the principle, but without drawings and diagrams, we cannot show the practical application of it. The principle may be briefly and popularly expressed by saying, that as the pressure of the air, externally, on anything containing air, is balanced by the resistance of the air within, if either be removed, the other acts according to its power. If all the air could be withdrawn from the human body, the effects of the tremendous pressure of the atmosphere, equal to that of several tons, would be at once shown. A common syringe, or squirt, may be made to illustrate this pressure, under several of its circumstances; the piston which moves up and down being supposed to be air-tight. Press this down, thus forcing out the air through the small end, and put that end into the water. The piston being drawn up, will be followed by the water. Boys say, it is sucked in. No. It is forced in by the pressure of the air on the water, which meets with no resistance when the piston is drawn back, as there is no air to oppose it. If a hole were bored through the piston, so that when it were drawn back air would be admitted and no vacuum formed, no water would be drawn up. Now, let another experiment be tried. Let the piston be pushed down, forcing out, as far as possible, all the air; and then let the end of the thumb be very closely applied to the small end, so as

to prevent the admission of air by it. This being done, try to draw the piston back. There will be a great difficulty in doing this, as the pressure of the atmosphere has to be overcome, and this is now unbalanced on the other side. But more than this. By drawing it back, while the small end is close shut, a vacuum is occasioned between that end and the piston. Still keep the small end closed, and leave hold of the piston. The pressure of the air will force it down; and if it were connected with something small and moveable on the outside, as the piston was forced down, that something would move with it. Conceive of this on a large scale. Here is a wide tube a mile long. A circular block is put in at one end, of the same diameter; sufficiently close to allow no air to pass, yet capable of being moved along the barrel. The air on the outside presses on it; but there is an equal pressure on the other side: the block, therefore, is stationary. But VOL. IX. Second Series.

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now, suppose that by some large machinery, acting like an air-pump, the air in the tube is withdrawn, and a vacuum formed. The pressure at the other end takes effect, and the block is forced along the whole length of the tube. And suppose a slit in the top of the tube, along which a peg, fastened to the top of the block, could move, that slit being so managed as to admit no air from without; as the block moved within the tube, the peg would move on the outside, and draw anything, not of too great a weight, along with it. GERVASIUS will thus see how atmospheric pressure can be made a moving power: but to describe the practical details, several and complicated diagrams would be necessary. We have done the best we can in giving him a popular (rather than scientific) illustration of the principle.

DIALOGUES ON CHEMISTRY. No. II.

BETWEEN THE TEACHER AND HIS PUPILS.

Pupil. To what branch of the study of chemistry are we to direct our attention in the present conversation?

Teacher. I have experienced considerable difficulty in preparing a reply to the question you propose, and which I, of course, anticipated. Almost all the larger, and more standard works that have recently been published on the subject, commence by treating on those forces, in their nature and cause, and in their operation, together with its rules, by which chemical action is excited, and chemical changes occasioned. Turner's "Elements," for instance,—and I cannot go higher in point of authority, as the edition to which I refer, (the seventh,) is given to the public by Liebig and Gregory,bring before us, for the first part, an account of Heat, Light, and Electricity; with the latter, Galvanism being connected. And the most recently published volume-" Manual of Elementary Chemistry, Theoretical and Practical. By George Fownes, Ph. D."-observes the same method: and, what is singular enough, I see, by noticing the pages respectively, that in both works nearly the same proportion between this part and the whole is observed. I shall have, unavoidably, to call your attention to these particulars, considering their

operation as chemical forces; but, on the whole, I think not only that you will like it better, but that it will agree best with the object and plan of these conversations, to notice at once the elements on which these forces operate, occasioning so many changes, and thus forming so vast a variety of bodies.

P. It seems so natural that the bodies operated upon should be considered before the forces operating, that we should like to know the precise cause of the difficulty you have experienced.

T. It is just here. Heat, light, and electricity have been known and studied so much longer than the elementary constituents of bodies. Even yet, it is far from being certain that chemists have completed their list of these. It is not only not unlikely, but very likely, that some of what are now considered as elements, may be, in the progress of experimental study, and with instruments more correct, or more powerful, ascertained to be themselves compounds. Still, as I am aiming at a popular view of the subject rather than a profound one, though I hope you will find it to be, as far as it goes, accurate,-I have decided on coming first to the elements. The list is correct and complete enough for my purpose; and when once you have it before you, I can, in subsequent conversations, refer to it with greater propriety than if I took up the subject of forces first, and when it was necessary to illustrate their operation, had to refer to what had not been before you at all. Even as it is, I foresee that I shall not be able to avoid this inconvenience altogether; but, on the whole, we shall find less of it on my present plan than if I had followed the more strictly scientific one.

P. You are going to converse with us, then, on what may, be called, we suppose, chemical elements?

T. Yes; and you must now allow me to request you to keep distinctly in view the account I have given you of the exact nature of chemistry, as relating to the combination, by means of their mutual affinities, of the original, the simplest, forms of matter. Masses, I have told you, are made up of particles, united by cohesion; and as is the mass, so also are all the cohering particles. For the sake of convenience, I

have chosen to use the word particle in this sense. Small as it may be,―reduce it infinitesimally, if you please,-if it was not elementary before, it does not become so by the reduction. Again to refer to the common illustration, the smallest conceivable particle of marble is marble; and as marble is not an element, neither is its smallest particle. Marble is composed of lime and carbonic acid: suppose, therefore, that you could arrive at the very smallest particle, that "integrant particle," (I use the phrase which I find in Turner,) being as perfect marble as the largest mass, would contain carbonic acid and lime. The senses seem bewildered in such speculations. That which you have reduced to the lowest state of division is yet a compound, and may therefore be decompounded. By this decomposition, you get at last to the elements from which all material masses are formed.

P. The smallest portions, then, of compound matter you call particles; while the smallest portions of simple, uncompounded matter you term atoms?

T. Just so and, as Prior says, "If 'tis not sense, at least 'tis Greek." Indivisible matter seems a contradiction in terms. But as in mathematical science we set out from a point, which is defined to be that which has neither length, breadth, nor thickness; so in chemistry we begin with atoms, so called because they cannot be divided. And, what is stranger still, you will find, by and by, that these atoms differ among themselves; the atoms of one element having more weight than the atoms of another element. How it is so we cannot comprehend; that it is so is proved by the results. Assuming this to be the fact, we reason upon it; and when the issues come to be cognizable by our senses, our reasoning is found to agree with what our observations and experiments discover. Thus, in the two great departments of science, as to the works of God, mathematical and physical, our foundations are laid in mystery, baffling (though not directly contradicting) our senses; teaching us that our knowledge is limited, that there are depths which we cannot explore, heights which we cannot reach. Let it not stumble us if we meet with similarly mysterious limitations to our knowledge, when we come to the word of God.

You are aware, I suppose, of the enumeration of the elements by the philosophers of antiquity?

P. They said there were four,-earth, air, fire, and water. T. Such was the extent of their knowledge; and many and great were the errors into which they consequently fell. One use, however, we may make of the arrangement. We must first remove one of the elements altogether, fire; or, to use the modern term, denoting the matter of heat, (to distinguish it from the sensation of heat,) caloric. Whatever this be, and though it occupies a most important position in chemical operations, it is not an element in the sense in which we are now using the word,-that of which bodies are compounded. Removing this, three remain,-earth, water, and air. This enumeration, though it by no means brings the elements before us, (for not only is it insufficient in point of number, but incorrect, so to speak, as to nature; it does not mention a single real element,) yet it does bring before us the three great forms in which bodies, as apprehended by our senses, exist. Earth may be taken as representing matter under its solid form; water, under its fluid form; air, under its gaseous form. In each case, the particular state has its own laws; and though the full examination of them belongs properly to physical science, chemistry requires some notice of them. At present, it is sufficient to remind you of the forms which the various combinations of the elements may

assume.

P. But has not this fourfold enumeration come down almost to our own times?

T. Indeed it has. It had not become antiquated when I went to school. My old master loved natural philosophy, as well as classical lore; and greatly were his scholars amused, as well as instructed, on those days when he took us to his apparatus-room, and set the electrical-machine to work. But the copy, Quatuor sunt elementa, ignis, aër, terra, aqua, had not been banished from the desk. True chemical discovery is only of comparatively recent date, because it is only latterly that true chemical analysis has been practised and understood. Bacon laid down the rule, that nature was to be examined for facts, and that from these the theory should