IV. THE PAST AND PRESENT OF CHEMISTRY.

By DR. HERRMANN KOPP,

Professor of Chemistry in the University of Heidelberg. CHEMISTRY is generally regarded as one of the youngest of the sciences, and as exhibiting most unmistakably several characteristics of youth. Some think that, whilst she has made rapid progress of late, her development has achieved, in some aspects at least, but little of a solid and lasting character. Others allege, as a characteristic of her youth, that she often presumptuously gives her opinion and advice. She brings her judgment, supported by knowledge only just acquired, to bear upon older sciences, whilst she claims to be heard in the discussion of subjects with which her elder sisters have earnestly occupied themselves for centuries. In fact the venerable science of medicine, with her continually changing aspect, the somewhat younger natural philosophy with her glorious modern developments,-these and many other sciences long held in respect, may assert that they were already well grown when chemistry was still in her babyhood, talking nonsense, and manifesting the most perverse tendencies,-that they can remember the time when they took the infant under their fostering care, and led her by the hand.

Chemistry cannot deny this. She even gratefully acknowledges it, notwithstanding that, under their care, she was at times somewhat grossly maltreated. She cannot avoid the confession that, in her present aspects and pursuits, she is still very young; on the other hand, however, she may fairly plead that she does not quite date from yesterday. She can prove by documents, which, though not altogether indisputable, have yet considerable claims to authenticity, that she was at least in existence 1,500 years ago. It must be admitted that this is a respectable age, although insignificant as compared with that of some other sciences whose birthdays are lost in the gray mists of antiquity.

How can this ripe age of chemistry be reconciled with the youthfulness to which she generally confesses, and on account of which she has not unfrequently to submit to many reproaches? The mystery is explained when we take into consideration that the science which was known as chemistry to the ancients occupied itself with the solution of problems of an entirely different nature from those which engage the attention of chemists at the present day. For a long period chemistry, with childish illusion, pursued a phantom, and attempted the solution of an insoluble problem. Comparatively recent is her occupation with the task which we now consider to be her legitimate employment, and which as regards her way of dealing with it, we now look upon as the correct

method of investigation. Chemistry had a long childhood, and the more mature phases of her life are compressed into a comparatively short and recent period. Chemistry in childhood and chemistry in youth are almost as two distinct individuals. Let us endeavour to compare the characteristics of this childhood and this youth-the problems of the one and the pursuits of the other-the past and present of chemistry.

If we search for a connecting link between early and modern chemistry, we find it in a problem, the solution of which has occupied the minds of chemists in all ages. This problem, always more or less prominently kept in view, is the composition of the different substances found in nature or produced by art-the different heterogeneous materials which can be extracted from, or made to combine and form, a homogeneous substance.

The ancients made hardly any attempt to ascertain the chemical composition of substances. First, among the Greeks, and later among the Romans, we find sagacious observations on the diversities of bodies, but it was rather the physical than the chemical differences concerning which scientific observations were made. According to the doctrines of Aristotle, which, promulgated 2,200 years ago, so long maintained their authority, the fundamental properties of everything corporeal and palpable were considered to be dryness or moisture (that is, solidity or liquidity), and warmth or coldness. These are obviously physical conditions and different degrees of a physical property, and on the occurrence and the proportion of certain of these fundamental properties, other qualities, such as density or lightness, hardness or softness, were thought to depend. The assumption of the four elements, Earth, Water, Air, and Fire, offers to the mind a representation of the simultaneous occurrence of these fundamental properties. To Earth, as the representative of all solids, were ascribed dryness and cold; to liquid Water, moisture and cold; to Air or vapour, moisture and heat; to Fire, heat and dryness. The four elements of Aristotle represented fundamental conditions of matter, and the properties of bodies were regarded as depending on the proportion in which they contained the elements-the producers of those qualities. The whole conception was formed from a physical point of view rather than from one having even the faintest approximation to that of chemistry. The elements of Aristotle, as such, were not regarded as contained in different substances -as combining to form them, or as separable from them by analysis. The assumption of their existence did not therefore involve even the merest rudiments of a chemical idea. They were not regarded as different kinds of matter, but as different fundamental conditions which, when added to indifferent matter, endowed it with various properties.

This view, that the difference of bodies depended essentially

upon their physical properties, was the natural consequence of the slight knowledge of chemical qualities which the ancients possessed. The idea of chemical composition had not yet been conceived except, perhaps, in its very crudest form, as suggested by the composition of metallic alloys artificially prepared. But the well-known metals were scarcely distinguished from each other; thus lead and tin were regarded by the Romans as differently coloured varieties of the same metal-as dark and light lead. Of the most important chemical problems, such as those relating to combustion, to the changes effected in metals by the action of fire, or the caustic qualities of quicklime, we find scarcely a single one proposed, still less any attempts made at their solution. In regard to their knowledge of chemistry the ancients may be compared to ignorant or half-educated tribes of the present age; knowledge of important chemical phenomena they did not lack, but they made no attempt whatever to discover the causes of these phenomena.

This almost total ignorance on the part of the ancients, especially the Greeks, of any of the aspects of chemistry, is due entirely to their method of scientific investigation. Chemistry is essentially an experimental science, but experimental methods were little known either to the Greeks or Romans. The favourite mode of investigation with the most highly-cultivated people of antiquity, consisted in attempts to attain, by a pure effort of the intellect, to the conception of an universal principle by means of which all phenomena might be predicted and explained. Such a method of research could not even enable them to approach the domain of a science like chemistry. A few centuries later, however, we find the art of experimenting more advanced, and a real, if somewhat vague attempt being made to obtain a knowledge of the chemical composition of at least one class of bodies. It is true that this knowledge was sought after, not for its own sake, but as an aid to the solution of the problem of the transmutation of the common into the nobler metals. Alchemy existed in the fourth century of our era, and for more than a thousand years presented almost the only, and certainly the most important field for the development of chemistry.

Our knowledge of the spread of alchemy, and of the resulting progress of chemistry, is very defective for the period between the fourth and the thirteenth centuries, and we know nothing whatever of the origin of alchemy, nor where it was first attempted. All that can with certainty be said is, that alchemy is undoubtedly older than the most ancient testimony that has reached us concerning it (from the fourth century), for this testimony does not speak of it as a new pursuit but as one which had long been carried on. The confident assertions regarding the practicability of alchemy do not now concern us, but it is important to a clear comprehension of the

present state of chemistry that we should know what were the grounds of the belief that one metal could be transformed into another.

The foundation of this belief was a theory of the cause of difference in matter, which finds its best expression in the doctrine of Aristotle already alluded to, that matter itself is everywhere one and the same thing, and that its varieties are produced solely by a variation of its qualities. Changes in the properties of substances, and especially of metals, were recognized at an early period. Thus it was known that, by the action of certain substances, copper could be made as yellow as gold, and, by that of others, as white as silver, the transformation being effected throughout the entire mass. We now know that when red copper is turned into yellow copper (brass) or into white copper (German silver), a change takes place in the composition as well as in the colour; but at the period when alchemy flourished, such a change was not thought of any more than we now consider it as taking place when soft steel is transformed into hard steel-a transformation with which the ancients were also acquainted. The hardness, colour, ductility, fusibility, and some other properties of certain metals could be altered, and hence it was thought that it must be possible so to change all the properties of one metal into those of another, that the one metal would really be transformed into the other. Until the fifteenth century, and even somewhat later, the idea underlying the pursuits of alchemy, was the existence of an universal matter which, endowed with various properties, forms all known substances, and consequently all the metals; in short, that a change in appearance constitutes a veritable transformation of a metal. If a fragment of iron be dropped into a solution of blue vitriol, the iron gradually disappears and in its place we find copper. We now know that the iron is dissolved and the copper contained in the vitriol precipitated; we do not regard the visibly iron-coloured metal, which is dropped into the solution, as including the same matter as the visibly coppercoloured metal which is afterwards found in its place, but in the alchemical stage of the science a totally different view prevailed, and the mythological nomenclature then used, such as Mars for iron, and Venus for copper, clearly expresses the then prevailing theory. The same material which in Mars's coat of mail appeared as iron, reappeared in the garb of Venus (as copper) after being acted upon by the vitriol.

In the sixteenth century chemistry emerged from its previous degraded condition, and passed into the hands of men who were not trammelled by the doctrines of Aristotle. The physicians who were the followers of Paracelsus paid but little respect to ancient authorities, and relied entirely upon their own observations and investigations. Three properties of matter especially excited their

attention, viz. :—first, that of enduring the action of fire without alteration; secondly, that of being volatilized by heat, in an unchanged condition; and thirdly, that of burning or undergoing change by fire. They assumed that an ideal something, which they called salt, was the cause of the property of resisting the action of fire; that another ideal, something which they termed mercury, was the cause of the property of volatility; and that a third substance, also ideal, and named sulphur, was the cause of combustibility. And as one at least of these properties occurs in every substance, their supposed causes came to be regarded as the constituent elements of all matter. If a substance were found to be combustible, it contained this ideal sulphur; if it volatilized without change, the ideal mercury was present, whilst if it left an unchanged residue, ideal salt was amongst its constituents.

These so-called elements-sulphur, mercury, and salt-the original notion of which may be traced farther back than the sixteenth century, played a most important part in the medical science of that and the seventeenth century. Physicians then believed that the health of the human body, as well as of its separate organs, depended on the combination in certain proportions of the before-mentioned elements, that disease was the result of a disturbance of these proportions, and that restoration to health was effected by their re-establishment. These applications, however, do not now immediately concern us, but they are important as indicating the gradual development of the theory that different substances differ, that is, exhibit different qualities, because they are composed of different ingredients, or of the same ingredients in different proportions; nevertheless the conception of this idea was still so crude that totally dissimilar substances, such as the incombustible portions of the most widely different bodies were all known as the salt constituent, the universally-accepted elements were purely fictitious, and that hardly any attempt was made to extract them from the substances in which they were supposed to exist.

But whilst chemists were thus busying themselves with these imaginary elements, they were also making gradual progress in the sounder recognition of the composition of a great number of substances. They discovered, for instance, that copper is present in substances which, to the eye, reveal no appearance of metallic copper, as in blue vitriol, and that vermillion contains mercury and sulphur-not the imaginary elements known by these names-but real mercury and real sulphur. Such knowledge as this steadily increased and became more prominent. Towards the end of the seventeenth century it had in fact made such progress, that the recognition of the substances which a body may be proved to contain, was considered as the only problem which chemistry should strive to solve, whilst investigations, involving the assumption of elements,