to empirical perception. In other words, they are merely conditions of the possibility of empirical knowledge. Now, such knowledge is called experience. Hence the categories have a share in the knowledge of those things only, that are objects of possible experience.

The above proposition is of the greatest importance, for it marks out the limits of the pure conceptions of understanding in their application to objects, just as Transcendental Esthetic marked out the limits of the pure forms of sensuous perception. Space and time are but the conditions under which objects that are relative to our senses are capable of being presented to us, and therefore they apply only within the limits of experience. Beyond those limits they have no meaning whatever, for they are only in the senses, and have no reality apart from them. The pure conceptions of understanding are free from this limitation, and extend to objects of perception of any kind, whether that perception is like or unlike ours, if only it is sensuous and not intellectual. But this extension of conception beyond our sensuous perception does not help us in the least. For, the conceptions are in that case quite empty, and we are therefore unable even to say that there are any objects corresponding to them. They are mere forms of thought without objective reality, for we have no perception at hand, and therefore no object, to which the synthetic unity of apperception, which is the sole content of those forms of thought, could be applied. Only our sensuous and empirical perception can give to them meaning and reality.

If I suppose an object of a non-sensuous perception to be given, I can, no doubt, think of it as having all the predicates implied in my presupposition. I can say that the object has none of the determinations proper to sensuous perception; that it is not extended or in space, that its duration is not time, that there is in it no change or succession of states in time, etc. But no real knowledge of an object is gained by merely indicating how it is not perceived, so long as I cannot tell what is the content of its perception. I cannot in that way understand even the possibility of an object to which my pure perception could apply, for I am unable to bring forward a perception corresponding to such an object, and can say only that my perception can never bring me into contact with it. But what most concerns us here, is that to a thing of that nature, not even a single category could be applied. I could not say, for instance, that such a thing is a substance, that is, a thing that can exist as subject, but never as mere predicate. For, how could I apply the conception of substance, when, in the absence of all empirical perception, I should not even know that anything corresponding to my idea could exist at all.— From Watson's Selections.

EIGHTEENTH CENTURY SCIENCE

THE SEVENTEENTH CENTURY had been made memorable in the sciences by the discoveries of Galileo and Kepler in astronomy, of Harvey, Leeuwenhoeck, Malpighi and Grew in biology, of Boyle in chemistry, and of Guericke, Newton, Huyghens and Roemer in physics. Thus astronomy was put upon a firm foundation and much was done for physics. The eighteenth century was to see in physics the opening up of the field of electricity, and the first beginning of the conception of heat in terms of motion; the foundation of modern chemistry and geology, and the development in astronomy of the nebular hypothesis of the growth of the universe.

The century opened (1701) with the first attempts in organic chemistry made by Boerhaave. He decomposed organic substances such as plants by sublimation and showed that the substances in the plants are taken up from the earth in which they grow after first being dissolved in the water that soaks down from the rains. He followed up these facts by showing that animals are made up of reorganized vegetable matter. His analyses were, of course, imperfect, because chemistry knew nothing as yet of even such elements as oxygen, hydrogen, nitrogen or carbon, the chief components of organisms. In fact, during the first half of the century, chemistry was retarded by the acceptance of Stahl's phlogiston theory, which was that burning is the release of an imaginary substance called phlogiston, supposed to exist in all things capable of combustion.

In botany the work of the century, was mostly descriptive and systematizing by Linnæus.

Haller and Hunter made a beginning in comparative anatomy by

trying to compare similar organs in different animals, and Buffon attempted to describe all the known animals of the globe.

In physics the latter half of the century opened with the demonstration of the identity of electricity with lightning by Franklin (1752). Hitherto electricity had been a plaything. Franklin showed that it is in reality a giant. In 1760 Black discovered latent heat and in 1765 Watt applied the principle to the construction of the first practical steamengine. Galvani (in 1789) found electricity to be present in animals and noted its effects in contracting the muscles. Volta in 1792 discovered chemical electricity and invented his battery, or "voltaic pile," to produce it. Rumford by studying the effect of motion in producing heat gave the old caloric theory that heat is a substance a severe shock, and opened the way for the nineteenth century conception of the conservation of energy.

In the latter half of the century Hutton and Smith laid the first foundation in geology by studying the formation of the earth.

In astronomy the century ended with a theory of the development of the universe. Lagrange (1736-1813) and Laplace (1749-1827) worked out mathematically the oscillations in the solar system caused by the interaction of its parts and showed its stability. Herschel (17381822) discovered the planet Uranus in 1781; found pairs of stars revolving round each other, thus demonstrating that the law of gravitation holds not only in our solar system but in the universe; showed that our solar system seems to be moving in a mass toward the far-off constellation of Hercules, and pointed out that some star clusters seem to consist of dispersed "star-matter" or gases. On these foundations Laplace built his hypothesis of the development of the universe from original gases.

Meantime, toward the close of the century, chemistry had supplied the conception of such gases. In 1756 Black by pouring acidulated water on limestone discovered carbonic acid gas; in 1766 Cavendish obtained hydrogen by pouring sulphuric acid and water on zinc. In 1774 Priestley procured oxygen by heating mercuric oxide, and later (1784) Cavendish combined this new gas with hydrogen by means of an electric spark passed through the mixture and found that they made water. Lavoisier revolutionized chemistry (1778) by showing that all combustion is the effect of combination with oxygen, thus overthrowing the old phlogiston theory. Chemistry had begun to grasp the elements of matter, and was on the high road toward being an exact science.

VI. 16.

BOERHAAVE

HERMAN BOERHAAVE was born near Leyden, December 31, 1668, and died Professor of Medicine and Botany at Leyden, September 23, 1738. In medicine he was the most representative man of his time, but he was something of an eclectic in his beliefs and stands for no great advance in the subject. In chemistry, as noted previously, he did a great deal toward starting the study of physiological chemistry by his investigations into the growth of plants as related to the substances absorbed from the soil.

PHYSIOLOGICAL CONCEPTIONS

In order to discover Truth in this manner by observation and reason, it is requisite we should fix on some principles whose certainty and effects are demonstrable to our senses, which may serve to explain the phenomena of natural bodies, and account for the accidents that arise in them; such only are those which are purely material in the human body, with mechanical and physical experiments; for we are not sensible of any other way of attaining to a true knowledge of the universal and particular affections of bodies.

Demonstration is an evident proof of some dubious proposition, so that nobody who admits the general principles, can deny their assent; these are purest in the mathematics, though there are many demonstrations no less evident in physic, especially those which are taken from anatomy. But there is no necessity for the principles of any art to be proved in that art, it is sufficient if their certainty is by any means. demonstrated in other arts.

These ought to be first adjusted with distinction, clearness, and certainty; with distinction, which points out one being from any other; as if one was to define a circle to be a right line continued upon a point till one end meets again with the other; with clearness, which consists of simple notions or ideas, easily conceived by any man in his senses, as

that two and two joined make four; with certainty, which cannot be denied by any reasonable person, or which must always appear true upon examination.

The universal laws of nature, or affections of all bodies, depend on mechanical and physical principles, upon which alone their actions are explicable; the same laws are also true in the human body, for its matter appears to be universally the same with that of all other bodies; so that what may be said to be true of all bodies, may be also affirmed true in our own. Thus, if one should affirm, that by the friction of two bodies would arise heat, the same will also be true upon the friction of solid parts in the human body. But then there are other principles not to be explained by these universal laws, but by some particular disposition in the certain body; these properties are called physical. But a physician ought to consider both the affections of bodies in general, as well as those only proper to the human body, that from a judicious comparison and just reasoning, he may never subject the human body to those laws only, to which the generality of, but not all, bodies are liable.

26. But as there are in the human body many other appearances not intelligible upon those principles, they therefore are not to be demonstrated and explained by such principles; if we would avoid error we must take a very different course for that purpose; this will readily appear to any one who considers and admits for true the following propositions, which are elsewhere demonstrated:

Such as memory, understanding, reason and the knowledge of past and future appearances; which are peculiar to the mind, a being without figure or extension, and conscious of pleasure and pain.

27. We are to consider (1) that Man is composed of a body and mind, united to each other; (2) that the nature of these are very different, and that therefore, (3) each has a life, actions and affections. differing from the other; yet (4) that there is such a reciprocal connection and consent between the particular thoughts and affections of the mind and the body, that a change in one always produces a change in the other, and the reverse; also (5) that the mind performs some actions by mere thought, without any effect upon the body; and that it has other thoughts, which arise barely from some change in the condition of the body; on the other hand, also, (6) that there are some actions performed by the body without the attention, knowledge, or desire of the mind, which is neither concerned therein as the cause or effect of those actions; that there are also some ideas formed in the