cal principles, adopted the notion of one elementary substance, charged, in the one instance, with the imaginary essence of inflammability, and free from it in the other. In these inquiries, he frequently verges upon more correct and refined views, but has no sooner entered the right path, than phlogiston, like an ignis fatuus, dances before his eyes, and leads him into the marshy mazes of error.

In the preceding investigations, Dr Priestley followed those methods of collecting aeriform fluids over water, which Hales and others had employed before him: he now ascertained that there were some gases absorbed by or soluble in water. Mr Cavendish, one of the most eminent Philosophers of that day, had announced this circumstance, and was puzzled by it; but Dr Priestley, with his usual and dexterous ingenuity, overcame the difficulty, by employing quicksilver instead of water, over which fluid metal he preserved and examined several kinds of air, which are instantly deprived of their elastic state by the contact of water.

The first permanently elastic fluid of this description which he examined, was the muriatic acid; he obtained it by heating copper in the fluid acid, or common spirit of salt, and called it marine acid air.

He immediately ascertained its absorption by water, and its powerful acidity; he found it incapable of supporting flame, and extremely destructive of animal life. He examined the action of a variety of substances upon this gas, and ascertained the remarkable rapidity with which it is absorbed by charcoal, and several vegetable and animal substances. Some unsuccessful attempts were made to ascertain the specific gravity of this gas, from which Dr Priestley correctly concluded, however, that it was a little heavier than air.

The success attending these experiments, and the readiness with which he procured and retained the gaseous muriatic acid, led him to extend his trials to other acids, when he found, that, by acting upon vitriolic acid by inflammable substances, he could procure from it a permanently elastic fluid, to which he gave the name of vitriolic acid air; he found that, like the marine acid air, it was rapidly absorbed by water, and must be collected and preserved over quicksilver; that it was nearly twice as heavy as atmospheric air; that it extinguished flame, and was instantly fatal to animal life; that it reddened vegetable blues, and destroyed most colours This air is, in fact, produced by burning sulphur in the atmosphere, and straw, wool, and other materials, are frequently bleached by exposing them to its fumes. 1

'Having elsewhere praised Dr Priestley's candour, I insert the following extract from his history of the

Having thus obtained permanent aeriform fluids, having acid qualities, it occurred to Dr Priestley, that the volatile alcali, the substance which gives pungency to salvolatile, spirit of hartshorn, and similar compounds, might be also procured in a pure and isolated gaseous form; and, after several unsuccessful trials, he succeeded, by heating a mixture of quicklime and sal ammoniac, when a great quantity of air escaped, permanent over quicksilver, but, like the acid gases, rapidly absorbed by water.

The odour of this gas was pungent in the extreme, and it possessed the property of salvolatile, smelling salts, and similar substances, of turning vegetable blues to green. After several experiments, in which the absorbing powers of different substances in regard to this air, were tried, Dr Priestley became impatient to discover the effect of mixing it with the acid airs just described,-he imagined that he should form a neutral air. On putting this notion, however, to the proof of experiment, he was surprised to observe that when marine acid air, and the volatile alcaline air, were mixed in due proportions, they were wholly condensed into a solid. And with sulphureous air a very similar result was afforded.

Dr Priestley concluded that alcaline air was considerably lighter than acid air, because, on mixing them over mercury, he observed the former to float above the latter;

discovery of Vitriolic Acid Air, to show the exactness with which he acknowledges the hints and assistance of others:

"My first scheme was to endeavour to get the vitriolic acid in the form of air, thinking that it would probably be easy to confine it by quicksilver, for, as to the nitrous acid, its affinity with quicksilver is so great that I despaired of being able to confine it to any purpose. I, therefore, wrote to my friend Mr Lane to procure me a quantity of volatile vitriolic acid," &c. "Seeing Mr Lane the winter following, he told me, that if I would only heat any oily or greasy matter with oil of vitriol, I should certainly make it the very thing I wanted, viz. the volatile or sulphureous vitriolic acid; and, accordingly, I meant to have proceeded upon this hint, but was prevented from pursuing it by a variety of engagements.

"Some time after this I was in company with Lord Shelburne, at the seat of Mons. Trudaine, at Montigny, in France; where, with that generous and liberal spirit by which that nobleman is distinguished, he has a complete apparatus of philosophical instruments, with every other convenience and assistance for pursuing such philosophical inquiries as any of his numerous guests shall chuse to entertain themselves with. In this agreeable retreat I met with that eminent philosopher and chemist, Mons. Montigni, Member of the Royal Academy of Sciences; and conversing with him upon this subject, he proposed our trying to convert oil of vitriol into vapour, by boiling it on a pan of charcoal in a cracked phial. This scheme not answering our purpose, he next proposed heating it together with oil of turpentine. Accordingly, we went to work upon it, and soon produced some kind of air confined with quicksilver; but our recipient being overturned by the suddenness of the production of the air, we were not able to catch any more than the first produce, which was little else than the common air which had lodged on the surface of the liquor, and which appeared to be a little phlogisticated by its not being much affected by a mixture of nitrous air."

on putting a lighted candle into alcaline air the flame was enlarged, and a portion of the air appeared to burn with flame.

We have now considered the principal discoveries of Dr Priestley, upon which his title to originality rests, and it must be allowed that they are not less important than numerous. If we even consider them merely as insulated facts, they are of a very superior character, and tended greatly to enlarge our knowledge of the chemical elements of matter; but the new views of many natural and artificial phenomena, which they exposed, and which before were buried in deep obscurity, confer upon them a more exalted aspect, and have obtained for them the deserved meed of universal admiration. In perusing Dr Priestley's tracts, we find the thread of the narrative occasionally knotted with conceit, and weakened by garrulity; but these blemishes are compensated by prevailing candour and perspicuity of style: he had greatly extended the boundaries of science, and was awake to the importance of his conquests; but resisted that febrile thirst of innovation and reform, which was endemic among contemporary Chemists.

"At present," says he, in the Preface to his third volume of Experiments and Observations, relating to various branches of Natural Philosophy, "At present all our systems are in a remarkable manner unhinged by the discovery of a multiplicity of facts, to which it appears difficult or impossible to adjust them: We need not, however, give ourselves much concern on this account. For when a sufficient number of new facts shall be discovered, towards which even imperfect hypotheses will contribute, a more general theory will soon present itself, and perhaps to the most incurious and least sagacious eye. Thus, when able navigators have, with great labour and judgment, steered towards an undiscovered country, a common sailor, placed at the mast head, may happen to get the first sight of land. Let us not, however, contend about merit, but let us all be intent on forwarding the common enterprise, and equally enjoy any progress we may make towards succeeding in it, and, above all, let us acknowledge the guidance of that great Being, who has put a spirit in man, and whose inspiration giveth him understanding." With this quotation, sufficiently characteristic of his general style, I shall take leave of Dr Priestley, and introduce another hero of chemical history, his contemporary and great rival, Scheele.

SECTION V.

DISCOVERIES OF SCHEELE AND CAVENDISH.

AMONG those whose names became eminent in the history of chemical science during the first half of the eighteenth century, Margraaf and Bergman are entitled to particular mention. The former was a pupil of the once celebrated Neumann,' a man whose works are now not much thought of, but who did considerable service to the Chemistry of his day, and was evidently possessed of great diligence and some capacity. In 1753, Margraaf pursued chemistry under Juncker at Halle, and, having returned to Berlin in 1738, we find several of his contributions in the Transactions of the Scientific Society of that capital. Subsequent to that period, his works were collected and published at Paris in 1762. They contain a great body of information, at that time novel and important, but they are chiefly entitled to notice as furnishing specimens of the art of analysis, which was afterwards carried to greater perfection by Bergman, who, indeed, may be considered as the first who pointed out the true objects of that branch of the science, and who aimed at conferring upon it the statical accuracy which has since rendered it so important and useful.

3

But Bergman was something more than a diligent experimentalist and acute reasoner; he was also an active patron of science, and had the merit of rescuing Scheele

1 Casper Neumann was born at Zullichau in Prussia, in 1682, and in 1705 we find him enjoying the patronage of the King of Prussia, by whom he was sent to complete his studies at the University of Halle. In 1711 he became a pupil of Boerhaave, and shortly after visited England, whence he accompanied George I. to Hanover in 1716. In 1723 he became Professor of Practical Chemistry in the Royal College of Berlin, where he died in 1737. His works consist chiefly in dissertations on various subjects of chemical inquiry, published in the Transactions of the Royal Society, and in the Miscellanea Berolinensia. His Lectures were not printed till after his death, and proved a valuable magazine of chemical knowledge. "The author," says Dr Lewis, who edited his works, "biassed by no theory, and attached to no opinions, has inquired by experiment into the proportions and uses of the most considerable natural and artificial productions, and the preparations of the principal commodities which depend on chemistry, and seems to have can didly and without reserve communicated all he discovered."

2 Born at Berlin in 1709, where he died in 1782.

3 Torbern Bergman was born in Sweden in 1735, and died in 1784. His principal chemical papers are contained in the Opuscula, published at Upsal in 1779. They contain much to admire, not merely as being rich in facts and discoveries, but also on account of the general views which he takes of the mode of prosecuting philosophical inquiry, and which is so ably set forth in the preliminary essay, De Indagando Vero. The Opuscula was translated into English by Dr Edmund Cullen in 1788.

from his obscure situation, and of discerning that talent and genius in the bud which afterwards was so vigorously fruitful.

If we compare Scheele with our own countrymen, we discern him possessed of the accuracy and cool judgment of Black, conjoined with the inquisitive and busy activity of Priestley, and his success in the pursuit of science was such as might be expected to flow from this happy and rare union of opposite talents. In the number of his discoveries, their weight, and novelty, he has indeed very few equals; nor has their splendour been tarnished by time, or dimmed by the brilliant light of modern investigation.

Scheele is among the fortunate few, who, starting from an obscure original, have attained the zenith of scientific eminence. He was born in 1742 at Stralsund, where his father was a tradesman. His youthful days were passed in the house of an Apothecary at Gottenburgh, where, by singular perseverance, and that kind of industry which is prompted by strong natural inclination, he acquired a valuable stock of chemical information. In 1773, having removed to Upsal, accident brought him acquainted with Bergman, who became his friend and patron, and to whose honour be it told, that, when Scheele's reputation afterwards rose to such a height as threatened to eclipse his own, instead of listening to the voice of jealousy, which, on such occasions, is too common a frailty, he became more zealous in behalf of his rival, and more indefatigable in the service of his friend. Scheele afterwards removed to Köping, in the neighbourhood of Stockholm, where he died in 1786.

1

No adventitious aid, however, can be said to have contributed to Scheele's greatness. On the contrary, obstacles were opposed to his progress which would have damped the ardour, and checked the flight, of less aspiring and persevering minds; and much of his useful life was spent," not in the soft obscurities of retirement, or under the shelter of academic bowers, but amid inconvenience and distraction, in sickness and in sorrow."

1

Scheele's first publication, which appeared in the Stockholm Memoirs for the year 1771, relates to the analysis of fluor spar. The peculiarities of this substance were first noticed in 1768 by Margraaf, but the discovery of the principle upon which they depend was reserved for the superior sagacity of Scheele, who demonstrated in it the existence of lime, and an acid till then unknown, which he called fluoric acid.

1 Scheele's Essays have been collected and translated into English by Dr Thomas Beddoes. London, 1786.