volatile compound of sulphur, hydrogen, and nitrogen, called of old "the fuming liquor of Mr. Boyle," still continues at times to offer up its sorry incense to his memory, But otherwise, his name is rarely referred to, except by professed historians of chemistry.

In natural philosophy, however, he retains, and will retain, a high place as an observer, especially in reference to pneumatics. The first to construct and employ an air-pump in England, a very little after the earliest airpump had been constructed in Germany, his name is inseparably connected with a department of knowledge which, dealing with the properties of the atmosphere, is indissolubly interwoven with every one of the physical sciences. We shall not, therefore, convey to the reader a false impression of the kind of reputation which Boyle possesses at the present day, if we refer to him as a natural philosopher, rather than as a chemist, although, did our limits permit, we should endeavor to show that he has done more for chemistry than most of his successors give him credit for. It would be a vain task, however, to condense six goodly folios into a few pages, and we have this additional reason, and it is our chief one, for selecting Boyle's pneumatics as the example of his scientific researches, that the early history of the air-pump in England has fallen into great and unaccountable confusion. The confusion is every day increasing, and cannot be remedied too speedily, so that a service will be rendered to present, as well as to past, science if we remove it. The subject, accordingly, is discussed somewhat fully in what follows.

Pneumatics, as a science, was little known to the ancients. An instrument corresponding to a very indifferent air-pump, was constructed by Hero of Alexandria, in which an imperfect vacuum could be produced by sucking out the air from the interior of a vessel by means of the mouth. The Alexandrian air-pump may be seen, at the present day, in the hands of our nursery maids who never heard of Hero or Alexandria. Children are amused by having a thimble or a nut-shell made to cling to the skin, after the air has been withdrawn from it by the action of the lips and cheeks. The thimble or the nut-shell vacuum is as perfect as Hero's can have been, and the mode of its production is probably as clearly apprehended in the nursery as it was in Hero's time, and for ages after. The Greeks and Romans had no air-pumps-not, however, because they had not sufficient ingenuity to

devise and construct them, for they used pumps to raise water; and an air-pump, though the cause of its efficiency in emptying a cavity of its contents is different, is merely a water-pump employed to withdraw air instead of water from a vessel. A false philosophy had taught them that nature abhorred a vacuum, so that a void was nonexistent and impossible, and those who had no faith in the possibility of a vacuum, were as little likely to try to produce one, as the scientific mechanicians of our day are likely to employ their ingenuity in endeavoring to realize perpetual motion. The world universally doubted or disbelieved that such a thing as literal emptiness could exist, till, in the early half of the seventeenth century, Galileo's celebrated pupil, Torricelli, demonstrated that it could. Nature may be truly said to abhor a vacuum, but she does not forbid one. A void is difficult to produce, and still more difficult to preserve. Absolute emptiness has perhaps never been realized, but a very near approach to it has been made, and the void may be retained for a long, though not perhaps for an indefinite period. Torricelli's vacuum, which exists in the upper part of every barometer, was produced by filling with quicksilver a glass tube, shut at one end, and more than thirty inches in length. The open end was then closed with the finger, and the tube was inverted and plunged with its mouth downwards below the surface of quicksilver contained in a basin. The finger was then withdrawn, the quicksilver immediately retreated from the closed extremity of the tube, which was held perpendicularly, and sank till it left a column of the liquid metal some thirty inches long. If the tube employed were three feet in length, a space six inches long would thus be abandoned by the mercury. This space, if the experiment were properly performed, was in winter, as nearly as possible, a perfect vacuum. In summer it contained a little of the vapor of mercury. In 1654, ten years after the Torricellian vacuum had been first produced, the famous consul of Magdeburgh, Otto von Guericke, remarkable as the inventor of the electric machine, as well as the air-pump, was led to the conclusion, whilst reflecting on Torricelli's experiment, that air in virtue of its elasticity would expand when relieved from pressure, and continue to abandon a hollow vessel connected with a pump put in action, till the vessel should become ultimately vacuous. After some preliminary trials, accordingly, of another kind, he connected a glass globe,

contrive an air-pump which should be more manageable than the German one, and free from its defects; and then adds, "after an unsuccessful trial or two, of ways proposed by others, the last-named person (R. Hooke) fitted me with a pump, anon to be described." (Birch's Boyle, vol. i. p. 7.) In

a manuscript which was not published till after his death, Hooke himself says, "in 1658 or '9 I contrived and perfected the air-pump for Mr. Boyle." (Waller's Life of Hooke, p. 3.)

This instrument consisted of two principal parts, a glass vessel, and a pump to draw the air out of it." The pump was so placed on a wooden tripod, as to have its mouth downwards, so that the piston-rod, or shank of the sucker, when, like the ramrod of a musket it was pushed home, ascended into the cylinder or barrel. The object of this invention was to allow the glass vessel, from which it emptied the air, to be placed in a vertical position above the pump. This glass vessel Boyle called the receiver, an apparently paradoxical title for a hollow globe, which was, if possible, to be emptied of its original contents, atmospheric air. The name, however, which is still retained, though modern air-pump receivers are differently constructed, was eminently significant, and marked an important difference between Boyle's airpump, and Otto von Guericke's.

The receiver was a globe, or rather a pear-shaped vessel, with a large aperture at its wider upper end, provided with an air-tight movable cover. Through this aperture the vessel could be made to receive any object, such as a burning candle, or a living animal, on which it was intended to try the effects of a vacuum. The hollow stalk of the pearshaped receiver terminated in a brass tube, provided with a stop-cock, and ground to fit into the upper end of the inverted cylinder. The latter had an opening in it close to the place where the stop-cock entered, which could be closed or opened by a brass plug, ground to fit it, and managed by the hand of the experimenter, or the worker of the pump. The piston, which had no aperture or valve in it, was not moved directly by the hand. The piston-rod had teeth cut on it at one side, so as to form a rack, which was raised or depressed by a handle acting on a pinion or toothed wheel, working into the teeth of the rack, as in the air-pumps of the present day. We shall not dwell more minutely on the peculiarities of the original English air-pump. An engraving of it will be found at the end of the first volume of Birch's Boyle, and in

It

the second volume of Shaw's Boyle, p. 472. It was necessary to describe it somewhat minutely, for a reason which will presently appear. The most important points to be noticed about it are, that unlike any later airpump, the cylinder and the receiver were directly connected, and, further, that it was provided with only one barrel or pump. appears to have been partly in reference to the former of those peculiarities, but also because he did not pretend to be able to produce an absolute vacuum, that Boyle named his instrument. He seldom calls it an air-pump. Once he speaks of Guericke's instrument as "the wind-pump, as somebody not improperly calls it." "Pneumatic pump" also but rarely occurs. The title he preferred for his instrument was that of pneumatical engine." Others called it the rarefying engine," and it was known over Europe as Machina Boyleana-Boyle's machine.

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It was strictly a pneumatical, not a rarefying engine. It could be used to condense air into the globular receiver, as well as to withdraw air from it, as Boyle showed, and was thus something else than a mere vacuumproducer. Vapors and gases could also be introduced into the globe, as they were, in many of the experiments made with it. It was thus best denominated an air or pneumatical engine.

At the present day it would be considered an awkwardly contrived, ill-proportioned, and imperfect instrument. It taught Boyle, however, and his contemporaries so much, achieved such wonders, was so difficult of construction, and so costly, that its possessor called it his "Great" Pneumatical Engine. He did not retain it long in his possession. With a rare and noble liberality, he presented it to the Royal Society in 1662, so that his poorer scientific brethren, who could not afford so expensive a piece of apparatus, might study pneumatics at his cost, and multiply experiments by means of the great engine. Acts as liberal have been done by many men on their death-beds, but seldom during their life-time; and wealthy philosophers have rarely descended from the height of advantage their riches gave them, to put into poor men's hands the means of rivalling and outstripping them in their favorite pursuits.

For six or seven years Boyle turned aside from pneumatic research altogether, and no one took his place, at least in Great Britain. Finding that few new experiments had been made in the course of many years, he resumed his inquiries into the properties of the air,

and began by constructing a new air-pump. His account of this, which he distinguishes as his "Second Engine," and of the experiments which he made with it, was published in the shape of a letter to his nephew, Lord Dungarvan, entitled "A Continuation of New Experiments, Physico-Mechanical, touching the spring and weight of the air, &c. &c, Oxford 1669." The letter is dated March 24, 1667, which we may consider the year in which the second English air-pump was constructed, though it may have been finished in the preceding year. Various considerations "invited me," says Boyle, "to make some alterations in the structure, some of them suggested by others, (especially the ingenious Mr. Hooke,) and some that I added myself, as finding that without them I could not do my work."

The second pneumatical engine, like the first, had a single barrel; but the mouth of the latter, from which the piston-rod projected, was turned upwards, and the barrel stood in a wooden box, or trough, filled with water, which rose above the mouth of the cylinder, so that the latter was entirely under water. The object of this arrangement was to keep the leather of the piston, or sucker, always wet, and, as a consequence, turgid and plump," so that it should move air-tight in the barrel. The piston, which was moved by a rack and pinion, had an aperture in it, which was closed and opened alternately, by thrusting in and pulling out a long stick, managed by the hand of the operator. But the great peculiarity and improvement in the engine was, that the receiver was not directly attached to the barrel. A tube, provided with a stop-cock, passed from the upper part of the side of the barrel, in a horizontal direction, along a groove, in a wooden board, covered by a thick iron plate, and was then bent up so as barely to project through the iron. The receiver was no longer a globe, or pear-shaped vessel, with various leaky apertures in it, but a bell-shaped, hollow, glass jar, which, turned with its mouth downwards, like an inverted drinking-glass, was, to use Boyle's homely but expressive words, "whelmed on upon the plate, well covered with cement." When the pump was wrought, the air in the bell-jar, or receiver, was drawn out through the horizontal tube. The reader familiar with pneumatics will recognize in the whole arrangement, a device which has been followed, with trifling alterations, in every later air-pump, down to the present day. Every modern air-pump has its "plate," made, however, not of iron, but of brass, or

of plate-glass; and the bell-jar receivers are whelmed on upon the air-pump plate, as they were in Boyle's day. One great advantage of this arrangement was the increased stability given to the apparatus, by transferring the heavy glass receiver, which in the first air-pump was fixed by a narrow tube to the barrel, to a flat support, on which it rested on a broad base. Another advantage was the avoidance of many apertures, which could not be kept air-tight, so that air should not leak into the receiver. For it must be remembered, that every pneumatic receiver, or other exhausted vessel, lies at or near the bottom of a deep sea of air, as a diving-bell does at the bottom of a sea of water; and the latter does not more readily rush into the bell, through the smallest fissure, than air forces its way along the most imperceptible channel, into the exhausted receiver. In the diving-bell there is air, at least, to resist the intrusion of water; but in the receiver there is a vacuum, soliciting the entrance of air. The fewer, therefore, the valves and stopcocks, the greater the chance of producing and preserving a good vacuum. A third advantage, to mention no more, was the facility which the plate afforded for placing on it any object, such as a candle, a barometer, a thermometer, a piece of clockwork, a growing plant, or the like; and when the object was exactly arranged, bell-jars, of various dimensions and shapes, could be laid over it, and the pump set working. In the first pneumatical engine, bodies intended to be subjected to a vacuum were awkwardly inserted by a large aperture at the top of the receiver, or suspended within it by strings.

In

Boyle published the account of the experiments he made with his second air-pump in 1669, and laid pneumatics again almost entirely aside for seven or eight years. 1676, however, he began to think of resuming the subject; and he was fixed in his resolution by a visit paid him by a very ingenious and inventive Frenchman, Denis Papin, whose name is still connected with one of his many devices, the Bone-Digester, a peculiar high-pressure steam-boiler, with which he effected strange triumphs in cookery. He has a place, and a high one, long overlooked, among the inventors of the steamengine; and it will presently appear that he has a claim, also overlooked, to a high place among the inventors of the air-pump. Papin came to England in search of some situation which might afford scope for his mechanical genius. Boyle had lost the services of Hooke, whom he generously released from his en

gagements with him, in 1662, in order that he might become Curator and Experimenter to the Royal Society. Papin, for a time, became assistant to Boyle, whose indifferent health prevented him from experimenting much himself, and a new series of pneumatic researches was undertaken. This was the more readily accomplished, that Papin had brought with him "a pneumatic pump of his own, made by himself," and much superior in efficacy to either of Boyle's pneumatical engines.

An engraving and minute description of Papin's air-pump are given in Boyle's tract, entitled, “A Continuation of New Experiments, Physico-Mechanical, touching the spring and weight of the Air, and their Effects, Second Part." The substance of this tract was first noted down in French, by Papin, who performed most of the experiments; then translated by Boyle, or under his superintendence, into Latin, in which the treatise was first published. Afterwards, this was translated, under Boyle's supervision, into English, in which it is reprinted in Birch's Boyle, vol. iv. p. 504. We cannot give the original date of the Latin or English editions of the tract, which must be regarded as the joint production of Boyle and Papin, but the experiments recorded in it are all dated. The first bears date July 11, 1676, (B. B. iv. 519,) the last, February 17, 1679, (B. B. iv. 593.) Papin's air-pump, which he brought with him, is, therefore, at least as old as 1676, which may be considered the date of its introduction into England. Its great peculiarity, as contrasted with former air-pumps, was, that it had two barrels. It was, according to Boyle, Papin's own contrivance. The former, referring to the use he made of the latter's mechanical devices in prosecuting his researches, says: "Not a few of the mechanical instruments, (especially the double pump and wind-gun,) which sometimes were of necessary use to us in our work, are to be referred to his invention, who also made some of them, at least in part, with his own hands." (B. B. iv. 506.)

Papin's air-pump was a curious machine; it had two pumps standing side by side, the mouths of the barrels being turned upwards. Each of the piston-rods terminated in a stirrup, attached to its upper end, and the stirrups were connected by a rope or cord, which passed over a vertical grooved wheel, or large pulley, fixed on a movable axis. Το

work the machine, the exerciser of the pumps, as he is called in the original account, put his feet into the stirrups, and holding on, as it

| should seem, by his hands, to the upper part of the frame-work of the pump, or leaning against it, (for the description is not precise on this particular,) moved his feet alternately up and down, as a hand-loom weaver does, or a culprit on the treadmill. The pistons, or suckers, which were bottomless brass cylinders, had valves opening upwards, like that of an ordinary water-pump; and similar valves were placed at the bottom of the cylinders, which were filled with water to a certain height, that the pistons might move air-tight in them. From the cylinders, tubes passed to a common canal, terminating in the airpump plate, on which receivers to be exhausted were laid, as in Boyle's second engine. The advantages of Papin's arrangement were very great. When a single pump is used, it becomes increasingly difficult, as the exhaustion proceeds, to draw out the piston against the pressure of the external air, which comes, towards the end, to oppose an unresisted force, equal to nearly fifteen pounds on each square inch, to the extrusion of the piston. When the piston, on the other hand, is pushed home, it is driven into the barrel with the same force which resists its withdrawal, and is liable to break the valves, or injure the bottom of the cylinder. But if the pistonrods of adjoining cylinders are balanced against each other, as those in Papin's machine were, so that the one ascends as the other descends, the evils described are all obviated. The resistance which the air offers to the ascent of the one piston is balanced, or nearly so, by the force with which it compels the other piston to descend, so that the two hang against each other almost in equilibrio. A very slight expenditure of force, accordingly, little more than is requisite to overcome the friction of the moving parts, suffices for the working of the pump. A double-barrelled air-pump not only exhausts twice as expeditiously as a single-barrelled one, but does double work for nearly the same expenditure of force. In this respect there is an essential difference between a double-barrelled airpump and a double-barrelled gun. In the latter, a double effect is gained only at the expense of a double expenditure of time and force. Two gun-barrels require twice the charge, loading, ramming, priming, and firing of one barrel, and take twice the time to load. In the air-pump, on the other hand, the working of the one piston renders much more easy the work of the other, and diminishes the time requisite for working both. The barrels of a musket are isolated, though lying side by side, and are not mutually dependent;