the cause per quam, or the archæus, and the cause ex gua, which he maintained was water. Even existing botanical works sometimes quote Van Helmont's experiment with the willow, which he imagined, of course quite erroneously, to obtain its wood, leaves, bark, &c., entirely from that fluid. The archæus he maintained was nearly analogous to the soul, and by the ferment that it produced the particles of water were so variously arranged as to produce every material form and structure. The manner in which Van Helmont explained details by means of his theory need not be here stated, and are only interesting to the scientific antiquarian. He founded, however, the school of iatro-chemistry, among the members of which we may mention our own Willis, the anatomist. This sect, essentially a methodic, and therefore of course a wrong one, held the preposterous notion that all the operations in the living body depended entirely upon non-vital or upon chemical and mechanical properties, and could be explained by and were analogous with the mere properties of dead matter. Nevertheless, for a considerable time all the physicians were more or less iatro-chemists. To this day in medical language, or, at any rate, in popular medical language, many expressions remain that have been clearly derived from this sect.

Properly speaking, it was Booerhaave, the great medical dogmatist, who put down this methodic school of iatro-chemists, although several movements in this direction had unquestionably been made before the publication of his work upon chemistry. This reform was effected by Booerhaave, not by rejecting theory, but by returning to the old plan of rational dogmatism. Some of his experiments, as those having reference to the impossibility of fixing mercury, were most elaborate, and extended over a period of fifteen years. Booerhaave, however, cannot be said to have added anything to chemistry, so much as to have restored it to its orthodox form. Among the members of his school, some of whom, in point of fact, really preceded him, but who did not attain to his authority, may be named Glauber, Lemery, Kunckel, and Geoffroy.

We must now go chronologically backwards. Towards the close of the seventeenth century Mr. Mayow of Oxford expressed his belief that there was a connection between combustion and respiration. He ascertained experimentally that air in a glass, in which a candle had been

burned, could no longer support combustion, and that an animal introduced into it died. Some time afterwards the subject of combustion excited the attention of Stahl, and he and his school founded the famous phlogiston theory. The original founder, indeed, of this very ingenious theory was certainly Beecher, but Stahl so refined upon it that he is entitled to be regarded as its founder. All combustible bodies, he taught, were compound ones, and during combustion one of the constituents was dissipated, while the other remained behind. Thus, when a metal was burnt, and a calx (or oxide) obtained, it was held that the metal, before being burnt, had been a compound, that in the burning it had parted with one of its constituents, (which received the name of phlogiston,) and, farther, that if the phlogiston could be restored to it, (the calx,) the original metal could be obtained. It will be observed that the constituent supposed to be expelled from combustibles during combustion was considered to be the same, namely, phlogiston in all combustibles.

We know now that this theory was not only imperfect but erroneous, but we perhaps do not sufficiently appreciate its beauty and its usefulness. And certainly it accorded exactly with all known chemical facts. For instance, if we take lead and expose it to a red heat, red lead as we call it, or lead calx, as it was called, is formed. Stahl's explanation of this was, that lead was a compound of lead, calx, and phlogiston, and that the latter had been by the heat expelled. If this calx be heated with charcoal, or any other combustible substance, we get metallic lead again. Stahl explained this by saying that the calx had acquired phlogiston from the combustible. We know that lead, when heated, becomes a calx or oxide, by taining oxygen from the air, and that the calx becomes a metal again from its oxygen combining with the carbon of the combustible. Our space does not permit us to give other illustrations, but it will be plain from what has been stated, how admirably the phlogiston theory explained an immense number of chemical phenomena.

The error committed by Stahl and his school was in not using the balance. When the lead was heated, if it parted with phlogiston to become a calx, it would of course become lighter, but the fact is the very reverse. On the other hand, if when the calx became metal again, it did so by obtaining phlogiston from the combustible, it would

become heavier, but it actually loses. And, in fact, as we shall immediately have occasion to notice, it was the balance that destroyed the phlogiston theory. The theory, however, did in its time good, was taught by eminent men, and was relinquished whenever disproved. It was a step in rational chemistry.

Our space forbids us to mention any one who kept up the credit of the Stahlian school, save that of Chargnier, whose Elements of Chemisty was long the standard text book, and the English translation was for long recommended by Dr. Black, who, about a century ago, made a discovery that was the first step in the foundation of the new rational school. It had long been known that when chalk, a solid substance, is heated to redness, it becomes very acrid. The explanation given of this was, that the chalk had acquired phlogiston, but Black ascertained that the change was owing to the fixed air, as carbonic acid gas was then called, being driven off. When, to the other alkalies carbonate lime was added, it was thought that the lime communicated causticity, but Black proved most satisfactorily that the real action that took place was, that the lime removed from them the carbonic acid, and which carbonic acid had prevented their properties from being developed. It is impossible to overrate the importance of this new doctrine of chemistry. Continuing his investigations regarding heat, Black developed his theory of latent heat, which certainly has been the parent of the steam engine. Black farther ascertained with regard to carbonic acid, that it would not support either combustion or respiration, that it was formed in the lungs of animals, and was also a product of fermentation.

Before the time of Black chemists had known that aeriform matter was often evolved from substance, but such were always considered to have the composition of air, and indeed to be that fluid. Black had, however, established the existence of fixed air, or carbonic acid. The properties of this were still more fully investigated by Cavendish, who likewise discovered the existence of a third separate gas, inflammable air, or as we now call it, hydrogen. Cavendish, for a very probable reason, the fallacy of which we cannot here enter into, was inclined to regard hydrogen as phlogiston. A little later, Rutherford of Edinburgh discovered the existence of another gas,

nitrogen, or mephitic air, as it was then named. The school of the pneumatic chemists was founded.

The greatest ornament of this school was unquestionably Priestley. His first discovery was that of nitrous acid, or at any rate he first attended to the properties of this gaseous compound, and employed it in the analysis of air. On the 1st August, 1744, he discovered oxygen, or diphlogisticated air, as it was termed. The reason that Priestley adopted this expression was, that he believed in the phlogiston theory, and seeing that the oxygen obtained from the air supported combustion so readily; he inferred that it had been deprived of its phlogiston, and that the cause of its supporting combustion so brilliantly, was owing to its great tendency to combine again with phlogiston. Dr. Priestley likewise first made known sulphurous acid, hydrochloric acid, ammoniacal gas, and other substances, and moreover, he was the first who pointed out that the fixed air, or carbonic acid that was the product either of combustion or respiration, was removed from the atmosphere by plants that actually obtained nutriment from it.

We cannot pass over this school of chemistry without a notice at least of Scheele and Bergmann. The former certainly discovered oxygen quite independently of any knowledge that he had of what Priestley had done, and the Essay on "Elective Attractions" of the latter, was the most successful piece of philosophical generalization of his age. He demonstrated that all bodies that can chemically unite together, have a fixed and definite attraction for one another, that one substance has various degrees of affinity for various substances, and that even when united to one for which it has a slight affinity, if a substance be presented to it for which it has a stronger affinity, decomposition takes place, and a new compound is formed. This doctrine has been a little modified, but is nevertheless the basis of all inorganic chemical philosophy.

Next came the French school, but their additions to chemical science, as well as those of their successors, demand, and shall obtain a separate review. One observation may here be allowed. The number of chemical facts that have accumulated has been, since the era of Lavoisier, so great, that there is again some little fear of both empiricism and methodism attacking chemistry. It

may seem paradoxical to say it, but perhaps the greatest blessing that could fall on modern chemistry would be, if it did not receive one solitary additional fact for the next dozen years. As too much food, instead of nourishing, produces disease, so the too eager research for, and too rapid recording of facts and detailed observations, produce a morbid state in science. Facts, indeed, only are of use in so far as principles may be deduced from them. And at present chemistry has a great many statements that are not quite certain, and a great lack of the generalization of its clearly established facts. When this is the case, history tells us that there is a great tendency to despise slow and rational deduction, and to fly off into methodical theories, and empirical practices. Chemistry is now perhaps in some danger of both, for much undoubtedly remains to be learned regarding the composition of compound bodies, and perhaps the number of the elementary ones, or so-called elementary ones, to be reduced. In the hands of a Faraday, allotropism will be philosophical, yet, if it fall into the hands of some, it may become merely a representation of the methodic alchemy of the Arabs. Empiricism, too, has of late years made some fierce assaults upon chemistry. The mere laboratorians, who live among retorts and furnaces, and who are dexterous at manipulating, and at repeating often before performed analyses, are becoming very much disposed to consider themselves as the chemical philosophers. The work-people in fact not only despise, but wish to depose their masters, and like the bellows-blower to Handel's organ, affirm that it is they who make the harmony. However, if chemistry has before it some little dangers, it has had many much more serious ere now, and has not only escaped from them, but has carried away spoil, and so doubtless will chemistry do again.