CHAPTER XVI. SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). Boyle's Law of the Compressibility of Gases-This same Law discovered independently by Marriotte-Hooke's theory of Air being the cause of Fire-Boyle's experiments with Animals under the Air-pump-John Mayow, the greatest Chemist of the Seventeenth Century-His experiments upon the Air used in CombustionProves that the same portion is used in Respiration --Proves that Air which has lost its Fire-air is Lighter-Mayow's 'Fire-air' was Oxygen, and his Lighter air Nitrogen-He traces out the effect which Fire-air produces in Animals when Breathing. Boyle's Law of the Compressibility of Gases, 1661.-The Hon. Robert Boyle, seventh son of the Earl of Cork, and one of the principal founders of the Royal Society, was born in 1626. He had very delicate health, and when quite young travelled much abroad and learned there a great deal about science even before he was eighteen years of age. He was deeply interested in Galileo's discoveries, and was in Florence when that great astronomer died in 1642. After his return to England, when he was at Oxford, he read an account of Guericke's air-pump, and was so delighted with this new discovery that he set to work at once to make one without ever having seen the original. He succeeded so well, with the help of his friend and assistant Dr. Hooke, that his air-pump became famous, and many writers have by mistake given him the credit of being the CH. XVI. COMPRESSIBILITY OF GASES. FIG. 18. 129 inventor. We have seen, however, that Guericke was the first to hit upon this instrument; Boyle only improved it, and made with it many very valuable experiments upon the weight and nature of air. These are too many and lengthy for us to examine here; but there is one law about the compression of gases which you will find connected with Boyle's name in all books on physics, and which you ought to understand. Boyle knew from Torricelli's experiment that the weight of the atmosphere upon the air, close down to our earth is about equal to the weight of 30 inches of mercury in a tube (see p. 118). Now he wished to find out how much air is compressed, or forced into a smaller space, when more weight is put upon it, and to prove this he devised the following experiment. He took a tube a', open at the long end and full of ordinary air, and by putting a little mercury into the tube and shaking it carefully till it settled at the bottom, he cut off a small quantity of air between band c. This air was of course still under the usual weight of the atmosphere, which pressed down upon the mercury through the open end of the tube. But the mercury did not add to -60 55 50 45 -40 35 Pressure of Mercury 30 Pressure 30 25 Atmosphere 26 -20 the weight because it stood at the same height on both sides of the tube, and so was evenly balanced. He next added more mercury, till it stood 30 inches higher in the long end than in the short one (as seen in, a2). The air between b and c was now pressed down twice as much as before, for it had the 30 inches of mercury weighing upon it, as well as the atmosphere, which equalled another 30 inches. Boyle found that this double pressure had squeezed it into half the space (b c, fig. A2); in other words, by doubling the pressure he had halved the volume of the air. He then poured in 30 inches more mercury, making the pressure three times as great as at first, and he found the air was now compressed into one-third of the space it had filled at first. And this he proved to be the law of compression of air and of all gases, that the volume of a gas (that is, the space it fills) is decreased in proportion as the weight upon it is increased. If you double the pressure you have the volume; if you halve the pressure you double the volume. This law of the compressibility of gases is known as Boyle's Law, or sometimes as Marriotte's Law, because a Frenchman named Marriotte also discovered it some years later without knowing that Boyle had done so. It is not always absolutely true, but we cannot stop to discuss the exceptions here; you will find them in books on physics and chemistry. Boyle and Hooke both gave much time to the study of chemistry. Hooke published a theory in 1665 that air acts upon substances when they are heated, and so produces fire; for, said he, in making charcoal, although the wood is intensely heated and glows brightly, yet so long as the air is kept away it will not be consumed. Boyle also proved that a candle will not burn, nor animals breathe, without air. He found that when he put mice and sparrows into his airpump, and then drew out the air, they died; and that flies, CH. XVI. JOHN MAYOW. 131 bees, and even worms, became insensible; while fish, though they lived longer than the mice, soon turned on their backs and ceased to live. He also put a bird under a glass vessel full of air, and it died after three-quarters of an hour. It was clear, therefore, that fresh air is necessary to life, and Boyle began to think that just as a candle-flame cannot be kept up without air, so there must be some vital fire in the heart which is extinguished when air is shut out from it. This opinion he discussed at the Oxford meetings, and a young physician named John Mayow listened very eagerly, and then went home and set himself to try and find out what this strange power in the air could be, without which neither fire nor animals could exist. Mayow's Experiments on Respiration and Combustion, 1645-1679.-John Mayow's private history is very short. He was born in Cornwall in 1645; he became a Fellow of All Souls', Oxford, and practised as a physician in Bath; and finally he died at the house of an apothecary in York Street, Covent Garden, in 1679, before he was thirty-four years of age. This is all we know about his life; but he must have been a diligent worker and a real lover of science, for though he died so young he left behind him an account of a number of experiments and discoveries which entitle him to be called the greatest chemist of the seventeenth century. I wish we could go through all his experiments, for they form a rost beautiful lesson of the earnest and painstaking way in which God's laws should be investigated. Mayow never made a careless experiment; he never thrust in his own guesses when it was possible to work out the truth; he went on patiently step by step, taking every care to avoid mistakes, and never resting till he had got to the bottom of his difficulties. Let us now take some of his experiments on combustion, or burning, and respiration, or breathing, and try and follow them as carefully as he did. It seemed to him clear from the experiments of Boyle and Hooke that there must be something in the air which gave rise to flame and breath, and that this could only be a small part of the air, since a candle when put under a bellglass went out long before all the air was gone. He first of all satisfied himself by experiments that this gas which burnt, and which he called fire-air, was not only in the atmosphere, but existed in nitre, or saltpetre, and also in many acids; and then he set to work to discover how much of it there was in ordinary air. To do this he took a piece of camphor, with some tinder dipped in melted sulphur, and placed it on a little platform hung inside a bell-jar (see Fig. 19). He then lowered the bell-jar into a basin of water, having first put a siphon or bent tube under the bell-jar to let enough air out for the water to rise. Then he took the tube out, leaving the water at the same height inside and outside the jar, while the rest of the jar above the water was Mayow's experiments on combustion and respiration (Yeats) full of air. He now held up a burning-glass, and brought the sun's rays to a focus upon the camphor and tinder till it grew hot and burst into a flame. As it burnt he noticed that |