vinced that it was their own natural cohesive attraction which produced this effect.

Mrs. B. Very probably it was so; for plate-glass has an extremely smooth, flat surface, admitting of the contact of a great number of particles, when two plates are laid upon each other.

Emily. But, Mrs. B., the cohesive attraction of some substances is much greater than that of others; thus glue, gum and paste, cohere with singular tenacity.

Mrs. B. Bodies which differ in their natures in other respects, differ also in their cohesive attraction; it is probable that there are no two bodies, the particles of which attract each other with precisely the same force.

There are some other modifications of attraction peculiar to certain bodies; namely, that of magnetism, of electricity, and of affinity, or chemical attraction; but we shall confine our attention merely to the attraction of cohesion and of gravity; the examination of the latter we shall resume at our next meeting.

52. What other modifications of attraction are there, besides those of cohe sion and of gravitation?

CONVERSATION IL

ON THE ATTRACTION OF GRAVITY.

ATTRACTION OF GRAVITATION, CONTINUED.-OF WEIGHT.—OF THE FALL OF BODIES. OF THE RESISTANCE OF THE AIR.-OF THE ASCENT OF LIGHT BODIES.

EMILY.

I HAVE related to my sister Caroline all that you have taught me of natural philosophy, and she has been so much delighted by it, that she hopes you will have the goodness to admit her to your lessons.

Mrs. B. Very willingly; but I did not think you had any taste for studies of this nature, Caroline.

Caroline. I confess, Mrs. B., that hitherto I had formed no very agreeable idea either of philosophy, or philosophers; but what Emily has told me has excited my curiosity so much, that I shall be highly pleased if you will allow me to become one of your pupils.

Mrs. B. I fear that I shall not find you so tractable a scholar as Emily; I know that you are much biased in favour of your own opinions.

Caroline. Then you will have the greater merit in reforming them, Mrs. B.; and after all the wonders that Emily has related to me, I think I stand but little chance against you and your

attractions.

Mrs. B. You will, I doubt not, advance a number of objections; but these I shall willingly admit, as they will afford an opportunity of elucidating the subject. Emily, do you recollect the names of the general properties of bodies?

Emily. Impenetrability, extension, figure, divisibility, inertia

and attraction.

Mrs. B. Very well. You must remember that these are properties common to all bodies, and of which they cannot be deprived; all other properties of bodies are called accidental, because they depend on the relation or connexion of one body to

another.

1. What are those properties of bodies called, which are not common to all?

Caroline. Yet surely, Mrs. B. there are other properties which are essential to bodies, besides those you have enumerated. Colour and weight, for instance, are common to all bodies, and do not arise from their connexion with each other, but exist in the bodies themselves; these, therefore, cannot be accidental qualities?

Mrs. B. I beg your pardon; these properties do not exist in bodies independently of their connexion with other bodies.

Caroline. What! have bodies no weight? Does not this table weigh heavier than this book; and, if one thing weighs heavier than another, must there not be such a thing as weight?

Mrs. B. No doubt: but this property does not appear to be essential to bodies; it depends upon their connexion with each other. Weight is an effect of the power of attraction, without which the table and the book would have no weight whatever.

Emily. I think I understand you; it is the attraction of gravity which makes bodies heavy.

Mrs. B. You are right. I told you that the attraction of gravity was proportioned to the quantity of matter which bodies contain: now the earth consisting of a much greater quantity of matter than any body upon its surface, the force of its attraction must necessarily be greatest, and must draw every thing so situated towards it; in consequence of which, bodies that are unsupported fall to the ground, whilst those that are supported, press upon the object which prevents their fall, with a weight equal to the force with which they gravitate towards the earth.

Caroline. The same cause then which occasions the fall of bodies, produces their weight also. It was very dull in me not to understand this before, as it is the natural and necessary consequence of attraction; but the idea that bodies were not really heavy of themselves, appeared to me quite incomprehensible. But, Mrs. B. if attraction is a property essential to matter, weight must be so likewise; for how can one exist without the other?

Mrs. B. Suppose there were but one body existing in universal space, what would its weight be?

Caroline. That would depend upon its size; or more accurately speaking, upon the quantity of matter it contained.

Emily. No, no; the body would have no weight, whatever were its size; because nothing would attract it. Am I not right, Mrs. B.?

Mrs. B. You are: you must allow, therefore, that it would be possible for attraction to exist without weight; for each of the

2. Why are they so called? 3. What is the cause of weight in bodies? 4. What is the reason that all bodies near to the surface of the earth, are drawn towards it?

particles of which the body was composed, would possess the power of attraction; but they could exert it only amongst themselves; the whole mass having nothing to attract, or to be attracted by, would have no weight.

Caroline. I am now well satisfied that weight is not essential to the existence of bodies; but what have you to object to colours, Mrs. B.; you will not, I think, deny that they really exist in the bodies themselves.

Mrs. B. When we come to treat of the subject of colours, I trust that I shall be able to convince you, that colours are likewise accidental qualities, quite distinct from the bodies to which they appear to belong.

Caroline. Oh do pray explain it to us now, I am so very curious to know how that is possible.

Mrs. B. Unless we proceed with some degree of order and method, you will in the end find yourself but little the wiser for learn. Let us therefore go on regularly, and make ourselves well acquainted with the general properties of bodies before we proceed further.

all you

Emily. To return, then, to attraction, (which appears to me by far the most interesting of them, since it belongs equally to all kinds of matter,) it must be mutual between two bodies; and if so, when a stone falls to the earth, the earth should rise part of the way to meet the stone?

Mrs. B. Certainly; but you must recollect that the force of attraction is proportioned to the quantity of matter which bodies contain, and if you consider the difference there is in that respect, between a stone and the earth, you will not be surprised that you do not perceive the earth rise to meet the stone; for though it is true that a mutual attraction takes place between the earth and the stone, that of the latter is so very small in comparison to that of the former, as to render its effect insensible.

Emily. But since attraction is proportioned to the quantity of matter which bodies contain, why do not the hills attract the houses and churches towards them?

Caroline. What an idea, Emily! How can the houses and churches be moved, when they are so firmly fixed in the ground!

Mrs. B. Emily's question is not absurd, and your answer, Ca

5. If attraction is the cause of weight, could you suppose it possible for a body to possess the former and not the latter property? 6. When a stone falls to the ground, in which of the two bodies does the power of attraction exist? 7. If the attraction be mutual, why does not the earth approach the stone, as much as the stone approaches the earth? 8. If attraction be in proportion to the mass, why does not a hill, draw towards itself, a house placed near it?

roline, is perfectly just; but can you tell us why the houses and churches are so firmly fixed in the ground?

Caroline. I am afraid I have answered right by mere chance; for I begin to suspect that bricklayers and carpenters could give but little stability to their buildings, without the aid of at

traction.

Mrs. B. It is certainly the cohesive attraction between the bricks and the mortar, which enables them to build walls, and these are so strongly attracted by the earth, as to resist every other impulse; otherwise they would necessarily move towards the hills and the mountains; but the lesser force must yield to the greater. There are, however, some circumstances in which the attraction of a large body has sensibly counteracted that of the earth. If whilst standing on the declivity of a mountain, you hold a plumb-line in your hand, the weight will not fall perpendicular to the earth, but incline a little towards the mountain; and this is owing to the lateral, or sideways attraction of the mountain, interfering with the perpendicular attraction of the

earth.

Emily. But the size of a mountain is very trifling, compared

to the whole earth.

Mrs. B. Attraction, you must recollect, is in proportion to the quantity of matter, and although that of the mountain, is much less than that of the earth, it may yet be sufficient to act sensibly upon the plumb-line which is so near to it.

Caroline. Pray Mrs. B. do the two scales of a balance hang parallel to each other?

Mrs. B. You mean, I suppose, in other words to inquire whether two lines which are perpendicular to the earth, are parallel to each other? I believe I guess the reason of your question; but I wish you would endeavour to answer it without my assist

ance.

Caroline. I was thinking that such lines must both tend by gravity to the same point, the centre of the earth; now lines tending to the same point cannot be parallel, as parallel lines are always at an equal distance from each other, and would

never meet.)

Mrs. B. Very well explained; you see now the use of your knowledge of parallel lines: had you been ignorant of their properties, you could not have drawn such a conclusion. This may enable you to form an idea of the great advantage to be derived even from a slight knowledge of geometry, in the study of natural philosophy; and if after I have made you acquainted with

9. How can the attraction of a mountain be rendered sensible? 10. Why cannot two lines which are perpendicular to the surface of the earth be pa rallel to each other?

C