CH. XIII. FRANCIS BACON.-'NOVUM ORGANUM.

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CHAPTER XIII.

SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). Francis Bacon, 1561-1626-He teaches the true method of studying Science in his Novum Organum'-René Descartes, 1596-1650-He teaches that Doubt is more honest than Ignorant AssertionWillebrord Snellius discovers the Law of Refraction, 1621Explanation of this Law.

Bacon's Influence upon Science. -Although this book is a history of scientific discovery and not of philosophy, yet we must now mention in passing two philosophers who lived about this time, and whose writings had great influence upon science. These were Francis Bacon in England, and René Descartes in France.

Francis Bacon, commonly known as Lord Bacon, was born in London in 1561, and died in 1626. He was made Lord Chancellor of England in 1618, in the reign of James I., with the title of Lord Verulam and afterwards Viscount St. Alban's, and was a great political character. Bacon devoted much of his time to science, and, like his namesake Roger Bacon in the fifteenth century, he seems to have foreseen many of the discoveries which were afterwards made. But his most useful work was a book called the Novum Organum,' or 'New Method,' published in 1620, in which he sketched out very fully how science ought to be studied. He insisted that no knowledge can be real but that which is founded on experience, and that the only

true way to cultivate science is to be quite certain of each step before going on further, nor to be satisfied with any general law until you have exhausted all the facts which it is supposed to explain.

For example, if you require to understand what heat is, and how it acts, you must not be satisfied, he says, by merely making a few experiments on the heat of the sun and that of fire, and trying from these to lay down some general rule of how heat works. 'No, you must examine it in the sun's rays both when they fall direct and when they are reflected; in fiery meteors, in lightning, in volcanoes, and in all kinds of flame; in heated solids, in hot springs, in boiling liquids, in steam and vapours, in bodies which retain heat, such as wool and fur; in bodies which you have held near the fire, and in bodies heated by rubbing; in sparks produced by friction, as at the axles of wheels; in the heating of damp grass, as in haystacks; in chemical changes, as when iron is dissolved by acids; in animals; in the effects of spirits of wine; in aromatics, as for example pepper, when you place it on your tongue. In fact, you must study every property of heat down to the action of very cold water, which makes your flesh glow when poured upon it. When you have made a list,' says Bacon, of all the conditions under which heat appears, or is modified, of the causes which produce it, and of the effects which it brings about, then you may begin to speak of its nature and its laws, and may perhaps have some clear and distinct ideas about it.'

You will see at once that this method of Bacon's had been followed already to a great extent by Copernicus, Tycho Brahe, Galileo, and Kepler; but Bacon was the first to insist upon it as the only rule to follow, and in doing this he rendered a great service to science.

CH. XIII.

WRITINGS OF DESCARTES.

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Descartes' Condemnation of Ignorant Assertion.-René Descartes, by his philosophy, assisted science in another way. He was a Frenchman, born in Touraine in 1596, and he became one of the most famous philosophers of France. He wrote a great deal on science, especially on mathematics and geometry, and also on the nature of man; but the point which we have to notice here was his belief that to arrive at the real truth was the only thing worth living for.

You will remember how the men of science of the sixteenth century had thought it a sufficient answer to Vesalius or to Galileo to say that Galen or Aristotle had decided questions of anatomy and physics ages ago; and how the judges of the Inquisition thought they had crushed the Copernican theory when they made Galileo recant. Authority was the idol to which these people bowed down, and they considered it rank heresy to doubt anything which had been taught by their forefathers. But Descartes said, 'It is not true to say we know a thing simply because it has been told us. It is a duty to obey authority, to submit to the laws and religion of our country and parents, and in matters where we are not able to judge, it is wise to receive what is told us by those who know more than we do. But to know anything requires more than this, and unless the reasons for any belief are so clear to our minds that we cannot doubt them, we have no right to say we know it to be true, but only that we have been told so.'

I think you can see how this rule of Descartes, that it is often more honest to doubt than to be quite sure without good grounds, would influence science. If scientific men in the time of Galileo, instead of saying 'We know that a heavy weight falls more quickly than a light one because Aristotle said so,' had said more modestly, 'We do not know, because

we have never tried, but we think it probable Aristotle was right until someone shows us that he was mistaken; '—if they had gone to the Tower of Pisa in this spirit, they would not have denied the truth of Galileo's experiment when it succeeded before their very eyes. And even now, in the present day, you will see that the greatest and best men who make the most discoveries, are those who are always willing to examine a new fact, even though it may contradict much that they have held before; and who never pretend to know for certain anything which they have not studied with sufficient care to be convinced of its truth.

These last few pages may be rather difficult for you to follow, but the chief lessons which it is necessary you should remember may be summed up in a few words. Bacon and Descartes both did great service to Science-Bacon by teaching that any true theory must be built up upon facts and careful experiments; Descartes by insisting that it is more honest to acknowledge we are ignorant, and to wait for more light, than to pretend to know that which we have not clearly proved.

Snellius Discovers the Law of Refraction, 1621.Among other things, Descartes wrote much upon Optics, and you will often see it stated that he discovered the law of refraction. This law had, however, been laid down before, in 1621, by a Dutch mathematician named Willebrord Snellius, and Descartes only stated it more clearly. You will remember that the Arab Alhazen first pointed out that rays of light are bent or refracted when they pass from a rarer into a denser substance or medium (see p. 47), as for instance from air into water; and that the denser the medium is into which they pass the more the rays are refracted. Vitellio and Kepler had measured some of the

CH. XIII.

THE LAW of refrACTION.

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angles at which rays are refracted in water and glass, but they did not know of any law by which they could calculate how much any particular ray would be bent out of its course.

For instance, in Fig. 12, suppose w w to be the surface of water in a glass vessel, upon which the rays A and B fall at

ww, Water. A A, B B, Rays passing from air into water. dd, Line from the ray A to the perpendicular, in the water, three-fourths the length of cc from the ray A in the air. d d', d d, Similar lines from the ray B.

the point o, and are refracted a to A' and B to B'. It is evident that B is bent much more out of its course than A, as you will see at once if you lay a straight ruler from end to end of each ray; and if we were to draw other rays between these they would all be refracted at different angles, those being most bent which were farthest from the perpendicular.

Now in making telescopes it is very important to know how much each ray is refracted; and as the rays are infinite in number, it was impossible to know this unless some