we now know that hydrogen is 14.4 lighter, but as his predecessors could not distinguish any difference between the gravity of hydrogen and the atmosphere, and believed all gases to be of equal weight, his approximate calculation is astonishing. But of course this was Cavendish's only function in life-to weigh things and measure them and analyze them.

Cavendish's discovery of the lightness of hydrogen was the nucleus around which developed a daring and noble art:

Among the picturesque hills of Annonay, dwelt old Peter Montgolfier, manufacturing paper-bags that brought him comfortable money. He had two sons, Stephen and Joseph, who helped him in the business, but also spent considerable time reading Priestley's and Cavendish's essays on different kinds of air,—and watching the clouds on high. They wondered what would happen if they could imprison a bag in a cloud: would it rise and float above their heads, sailing beyond the church-steeple and over the tree-tops.

It was not so easy to catch a cloud, but since hydrogen was so very light, why not try that? They did make the attempt, tho an unsuccessful one, for the paper proved permeable to the gas. But the sons of Peter Montgolfier were prejudiced in favor of pulp, and instead of abandoning the paper as an unsuitable envelope for the gas, they sought for another gas more suitable for the paper.

Smoke ascended: every naked savage knew that. If a bag should be filled with smoke, would it not also ascend? How strange that some restless intellect, an Aristotle or a Paracelsus or a Newton, had never thought of it!

The Montgolfier brothers filled a bag with smoke, and the bag arose; they raised their hands, but the heated air carried the bag beyond their reach-higher than any house in the village. So the first balloon sailed in a little town that had nothing to make it famous except a Gothic church built in the fourteenth century.

The Montgolfiers came to Versailles to amuse the French court; and as Louis XVI and Marie Antoinette required all

they saw to be gorgeous, like their too-ornate palaces and over-gilt carriages, the balloon was showily painted with ornaments in oil. An osier cage was suspended at the bottom, and in it were placed a sheep, a cock, and a duck: the first aërial passengers except of course the birds that travel on the wing. The balloon mounted to a height of 1500 feet and remained in space till the hot air acquired the temperature of the surrounding atmosphere-in eight minutes- and then descended in the wood of Vancresson, two miles distant. The sheep kicked the cock, hurting its right wing, but otherwise there were no accidents. Thus, amid the masque of powder and the scent of patchouli, was born the art of aëronautics.

Naturally these experiments excited every alert mind: Would man learn to aviate the atmosphere? Would the fabled wings of Dædulus become realities? Young Jenner made a balloon of his own, and old Euler was all agog. Leonard Euler was the greatest mathematician of the century. The hand of the potter did not shake when he was molded: he had a giant's strength and an intellect that matched. But Euler abused his privileges. In three days he solved a problem that took other experts months. He found the answer, but nature protested. Euler sank to his bed in a fever, and when he rose, there was no sight in his right eye. The mighty man laughed, and continued his work. A cataract formed in his left eye, making him practically blind. But Euler did not cease his investigations. Then Wenzell came to the mathematician, and couched his cataract, and Leonard Euler once more saw his wife and children. Take care,' said Wenzell, but Euler studied day and night, till sight left him again. But Leonard Euler worked on. He was throwing light on every phase of mathematics - how could he stop for his own physical darkness? Age advanced upon him, but he continued his researches, dictating memoirs on planetory perturbations, perfecting integral calculus, creating the subject of partition of numbers, inventing the calculation of sines, nearly squaring the circle, winning prizes from even the Ber

nouillis. Scornful of infirmities, his magnificent activities would have exhausted a score of lesser men. He had long passed the allotted years of the psalmist when he heard of the ascensions at Annonay. The mathematical theory of the motion of a balloon engrossed à mind that knew so well the motions of planets. The skilled hand grasped the chalk, and when great Leonard Euler was dead,- having ceased to calculate and to breathe at the same moment his blackboard was found covered with his last investigation: the rate of ascent of a balloon.

-

At this time a balloon was constructed in Paris by the brothers Robert, under the direction of Jacques Charles. The latter gentleman was a professor of natural philosophy, and desired that his balloon,- which was of silk, and varnished with a solution of elastic gum-be filled with hydrogen instead of hot air. For a few days the gas was prepared by throwing five hundred pounds of dilute sulphuric acid on a thousand pounds of iron filings, but one day the balloon disappeared from its accustomed spot, the Palace des Victoires. Too great a crowd had gathered there, and during the night, preceded by torches and guarded by soldiers, the precious object had been conveyed to the largest open space in Paris, the Champ de Mars. The next day a cannon-shot gave the signal for the ascent, and the balloon circled three thousand feet above the immense sea of faces that watched below. The rain descended, wetting the balloon and drenching fair ladies, but no one paid any attention to the shower. For three quarters of an hour the balloon sailed in the elastic fluid, finally falling in a field where some frightened peasants tore it to fragments.

But the superiority of inflammable air-as hydrogen was then called over hot air was demonstrated, and the future of balloonery was assured. As far as science is concerned, this was important for at least two reasons. In the first place, some scientific experiments of value were performed by means of the balloon, as can be seen without leaving Cavendish's own work: Cavendish, as mentioned above, determined the

composition of the atmosphere, showing the exact relation between its two most important ingredients, nitrogen and oxygen. He made hundreds of experiments which proved that the composition of the atmosphere is constant, it being immaterial whether the air to be analyzed is collected on fair days or foggy, or from the pure country or the sooty city.

This led some chemists, such as Prout and Thomson, to maintain that the air must be a chemical compound. But John Dalton, discoverer of the Atomic Theory, correctly insisted that the air is merely a mechanical mixture of constant composition. He believed, however, that since nitrogen is lighter than oxygen, the relative amount of the two gases varies at different heights from the earth's surface, the nitrogen increasing and the oxygen diminishing as we ascend.

To see if this were true, Gay-Lussac collected the atmosphere in the streets of Paris, and also gathered air in a balloon at an elevation of seven thousand meters, for the chemist who climbed Vesuvius when the volcano was vomiting violently, was likewise the most enthusiastic aëronaut of the day. His analysis, which has since been placed beyond doubt, proved there was no change in the proportion of nitrogen and oxygen. In this work, Gay-Lussac was aided by Thenard, the discoverer of hydrogen peroxide. We do not believe with the alchemists that the baser metals can be transmuted into gold, but we know that the application of hydrogen peroxide turns a brunette into a blonde.

In the second place, ballooning accustomed men to navigate the atmosphere, and thus prepared the way for the introduction of the aëroplane,— and for Langley, Zeppelin, Bleriot, Farman, Santos-Dumont, Arch Hoxsey, Hugh Latham, Glen Curtiss, the Wright brothers, and all the glorious bird-men who yesterday conquered the empire of the air.

The laboratory method of Cavendish was not only orderly, but organized. He did not leap from subject to subject with the speed that Priestley forsook one road of research for a more novel path. Priestley performed his experiments with

enthusiasm and ease.

one.

He was a great man and a careless

He would announce a new theory before breakfast, reject it at lunch, and formulate another at the supper-table. He made discoveries or mistakes with equal nonchalance.

Priestley was like a wonderful child amusing himself with apparatus. For the 'entertainment of a few philosophical friends' he performed what he called 'a random experiment,' which consisted in exploding hydrogen and oxygen in Volta's electric eudiometer. After the spark had passed, a dewy deposit was found on the sides of the glass, but Priestley paid no attention to the moisture. This was by no means the only occasion on which the versatile doctor had an important discovery nearly in his hands, and brushed it aside for a more cautious observer to pick up.

Among the philosophical friends who witnessed Priestley's experiment was John Warltire, who wanted to know whether heat is heavy, and with this object in view he burned the gases several times, and weighed the flask after the explosion and after it cooled, reaching however the incorrect conclusion that heat is a ponderable body.

At this time Cavendish was working with the air, and as soon as he heard of Priestley's and Warltire's experiments, he repeated them. He took greater precautions than Warltire, and finding no difference between the first and second weighing, satisfied himself that heat is not a material entity.

Nor did he disregard the deposit of dew which Priestley passed by so heedlessly. On the contrary, Cavendish considered it as 'likely to throw great light on the subject and well worth examining more closely.'

Various explosions were now heard in the Cavendish laboratory, till it was ascertained that when one volume of oxygen is detonated with two of hydrogen, the gases combine to form a liquid which proved to be water!

Since the dawn of inquiry, when man first speculated on the composition of substances, water was considered an element. Egyptian magician, Chaldean priest, Greek philosopher,