important ever discovered in natural science, were at first treated with neglect, until John Dalton investigated these laws and discovered a simple explanation of them.

Dalton investigated two gaseous compounds of carbon and hydrogen, and found that the so-called heavy carburetted hydrogen now called ethylene contained exactly half as much hydrogen combined with one part by weight of carbon as is the case in light carburetted hydrogen or marsh-gas. To explain this and similar observations concerning the oxides of nitrogen Dalton made use of an old and much-disputed hypothesis. He assumed that all elements consist of very minute indivisible particles, having a definite weight, termed atoms,' and that chemical compounds are produced by the union of these atoms.

This hypothesis was by no means new. More than two thousand years ago the Greek philosophers energetically debated the continuity of matter-whether matter completely fills the space it occupies, or whether it is composed of very minute individual particles separated from each other by spaces. These particles were termed atoms on account of their indivisibility.

Democritus and many others based their system of natural philosophy on these hypothetical atoms, and attempted to explain the transformations of the universe as a result of their properties and rapid movements. Aristotle and his followers could not tolerate the idea of the existence of an empty space between the atoms, but maintained that the whole space is completely filled with matter. This difference of opinion survived till recent times, but at the present day the truth of the atomic theory is no longer a matter of dispute. Dalton does not appear to have troubled himself about this discussion. He made use of the atomic theory because it enabled him to explain without difficulty the fact that the elements combine only in definite proportions by weight, and that if certain elements unite together in several different proportions these proportions bear a simple relation to each other. We assume that all the atoms of one and the same element have the same weight, but that this weight varies for different elements in the proportion of their stœchiometric quantities. Let A be the weight of the

1ǎToμos, the indivisible.