the existence of which cannot be demonstrated, and which, when more closely examined, prove to be imaginary, was regarded as a mischievous fallacy. It was Robert Boyle, who, about the year 1660, first expressed himself decidedly on this subject, and so laboured for the recognition of this truth, that we must regard it as having been established by him. From that time forth those substances only were regarded as the constituents of bodies, which could be extracted from them by analysis, or which had been added to them by synthesis, and those substances only were considered to be elements which defied all known means of analysis. Further, these chemical elements were regarded as fundamentally different kinds of matter of the simplest constitution then known.

What a contrast is exhibited between the ancient idea of the cause of difference in various forms of matter and that which obtained from the time of Boyle! If we consider these two opposite conceptions historically, and the transition from one to the other, they appear like two totally dissimilar pictures; but, like dissolving views, changing, the one into the other by slow degrees. In the first place we have the Aristotelian idea, according to which, matter itself devoid of properties, becomes endowed with characteristic qualities by the addition of properties, and forms, when invested with these properties, the various substances known in nature; then this idea passes gradually into that of the alchemists, but becomes confused in the transition, inasmuch as the differences of physical conditions and properties are no longer regarded as the only causes of variety in substances; the difference in chemical properties receives more attention, the existence of elements, the producers of such properties is assumed; and thus the path is prepared which leads to the idea of chemical composition. Then we see the Aristotelian theory gradually becoming indistinct, whilst the idea of the importance of the chemical deportment and composition of bodies assumes prominence, and at last we see clearly that the differences between the substances which nature presents to us in such overpowering numbers, or which we have ourselves formed artificially, depend upon differences in their chemical composition. The idea of chemical composition, which makes its first appearance indistinctly in the history of the chemistry of the middle ages, now forms the foundation of the science. In modern times a clear comprehension of it is the essential condition of all progress.

Turning to modern chemistry: we now recognize sixty-three different simple forms of matter which we term elements, and which, when combined, form all known compound substances. The dissimilarity of these elements and the difference in the modes and proportions according to which they are capable of combining, are now believed to be the causes of dissimilarity in those substances which are regarded as chemically different. Such dissimilar com

pounds are regarded as being composed either of different materials, or of the same materials in different proportions. Tin, silver, mercury, and sulphur are regarded as fundamentally dissimilar substances; but in wood, in alcohol, in acetic acid, and in many other bodies, the same substances, viz. carbon, hydrogen, and oxygen, are contained in different proportions.

Until about the year 1830 it was believed that the chemical differences of bodies depended solely upon the causes just enumerated. It was supposed that the same element could only present itself in one form, endowed with one invariable set of properties, and that from the combination of the same elements in the same proportions, only one and the same substance could possibly result. But at this period facts became known which rendered this opinion untenable. At first these facts were few, and were regarded as exceptions to an essentially valid law, but their number rapidly increased, so that what had previously been considered as an invariable law was now shown to have owed its invariability to limited knowledge, and it became obviously insufficient to explain the chemical differences of bodies. Vinegar and sugar are, even from a purely chemical point of view, widely different substances, but they are composed of precisely the same elements, viz. carbon, hydrogen, and oxygen, combined in exactly the same proportions. An extinguished spirit-lamp, the wick of which is still red, emits an intolerably pungent smell; the very volatile liquid which is here produced and which emits this suffocating odour, has the somewhat uncouth name, aldehyde. Acetic ether, which has a delicious and refreshing smell, possesses entirely different chemical properties. Again, butyric acid, which is contained in rancid butter, and which has a most disgusting odour, is a body differing widely from aldehyde and acetic ether. Nevertheless, these three substances, which differ so markedly, both as regards chemical and physical properties, consist of the very same elements, viz. carbon, hydrogen, and oxygen united in precisely the same proportions.

In the case also of some elementary bodies such a variation of properties has been observed, that it would be almost impossible à priori to imagine that it is one and the same substance which appears in such various forms. The elementary body, phosphorus, which has been known for nearly 200 years as a soft, yellow substance, fusing easily, exceedingly inflammable, and undergoing rapid change when exposed to the air, appears also in the form of a red, brittle material, capable of enduring a high degree of heat without inflaming or undergoing any alteration, even when simultaneously exposed to the air. Again, the element oxygen, contained in large quantities in the air, which has no smell, acts upon certain substances only at a high temperature, and may remain for a long time in contact with moist silver without producing any change in the

metal; this very element, when in the condition in which it is called ozone, possesses properties as utterly different as we should expect to find only in a totally distinct substance. It is now a powerfully pungent gas, violently attacking at the common temperature the very same substances upon which ordinary oxygen has no action, and causing moist silver rapidly to become black with rust; in both these cases we perceive the existence of what appear to be different substances of undoubtedly similar composition.

Do not such facts appear to contradict the thesis advanced above as the acquisition of modern chemistry? They certainly do contradict the earlier and narrower conception of it; but they only serve to confirm the wider and more modern view, and to maintain it as the fundamental doctrine of chemistry. A few words of explanation will serve to render this clear. Science can no longer dispense with the hypothesis, that bodies consist of very small ultimate particles, which cannot be further divided without the production of something totally different from the matter subjected to this division. These homogeneous particles, of which a substance is built up, are called the physical atoms or molecules of that substance. A piece of copper is composed of copper molecules, the smallest perceptible quantity of oxygen or alcohol, of oxygen or alcohol molecules. A distinction is made between the molecules of bodies, i.e. the smallest particles which can be conceived as capable of independent existence, and atoms, i.e. the smallest particles which can enter into a chemical compound or contribute to the formation of a molecule. The molecules are composed of atoms. The molecules of compound bodies are composed of dissimilar atoms, atoms of different elements, whilst we assume that the molecules of undecomposable bodies-of elements--are built up entirely of the same kind of atoms. We have no knowledge of the absolute number of atoms which unite to form the molecules of different substances, whether a copper molecule, for instance, consists of 2 or 10 or 100 atoms of copper. But we do know or at least we can form very probable conjectures concerning the proportion in which the atoms unite to form a molecule; for instance, in what proportion the carbon, hydrogen, and oxygen atoms are combined in a molecule of alcohol, or what is the proportion between the number of oxygen atoms contained on the one hand in a molecule of alcohol, and on the other in a molecule of common oxygen.

It will now be apparent how the different conditions of the same elementary substance may depend upon differences in its composition. The same elementary atoms may obviously, by combining in different numbers to form molecules, give rise to dissimilar molecules. We know with almost absolute certainty that a greater number of oxygen atoms is contained in a molecule of ozone than in a molecule of ordinary oxygen. We can assume with great

probability that the number of oxygen atoms in a molecule of ozone stands to the number in a molecule of ordinary oxygen in the proportion of three to two. Thus, ozone and ordinary oxygen consist of molecules, the composition of which differs, not in respect of the quality but of the number of atoms which they contain. Imagine the oxygen atoms as so many soldiers belonging to one army, although this comparison be a rough one, it is not so inappropriate as might be imagined; for in fact, in chemical action, these atoms do fight against the alliance between other kinds of atoms, and must vanquish their opposition before they can take up and maintain new positions. A certain weight of oxygen consists of an unspecified number of these atoms or soldiers; but in equal weights of ordinary and of ozonized oxygen, the same number is arranged in a different manner. A given weight of common oxygen contains a certain number of these atoms or warriors, which are marshalled in a certain number of battalions or molecules; an equal weight of ozone contains the same number of exactly similar atoms or warriors, which are, however, placed in a smaller number of molecules or battalions. Á molecule of ozone is numerically a stronger battalion than a molecule of oxygen. By the help of this crude simile, it is easy to understand how the same element can pass from one of these conditions into the other, and how, according to the different marshalling of the said atoms, the same element may have different chemical effects upon other bodies-may attack the molecules or battalions of which they consist in a different manner.

The explanation just offered of the possibility of similar atoms being arranged in different kinds of molecules, is manifestly applicable also to dissimilar atoms. Imagine different kinds of elementary atoms-carbon, hydrogen, oxygen atoms for instance, to be represented by different kinds of soldiers, as infantry, cavalry, riflemen. Equal weights of acetic acid and sugar contain the same quantities of carbon, hydrogen, and oxygen, i.e. the same number of atoms of the three elements, or of warriors of the three different arms. But the arrangement of these warriors in military order in battalions or regiments-is dissimilar in the two bodies. A molecule of sugar contains a greater number (at least thrice, perhaps six times as great) of atoms of carbon, hydrogen, and oxygen as a molecule of acetic acid. Although the composition of acetic acid is identical with that of sugar, both as regards quantity and quality of the contained elements, yet the molecules of the two substances are of dissimilar formation, i.e. they contain the same elementary atoms in different numbers though in the same proportions, and we know with still greater certainty that in aldehyde, only half as many atoms of carbon, hydrogen, and oxygen are united to form a molecule, as in acetic ether or butyric acid, which in all other respects have exactly the same composition, containing the same elements in the same proportions.

But what is the cause of the difference between the two lastnamed substances-butyric acid and acetic ether, the molecules of which consist of equal numbers of the same kinds of atoms? Obviously the atoms contained in a molecule may differ, not only in number and quality, but also in the mode in which they are grouped together. To employ the simile once more, the battalions are divided into companies, and it is evident that this mode of division may differ, whilst the number and kind of elementary atoms remain the same. Two battalions containing equal numbers of warriors of three kinds and in like proportions may have very different internal formations. The soldiers of each class may, for instance, be massed in separate companies, or they may be mingled indiscriminately throughout the whole battalion; again, the number of companies and the mode in which the different classes of warriors are arranged may differ widely in the two battalions. These differences of internal arrangement may greatly affect their respective aggressive movements and powers of resistance, and when the battalion is vanquished and dispersed, its previous formation will affect the combinations which may be formed from its fragments.

Thus, even in modern chemistry, the fundamental idea that the varieties of matter depend upon differences of chemical composition, is still maintained, although not precisely in the same sense in which it was understood forty years ago; its scope is wider, and it has received new developments in special directions. Pure chemistryas distinguished from its technical applications-is at present occupied with the working out of this idea in the most varied directions. The different bodies found in nature are being interrogated with the view of ascertaining if any description of atom can be found which has not hitherto been met with, and to render the list of elementary substances more complete, new and more searching methods of investigation are being devised, in order to discover any kind of matter which has hitherto remained hidden or unnoticed. Many laws relating to the method of arrangement of the elementary atoms in molecules have been already discovered, and the existence of others foreshadowed. Certain peculiarities of the different kinds of elementary atoms are becoming more and more apparent-for instance, that some will enter in couples only into the composition of a molecule, and that elementary atoms present different numbers of sides, so to speak, for the attachment of other atoms. How the elementary atoms are grouped into the proximate constituents of various complex substances, and how certain chemical properties, such as those of acids, depend upon a special arrangement of atoms; these are examples of the problems which are now being assiduously investigated. Also the dependence of many of the physical properties of substances on their chemical composition, in the widest sense of the term, has been proved, and has been the subject of continual investigation, resulting in ever-extending knowledge. The