other was still breathing. This proved that the air which had lost its fire-air particles was lightest and rose to the top, so that the top mouse could no longer breathe. By these and a great many other experiments Mayow proved that air is made up of two portions-one heavy, which supports flame and life; the other light, and which is useless for burning or breathing, and this last was the largest portion. I want you to notice this particularly, because you will see by-and-by that Mayow had really discovered and described two gases. The one which he called fire-air was oxygen, which was not known to other chemists for more than one hundred years later, and the other and lighter one is now called nitrogen.

Having now proved that an animal in breathing uses up the same part of the air which a candle does in burning, Mayow wanted next to know what this fire-air did inside the animal. Harvey, as you remember, had proved that the blood passes through the lungs and there meets the air which we draw in at each breath. Here then, said Mayow, the fire-air particles must come in contact with the blood, and, joining with it in the same way as they do with the fat of a candle, must cause the heat of the blood. If anyone wants to prove this let him run fast. He will find that he is obliged to breathe more quickly and draw more air into his lungs, which will soon make his blood hotter and move more quickly, till his whole body glows with warmth. But if this mixture of the air with the blood does really take place, the arteries into which blood has just flowed from the lungs and heart ought to be full of air; and this is easily proved to be the case by putting warm arterial blood under an airpump, where, as soon as the pressure of the outside air is taken off, innumerable bubbles rise out of the blood as fast as they can come.

CH. XVI. BECHER AND STAHL-' PHLOGISTON.

135

In this way, by careful experiments and reasoning Mayow succeeded in proving that fire-air (or oxygen) is the chief agent in combustion and respiration. If he had not died so young he might have become more known, and men might have studied his discoveries, which he published in 1674. Unfortunately, however, he did not live to spread his knowledge, and a false theory of combustion caused his work to be forgotten for many a long year.

Theory of Phlogiston,' 1680-1723.-This mistaken theory was proposed by two very eminent chemists, John Joachim Becher (1625-1682) and Ernest Stahl (16601734). Ernest Stahl in particular was a man of great talent and perseverance, and he did a great deal for the study of chemistry by collecting a great number of facts about the way in which different substances combine together, and by arranging these facts into a system. But his theory of combustion was quite mistaken, and it seems very surprising that it should have been received by the chemists of that day in the face of the facts so carefully proved by Mayow. Stahl imagined that all bodies which would burn contained an invisible substance which he called 'Phlogiston,' and that when a body was burnt it gave up its phlogiston into the air, and could only regain it by taking it out of the air or some other substance. It would only confuse you to try and understand how this theory explained some of the facts of chemistry. You will see at once one which it did not explain, namely, why a body should grow heavier when it is burnt, as Geber, 1,500 years before, had shown it does. This fact alone ought to have been sufficient to prevent the theory gaining ground; but Stahl's fame was so great, and his imaginary Phlogiston' seemed to answer so well in a great many problems, that chemists believed in it

for nearly a hundred years, and Mayow's true explanation was forgotten till the eighteenth century, when fresh experiments proved Stahl's theory to be false.

་

Chief Works consulted.-Brande's 'Manual of Chemistry '—Introduction; Rodwell's Birth of Chemistry;' Yeats On Claims of Moderns to discoveries in Chemistry and Physiology,' 1798; Birch's 'Life of Boyle,' 1744; Shaw's 'Philosophical Works of Boyle,' 1725.

CH. XVII. FIRST USE OF THE MICROSCOPE.

137

CHAPTER XVII.

SCIENCE OF THE SEVENTEENTH CENTURY (CONTinued).

Malpighi first uses the Microscope to examine Living Structures-He describes the Air-cells of the Lungs-Watches the Circulation of the Blood -Observes the Malpighian layer in the human Skin-Describes the structure of the Silkworm-Leeuwenhoeck discovers Animalcules-Grew and Malpighi discover the Cellular Structure of Plants-The Stomates in Leaves-They study the Germination of Seeds-Ray and Willughby classify and describe Animals and Plants -The Friendship of these two Men.

Use of the Microscope by Malpighi, 1661.-We have now fairly left behind us the first fifty years of the seventeenth century; indeed, the experiments of Boyle and Mayow were all made after 1650. But I wish especially here to remind you that we have just begun the second half of the century, because it will help you to remember an important study which began very quietly about this time, but which has in the end opened out to us an entirely new world of discovery. In the year 1609, at the beginning of the century, Galileo brought distant worlds into view by the use of the telescope; and in like manner in the year 1661, or about the middle of the century, Malpighi, by the use of the microscope, revealed the wonders of infinitely minute structures, or parts of living bodies; enabling men to see fibres, vessels, and germs, which were as much hidden before by their minuteness as the moons of Jupiter had been by their distance. It is not quite certain who invented the microscope (μкpòs, little ; oкonέw, I look); but as the first which were made were only telescopes

(see p. 97), with lenses of such a focus as to look at an object near instead of far off, anyone may easily have hit upon the idea. The important point was the use made of them, and this, as far as regards the structure of living beings, we owe to Malpighi.

Marcello Malpighi was born at Crevalcuore, near Bologna, in the year 1628; he became Professor of Medicine at the University of Bologna in 1656, and was early distinguished for his discoveries in Anatomy, made chiefly by the use of the microscope. It is not possible for us, without a knowledge of anatomy, to understand thoroughly the structures which he described, but we may be able to form a general idea of the work he did.

One of his first experiments was the examination of the general circulation of the blood in the stomach of a frog, and he succeeded in demonstrating the fact that the arteries are connected with the veins by means of minute tubes called capillaries, thus proving beyond doubt the truth of Harvey's doctrine. His next work was to study the passage of the blood through the lungs (see p. 113), and to describe the air-cells from which the blood derives its oxygen. If you can get anyone to show you properly under the microscope a section of a frog's lung, you will see a number of round spaces bordered by a delicate partition; these are sections of air-cells, and round them you will see a network of minute tubes. Through these tubes or capillaries the blood flows in a living creature, and takes up oxygen from the air through the coverings or membranes of the aircells and capillaries, giving back in exchange carbonic acid to be breathed out into the atmosphere. Malpighi was the first to point out these air-cells and to describe the way in which the blood passes over them. After this he turned his