of the particles of which it is composed, offers an accumulative obstacle to the passage of water downwards, more than balancing the hydrostatic pressure, and amounting at length to an absolute stoppage. This is a conclusion which wells in the New Red Sandstone-itself a very porous rock-seem to offer, for after a certain depth has been reached, the increase of supply appears to take place in a diminishing ratio to the depth, and we may suppose a zero point would ultimately be reached.

*

As far, then, as our experience goes, it is very probable that at depths greater than 2,000 feet, no water would be found in ordinary coal-measure strata, and thus by sinking in this formation a principal source of error would be avoided. Other sources of error (such as those arising from differences in density, and conducting power in the several varieties of strata) being of less moment, would disappear, or become inappreciable, on taking the mean temperature of very deep borings.

After much consideration, the plan which we venture to recommend, in case of experiments being undertaken by the British Association, or any other scientific society, would be, not to commence at the surface, but at the bottom of a coal-mine, of not less depth than 600 yards.

There are several collieries, particularly in Lancashire and Cheshire, sufficiently deep for the purpose. It would be an easy matter to excavate a chamber in the coal and its roof, where the borings might be carried on. The chamber ought to be a short distance from the bottom of one of the shafts, and out of the way of mining operations. As the process of boring progressed, observations should be taken at every 10 yards, and at every change of strata from sandstone to shale, or coal. The boring might be carried down at least to a total depth of 1,000 yards from the surface, and having been completed under proper supervision, could not fail to give results of value to science. It is also probable that a proprietor of some colliery of the required depth would willingly afford the facilities for carrying on the experiment for the sake of the information he would derive regarding the minerals underlying the coal-seam then being worked.

Before closing these observations, some further explanation is required regarding the depth of invariable temperature, or, in the words of Humboldt, the depth of "the invariable stratum." The surface of the earth undergoes a change of temperature according to the season of the year. In summer the rays of the sun and the warmth of the atmosphere affect the surface and penetrate gradually downwards with decreasing intensity, while in winter the cooling

As may be gathered from the account of the sinkings at the Green Lane Well of the Liverpool Corporation Waterworks. See Quarterly Journal of Science,' vol. ii., p. 421.

influences prevail from the surface downwards, and the result is, the formation of an envelope or stratum of invariable temperature at a short and variable distance from the surface all over the globe. The temperature of the invariable stratum approximates to that of the mean annual temperature of the place, and, according to Humboldt, its depth is regulated by the latitude (increasing from the Equator towards the Poles), by the conducting power of the rock or soil, and by the amount of difference between the temperatures of the hottest and coldest seasons. At Greenwich, the mean temperature is 49.5°, and that of the invariable stratum about 50.5 at a depth of about 50 feet from the surface.

It will be evident, on reflection, that the stratum of invariable temperature is the standard of departure for all measurements at greater depths, and this point would require a separate series of experiments for its determination at the locality selected for the deep borings; but for ordinary purposes it is probable 50° Fahr., at a depth of 50 feet, may be taken as the temperature and depth of the invariable stratum over the greater part of Central England.* In the latitude of Paris (48° 50') the depth and temperature of the Caves de l'Observatoire (86 feet and 53-30 Fahr.) are generally regarded as those of the invariable stratum.

It is scarcely necessary to point out the questions on which an accurate series of observations extending to great depths might throw light. As one illustration, we may select the subject of metamorphism of rocks, which is full of difficulties requiring solution, such as that presented by the rocks of the highlands of Scotland, where we find the metamorphosed Lower Silurian rocks reposing on unchanged Cambrian sandstones. Again, the change of bituminous coal into anthracite is only as yet partially explained in such instances as those of South Wales and the coal-field of the Don, where the same beds occur in both forms at opposite sides of the field. The nature and the fluidity of the interior of the earth itself is also at this moment (and probably ever will be) a matter of controversy among physical philosophers,† and one of those questions on which possibly some light might be thrown by the proposed experiments, though considering the small fraction of the earth's radius which comes within reach of man's feeble operations, one cannot be very sanguine on this head. It is to be remembered, however, that in prosecuting physical researches, it is not necessary to have a definite end in view, except that of adding to our knowledge. No new and well-certified observation will be allowed to lie for ever useless.

*See Professor Forbes' "Experiments on the Temperature of the Earth at different Depths."-Trans. Royal Society of Edinburgh,' vol. xvi., 1849.

† I refer more particularly to the views stated by Dr. Sterry Hunt in his lecture before the Royal Institution, London; and the reply thereto by Mr. D. Forbes.— See 'Geological Magazine,' vol. iv., pp. 357 and 443.

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 essen

tially 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