ATTRACTION OF MATTER.

13

matter in the universe, let two particles of matter be called into existence, and observed first at the distance of five miles, and then at three miles from each other. Their attraction for each other is greater at the distance of three miles than at the distance of five miles; but the greater attraction is not represented by 5 and the less by 3, but by the squares of those numbers, that is, by the one and the other of these numbers multiplied each into itself, the products of which multiplication are 25 and 9. Thus the attraction between these two particles at five miles' distance is represented by 9, and at the distance of three miles by 25. The law does not indicate the velocity with which two such particles will approach each other; but did we know what proportion each bore to the whole mass of the earth, then it might be discovered by reference to the velocity of bodies falling near its surface-sixteen feet in the first second.

We may here remark how the laws of motion mix themselves up with the law of gravitation, the same supposition being continued as to the absence of all other matter in space. If, after these two particles had approached to within three miles of each other, one of them were annihilated, all attraction would of course cease; but the other particle, in accordance with the first law of motion, would continue to move onwards in a straight line with the velocity which it had acquired at the moment of the extinction of the other.

Attraction. To express the attraction exercised by the particles of the sun over the particles composing each of the planets, numbers must be fixed upon which express, in some kind of dimension, the distances of each of these from the sun, and these numbers being squared we shall obtain a series denoting their relative attractions. To keep down the number of figures, it is best to choose some large measure, for example, the distance of the moon from the earth, or 240,000 miles.

In the following table are set down the squares of the distances of the old planets from the sun, expressed in numbers, denoting how many times each planet is more distant from the sun than the moon is from the earth.

These numbers, however, do not express the actual attraction between the sun and these several planets; but only what their relative attractions would be, if each contained the same number of particles. But where an estimate is already formed of the quantity of matter in any planet, and that quantity is considered in connection with the estimate of the quantity of matter in the sun, and the actual velocity in bodies falling near the surface of the earth, then the elements are afforded for calculating the actual force of gravity between the sun and that planet. The roots corresponding to the numbers in the above table denote the actual distances of the planets from the sun, as measured by the distance of the moon from the earth,—namely, for Mercury, 160; for Venus, 280; for the Earth, 400; for Mars, 600; for Jupiter, 2,100; for Saturn, 3,600; for Uranus, 7,600; or nearly as 1, 2, 3, 4, 15, 28, 54.

It is easy to see that the law of gravitation is sufficiently stated, when made to refer to particles of matter, by simply saying that the particles attract each other inversely as the squares of the distances. For it follows, as a necessary consequence, when a number of particles are collected into one mass, and a lesser number of particles into another mass, that the sum of the attractions in the one shall be to the sum of the

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MOMENTUM AND VELOCITY.-PHYSICS,

attractions in the other, directly as the number of particles in the one, is to the number of particles in the other. Again, when two bodies of the same bulk exhibit exactly the same attraction the one for the other, and under the same circumstances, we conclude that the number of particles in each is the same; and this is what is signified when it is said that two bodies have the same density. Moreover it can be proved that the attraction between the centres of two spherical masses of matter is the same as if the whole particles of each mass were collected within their respective central points.

The attraction between two bodies, or masses of particles, is measured, not by the mere velocity acquired by each, but by the amount of motion, or the momentum which each exhibits. When two masses of matter, different in the number of their particles, are supposed to come into existence in free space at some distance from each other, the quantity of motion produced in each is the same. That which contains the greatest number of particles would move with less velocity; that which contains the less number of particles with greater velocity; but the momentum, or quantity of motion, in each will be the same.

It is easy, then, to understand, on the principle of gravitation, why two bodies-for example, a pillow and a piece of lead equal to the pillow in weight—were there no atmosphere, would fall to the ground from a given height in the same time. Both would have the same momentum: but the momentum or impulse of the piece of lead would be impressed on a small portion of the surface, while that of the pillow would extend over a large surface, so that each point of that surface would be less affected.

At first consideration, it may be somewhat difficult to see clearly that this great law of gravitation essentially differs from a mathematical proposition, as resting not on intuitive convictions but on observed facts. But a closer view of the whole subject satisfies the inquirer that no law of this kind could have been predicted à priori; that is, from any natural or intuitive conviction of the human mind. Such knowledge has no other foundation than observation. What confuses the mind is the large extent to which mathematical investigation is employed for the assistance and perfection of observation. Here, however, mathematical investigation serves merely the office of an instrument, by which, indeed, the dominion of the senses over nature is almost immeasurably

increased.

Physics.-The several subjects just noticed fall strictly under the head of Natural Philosophy or Physical Science, and indeed merely afford examples of the kind of knowledge which belongs to that great department. But when we consider that Natural Philosophy is ancillary to the great objects of Mechanical Science-to the construction of Time-keepers, the Hydraulic Press, the Steam Engine, Artesian Wells, Gunnery, the Pendulum, Telescopes, Microscopes, the Barometer, the Tides, Railways, &c. —we shall be able to estimate the vast importance of a knowledge of its various subdivisions to men, particularly to those living in countries newly settled, and where the division of labour has not yet been carried sufficiently far to save every man from the necessity of being his own engineer and overseer. Even in the long-established social communities of modern Europe, we have but to glance the eye over the career of individuals of great activity of mind rather than of solid education, to discover how much time and money are annually wasted in the vain hope of accomplishing what is unattainable. Many a man of genius in former times, unenlightened by the knowledge this Work is intended to convey, has wasted his life and fortune in fruitless efforts to discover the perpetual motion. And although this is not often now the object to which

CHEMISTRY AN INDUCTIVE SCIENCE.

15

uninstructed ingenuity is directed, there is still as much health, as much genius, as much industry, as much wealth consumed on things unattainable as in former ages.

It is

Electric Sciences.-The fact that amber, after being rubbed upon woollen cloth, first attracts light bodies and then repels them, and upon which the Science of Electricity rests, derives all the evidence of its truth from observation. The same may be said of all the discoveries hitherto made in Electricity. There is no principle in the whole subject which could have been inferred independently of observation. purely a science of induction; and the same remark may be made of Galvanism. It was as impossible to predict, à priori, the decomposition of water, and the other surprising effects of Galvanism, by the mere approximation of two metallic plates immersed in an acid solution, as it is to establish, à priori, after the effect is witnessed, that it is really due to the apparatus employed. Of Magnetism, what more can be said than that certain facts have been ascertained by observation? And although it is now sufficiently apparent that Electricity, Galvanism, and Magnetism are merely different forms of one more general science, that conclusion has been deduced, not from any à priori reasoning, but simply from the accumulation of facts, and the inference of principles from these by the common process of induction.

Under the heads just noticed, together with those of heat and light, how many subjects fall, of surpassing interest and of the most direct use to men in every situation of life! Some years ago, when the number of steam-boat accidents in the United States attracted public attention, an American writer successfully showed that as many persons every year lost their lives by lightning, within the Union, as by the bursting of steam-boilers. Increased knowledge and attention on the part of engineers have very much diminished the annual mortality from steam-boat accidents; and surely it is not too much to expect that the great annual loss of life by lightning may be materially circumscribed by a better acquaintance with the nature of the electric fluid, and the precautions which such a knowledge may suggest for avoiding danger during the violence of a thunderstorm.

Chemistry.-But Chemistry supplies the best example of a purely inductive science; and the progress which Chemistry has already made is sufficient to make known the final composition of the bodies which man sees on every side around him. It teaches that, out of sixty-three simple substances, all these bodies are constituted. It shows him how to obtain each of those simple substances in a state of purity; and, when it is required, it points out how these simple substances are to be converted into such compound bodies as are necessary to the arts and conveniences of life.

In Chemistry there are no original à priori rules. There are no facts or laws discoverable by the mere light of thought, independently of experiment and observation. All that the exercise of genius can do in Chemistry is to suggest new paths to be explored. Chemistry, therefore, is a science which enables us to understand both the extent and the limits of the Baconian precepts. It is wholly inductive; and yet the principles which induction has here afforded, while they are numerous and most available, are, as laws of nature, neither free from exception nor very comprehensive.

It is by the study of the mere properties of substances that chemists have achieved most of their success. The early progress of Chemistry was tardy in the extreme, until gaseous bodies fell under rigid examination; and from that date its progress has been almost incredible. Chemists for ages knew of several sorts of air; but they seem never to have arrived at the idea, that by determining the several peculiar properties of these airs they might be able to distinguish them from each other. Hydrogen gas has

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RELATION OF ART TO CHEMISTRY.

been known from time immemorial as an inflammable vapour, which played about the apparatus whenever sulphate of iron was directly made by the addition of dilute sulphuric acid to iron filings. But although its peculiar inflammable character was known, and even its smell in this way of producing it, and also that it did not appear unless a large proportion of water was added to the acid; yet no one thought of seeking the means of identifying it when otherwise produced, until Cavendish noticed its extreme levity.

There was no deficiency of genius or industry among chemists during the period of this slow progress; but with all their solicitude to pursue the precepts of Bacon, they do not appear to have sufficiently felt the necessity of an exact knowledge of the peculiar properties of every substance, and the means of its identification when present in minute quantity. The only efficient aid which chemistry has derived from exact knowledge is the homely aid of the balance. Until recently, chemical operations were too rude to admit of much advantage from the nice determination of the weights of the substances employed in experiments; otherwise, how many difficulties of former times might have been solved without delay!

In the experiment of burning hydrogen gas with oxygen gas, it was remarked, at an early period, that the apparatus became bedewed with moisture. The gases shrank into nothing, and moisture was found upon the apparatus. Yet it was a long time before the conclusion was drawn, that the water was the product of the combustion. The balance would at once have settled this point, by showing that the water produced equalled the sum of the weights of the two gases exploded.

The subjects which Chemistry embraces are so many necessities of man in his social life. A few examples of the departments of art founded on Chemistry will suffice to show how desirable a knowledge of Chemistry is to every man, whatever his occupation in life. Among these stand prominent the extraction of metals from their ores; the subject of artificial light, or the various modes of artificial illumination; the arts of dyeing and bleaching; the substances fit for fuel; the nature of firedamp and choke-damp in mines; the artificial production of ammonia, in reference to agriculture; gunpowder; artificial minerals; chemical tests, and the detection of poisons; ventilation, and disinfecting agents; cements; artificial minerals; pigments; metallic alloys; and other subjects which it is needless here to enumerate.

Physiology. Next in order to Chemistry stand the Physiological Sciences. The discoveries in this science are to a great extent peculiar laws of nature, while many of the phenomena of living bodies are physical, chemical, and electrical. When the muscular fibre shortens itself on the application of a stimulus, it is in obedience to a pure Physiological law. When the impression conveyed from the surface of the body by a reflex nerve is succeeded by an influence transmitted to an organ of motion, it is in obedience to another distinct physiological law.

Certain laws of nature acting together with the laws of motion produce the planetary movements, so strikingly remarkable for symmetry and harmonious union with each other. On the other hand, certain laws of Physiology, in apparent opposition to the laws of physical nature, and to the ordinary laws of Chemistry, produce effects in every way so surprising, as to have engaged the attention of men in all ages, upon the very peculiar nature of the influences by which such effects can be called forth and sustained with an almost unerring uniformity, during the various limited periods to which the existence of individuals in the two organic kingdoms extends.

There is nothing, in the whole character of physiological science, more at variance

OBJECTS OF PHYSIOLOGY.

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with the general economy of nature than the limited duration, in each individual, of the phenomena which constitute animal or vegetable existences;-and the complete isolation, throughout its whole existence, of each individual from other portions of the organic world, after the first separation from the parent organism, is another most striking and peculiar feature of physiological science. The innumerable forms which organism assumes, in the varieties of animal and vegetable species, set at nought every possible idea of their source being a mere physical force of development, under the limitation of a few overruling influences. And what is not less remarkable than the characters already stated, is the manifestation, at every step, of the nice accommodation of means to peculiar ends, in the structure and economy of organic bodies, which renders it impossible to seize the mere inductive laws of Physiology, without a perpetual reference to final causes.

If it be said that the animal or plant could not have existed without certain organs adequate to certain ends-and therefore that such contrivances are merely indispensable conditions of existence, the answer is, that organic nature is not a necessary part of the economy of the universe; that the material world, without the organic, was complete in itself; and therefore it is to be concluded, because the organic world exists with marks of design, such as characterise the works of man on earth, as distinguished from the works of nature, that in the origin and maintenance of the organic world there is manifested a special intelligence and wisdom, without continual reference to which Physiology will fail to make the progress of which it is susceptible.

The knowledge of Physiology opens up a new field of human thought. In it we trace the wisdom of the Creator, as in Astronomy we discover manifest proofs of his power. Galen said with truth,-" The study of Anatomy is the use of a hymn in praise of the wisdom of God." This is, indeed, the most dignified office of Physiology; and it is in this light that it exhibits its greatest glory. But to how many subordinate uses is it also subservient! Under Physiology, in its largest sense, stand Medicine and Surgery. In proportion as the knowledge of even a rude Physiology has diffused itself, has the value of human life increased. Both Medicine and Surgery are but handmaidens of Nature; but how ineffectual-nay rather, how pernicious-were man's natural modes of treating discases and injuries, until the knowledge of Physiology had enlightened him. One great use of a knowledge of Physiology is to teach men what they should avoid doing when diseases have arisen, or injuries have been sustained. He who understands something of the animal economy, knows with what precaution he should employ less known remedies; while he knows also, that even good remedies are only good when seasonably used. And this knowledge, so far from unfitting him for finding new remedies among the natural productions of a strange place, affords him an infinite advantage over every one who, without such knowledge, ventures to experiment upon a disease. Let a man understand the general scope of Physiology, and he becomes, under sickness or injury, a safe guide in the wilds of Australia; while he who is ignorant of the animal economy, if he uses remedies at all, uses them as much at random as in the days of spells, amulets, and charms. If he has studied Botany, he knows, as we shall presently see, from which families of the vegetable kingdom safe drugs may be taken, and from which poisonous substances may be feared.

Man, in every country, acquires the most part of his knowledge by experience; but in every complex kind of knowledge, like that which relates to man, animals, and plants, his experience deceives him, unless he be previously acquainted with the general scope of nature in that department. Hence a new settler in a strange country,

B