Part 2

_Mrs. B._ Invisible, I allow; but we must not imagine that what we no longer see no longer exists. Were every particle of matter that becomes invisible annihilated, the world itself would in the course of time be destroyed. The particles of smoke, when diffused in the air, continue still to be particles of matter as well as when more closely united in the form of coals: they are really as substantial in the one state as in the other, and equally so when by their extreme subdivision they become invisible. No particle of matter is ever destroyed: this is a principle you must constantly remember. Every thing in nature decays and corrupts in the lapse of time. We die, and our bodies moulder to dust; but not a single atom of them is lost; they serve to nourish the earth, whence, while living, they drew their support.

The next essential property of matter is called _inertia_ or inactivity; this word expresses the resistance which matter makes to a change from a state of rest, to that of motion, or from a state of motion to that of rest. Bodies are equally incapable of changing their actual state, whether it be of motion or of rest. You know that it requires force to put a body which is at rest in motion; an exertion of strength is also requisite to stop a body which is already in motion. The resistance of the body to a change of state, in either case, arises from its _inertia_.

_Emily._ In playing at base-ball I am obliged to use all my strength to give a rapid motion to the ball; and when I have to catch it, I am sure I feel the resistance it makes to being stopped. But if I did not catch it, it would soon fall to the ground and stop of itself.

_Mrs. B._ Matter being inert it is as incapable of stopping of itself as it is of putting itself into motion: when the ball ceases to move, therefore, it must be stopped by some other cause or power; but as it is one with which you are yet unacquainted, we can not at present investigate its effects.

The last property which appears to be common to all bodies is _attraction_. All bodies consist of infinitely small particles of matter, each of which possesses the power of attracting or drawing towards it, and uniting with any other particle sufficiently near to be within the influence of its attraction; but in minute particles this power extends to so very small a distance around them, that its effect is not sensible, unless they are (or at least appear to be) in contact; it then makes them stick or adhere together, and is hence called the _attraction of cohesion_. Without this power, solid bodies would fall in pieces, or rather crumble to atoms.

_Emily._ I am so much accustomed to see bodies firm and solid, that it never occurred to me that any power was requisite to unite the particles of which they are composed. But the attraction of cohesion does not, I suppose, exist in liquids; for the particles of liquids do not remain together so as to form a body, unless confined in a vessel?

_Mrs. B._ I beg your pardon; it is the attraction of cohesion which holds this drop of water suspended at the end of my finger, and keeps the minute watery particles of which it is composed united. But as this power is stronger in proportion as the particles of bodies are more closely united, the cohesive attraction of solid bodies is much greater than that of fluids.

The thinner and lighter a fluid is, the less is the cohesive attraction of its particles, because they are further apart; and in elastic fluids, such as air, there is no cohesive attraction among the particles.

_Emily._ That is very fortunate; for it would be impossible to breathe the air in a solid mass; or even in a liquid state.

But is the air a body of the same nature as other bodies?

_Mrs. B._ Undoubtedly, in all essential properties.

_Emily._ Yet you say that it does not possess one of the general properties of bodies--attraction.

_Mrs. B._ The particles of air are not destitute of the power of attraction, but they are too far distant from each other to be influenced by it so as to produce cohesion: and the utmost efforts of human art have proved ineffectual in the attempt to compress them, so as to bring them within the sphere of each other's attraction, and make them cohere.

_Emily._ If so, how is it possible to prove that they are endowed with this power?

_Mrs. B._ The air is formed of particles precisely of the same nature as those which enter into the composition of liquid and solid bodies, in each of which we have a proof of their attraction.

_Emily._ It is then, I suppose, owing to the different degrees of cohesive attraction in different substances, that they are hard or soft, and that liquids are thick or thin.

_Mrs. B._ Yes; but you would express your meaning better by the term _density_, which denotes the degree of closeness and compactness of the particles of a body. In philosophical language, density is said to be that property of bodies by which they contain a certain quantity of matter, under a certain bulk or magnitude. _Rarity_ is the contrary of density; it denotes the thinness and subtilty of bodies: thus you would say that mercury or quicksilver was a very dense fluid; ether, a very rare one. Those bodies which are the most dense, do not always cohere the most strongly; lead is more dense than iron, yet its particles are more easily separated.

_Caroline._ But how are we to judge of the quantity of matter contained in a certain bulk?

_Mrs. B._ By the weight: under the same bulk bodies are said to be dense in proportion as they are heavy.

_Emily._ Then we may say that metals are dense bodies, wood comparatively a rare one, &c. But, Mrs. B., when the particles of a body are so near as to attract each other, the effect of this power must increase as they are brought by it closer together; so that one would suppose that the body would gradually augment in density, till it was impossible for its particles to be more closely united. Now, we know that this is not the case; for soft bodies, such as cork, sponge, or butter, never become, in consequence of the increasing attraction of their particles, as hard as iron?

_Mrs. B._ In such bodies as cork and sponge, the particles which come in contact are so few as to produce but a slight degree of cohesion: they are porous bodies, which, owing to the peculiar arrangement of their particles, abound with interstices, or pores, which separate the particles. But there is also a fluid much more subtile than air, which pervades all bodies, this is _heat_. Heat insinuates itself more or less between the particles of all bodies, and forces them asunder; you may therefore consider heat, and the attraction of cohesion, as constantly acting in opposition to each other.

_Emily._ The one endeavouring to rend a body to pieces, the other to keep its parts firmly united.

_Mrs. B._ And it is this struggle between the contending forces of heat and attraction, which prevents the extreme degree of density which would result from the sole influence of the attraction of cohesion.

_Emily._ The more a body is heated then, the more its particles will be separated.

_Mrs. B._ Certainly: we find that bodies not only swell or dilate, but lose their cohesion, by heat: this effect is very sensible in butter, for instance, which expands by the application of heat, till at length the attraction of cohesion is so far diminished that the particles separate, and the butter becomes liquid. A similar effect is produced by heat on metals, and all bodies susceptible of being melted. Liquids, you know, are made to boil by the application of heat; the attraction of cohesion then yields entirely to the repulsive power; the particles are totally separated and converted into steam or vapour. But the agency of heat is in no body more sensible than in air, which dilates and contracts by its increase or diminution in a very remarkable degree.

_Emily._ The effects of heat appear to be one of the most interesting parts of natural philosophy.

_Mrs. B._ That is true; but heat is so intimately connected with chemistry, that you must allow me to defer the investigation of its properties till you become acquainted with that science.

To return to its antagonist, the attraction of cohesion; it is this power which restores to vapour its liquid form, which unites it into drops when it falls to earth in a shower of rain, which gathers the dew into brilliant gems on the blades of grass.

_Emily._ And I have often observed that after a shower, the water collects into large drops on the leaves of plants; but I cannot say that I perfectly understand how the attraction of cohesion produces this effect.

_Mrs. B._ Rain, when it first leaves the clouds, is not in the form of drops, but in that of mist or vapour, which is composed of very small watery particles; these in their descent mutually attract each other, and those that are sufficiently near in consequence unite and form a drop, and thus the mist is transformed into a shower. The dew also was originally in a state of vapour, but is, by the mutual attraction of the particles, formed into small globules on the blades of grass: in a similar manner the rain upon the leaf collects into large drops, which when they become too heavy for the leaf to support, fall to the ground.

_Emily._ All this is wonderfully curious! I am almost bewildered with surprise and admiration at the number of new ideas I have already acquired.

_Mrs. B._ Every step that you advance in the pursuit of natural science, will fill your mind with admiration and gratitude towards its Divine Author. In the study of natural philosophy, we must consider ourselves as reading the book of nature, in which the bountiful goodness and wisdom of God are revealed to all mankind; no study can tend more to purify the heart, and raise it to a religious contemplation of the Divine perfections.

There is another curious effect of the attraction of cohesion which I must point out to you; this is called capillary attraction. It enables liquids to rise above their ordinary level in capillary tubes: these are tubes, the bores of which are so extremely small that liquids ascend within them, from the cohesive attraction between the particles of the liquid and the interior surface of the tube. Do you perceive the water rising in this small glass tube, above its level in the goblet of water, into which I have put one end of it?

_Emily._ Oh yes; I see it slowly creeping up the tube, but now it is stationary: will it rise no higher?

_Mrs. B._ No; because the cohesive attraction between the water and the internal surface of the tube is now balanced by the weight of the water within it; if the bore of the tube were narrower the water would rise higher; and if you immerse several tubes of bores of different sizes, you will see it rise to different heights in each of them. In making this experiment, you should colour the water with a little red wine, in order to render the effect more obvious.

All porous substances, such as sponge, bread, linen, &c. may be considered as collections of capillary tubes: if you dip one end of a lump of sugar into water, the fluid will rise in it, and wet it considerably above the surface of the water into which you dip it.

_Emily._ In making tea I have often observed that effect, without being able to account for it.

_Mrs. B._ Now that you are acquainted with the attraction of cohesion, I must endeavour to explain to you that of _Gravitation_, which is probably a modification of the same power; the first is perceptible only in very minute particles, and at very small distances; the other acts on the largest bodies, and extends to immense distances.

_Emily._ You astonish me: surely you do not mean to say that large bodies attract each other?

_Mrs. B._ Indeed I do: let us take, for example, one of the largest bodies in nature, and observe whether it does not attract other bodies. What is it that occasions the fall of this book, when I no longer support it?

_Emily._ Can it be the attraction of the earth? I thought that all bodies had a natural tendency to fall.

_Mrs. B._ They have a natural tendency to fall, it is true; but that tendency is produced entirely by the attraction of the earth: the earth being so much larger than any body on its surface, forces every body, which is not supported, to fall upon it.

_Emily._ If the tendency which bodies have to fall results from the earth's attractive power, the earth itself can have no such tendency, since it cannot attract itself, and therefore it requires no support to prevent it from falling. Yet the idea that bodies do not fall of their own accord, but that they are drawn towards the earth by its attraction, is so new and strange to me, that I know not how to reconcile myself to it.

_Mrs. B._ When you are accustomed to consider the fall of bodies as depending on this cause, it will appear to you as natural, and surely much more satisfactory, than if the cause of their tendency to fall were totally unknown. Thus you understand that all matter is attractive, from the smallest particle to the largest mass; and that bodies attract each other with a force proportional to the quantity of matter they contain.

_Emily._ I do not perceive any difference between the attraction of cohesion and that of gravitation; is it not because every particle of matter is endowed with an attractive power, that large bodies consisting of a great number of particles, are so strongly attractive?

_Mrs. B._ True. There is, however, this difference between the attraction of particles and that of masses, that the former takes place only when the particles are contiguous, whilst the latter is exerted when the masses are far from each other. The attraction of particles frequently counteracts the attraction of gravitation. Of this you have an instance in the attraction of capillary tubes, in which liquids ascend by the attraction of cohesion, in opposition to that of gravity. It is on this account that it is necessary that the bore of the tube should be extremely small; for if the column of water within the tube is not very minute, the attraction of cohesion would not be able either to raise or support it in opposition to its gravity; because the increase of weight, in a column of water of a given height, is much greater than the increase in the attracting surface of the tube, when its size is increased.

You may observe also, that all solid bodies are enabled by the force of the cohesive attraction of their particles to resist that of gravity, which would otherwise disunite them, and bring them to a level with the ground, as it does in the case of a liquid, the cohesive attraction of which is not sufficient to enable it to resist the power of gravity.

_Emily._ And some solid bodies appear to be of this nature, as sand, and powder for instance: there is no attraction of cohesion between their particles?

_Mrs. B._ Every grain of powder, or sand, is composed of a great number of other more minute particles, firmly united by the attraction of cohesion; but amongst the separate grains there is no sensible attraction, because they are not in sufficiently close contact.

_Emily._ Yet they actually touch each other?

_Mrs. B._ The surface of bodies is in general so rough and uneven, that when in apparent contact, they touch each other only by a few points. Thus, when I lay this book upon the table, the binding of which appears perfectly smooth, so few of the particles of its under surface come in contact with the table, that no sensible degree of cohesive attraction takes place; for you see that it does not stick or cohere to the table, and I find no difficulty in lifting it off.

It is only when surfaces, perfectly flat and well polished, are placed in contact, that the particles approach in sufficient number, and closely enough, to produce a sensible degree of cohesive attraction. Here are two plates of polished metal, I press their flat surfaces together, having previously interposed a few drops of oil, to fill up every little porous vacancy. Now try to separate them.

_Emily._ It requires an effort beyond my strength, though there are handles for the purpose of pulling them asunder. Is the firm adhesion of the two plates merely owing to the attraction of cohesion?

_Mrs. B._ There is no force more powerful, since it is by this that the particles of the hardest bodies are held together. It would require a weight of several pounds to separate these plates. In the present example, however, much of the cohesive force is due to the attraction subsisting between the metal and the oil which is interposed; as without this, or some other fluid, the points of contact would still be comparatively few, although we may have employed our utmost art, in giving flat surfaces to the plates.

_Emily._ In making a kaleidoscope, I recollect that the two plates of glass, which were to serve as mirrors, stuck so fast together, that I imagined some of the gum I had been using had by chance been interposed between them; but I am now convinced 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.

Questions

1. (Pg. 10) What is intended by the term _bodies_?

2. (Pg. 10) Is the term _matter_, restricted to substances of a particular kind?

3. (Pg. 10) Name those properties of bodies, which are called inherent.

4. (Pg. 10) What is meant by impenetrability?

5. (Pg. 10) Can a liquid be said to be impenetrable?

6. (Pg. 11) How can you prove that air is impenetrable?

7. (Pg. 11) If air is impenetrable, what causes the water to rise some way into a goblet, if I plunge it into water with its mouth downward?

8. (Pg. 11) When I drive a nail into wood, do not both the iron and the wood occupy the same space?

9. (Pg. 11) In how many directions, is a body said to have extension?

10. (Pg. 11) How do we distinguish the terms height and depth?

11. (Pg. 12) What constitutes the _figure_, or _form_ of a body?

12. (Pg. 12) What is said respecting the form of minerals?

13. (Pg. 12) What of the vegetable and animal creation?

14. (Pg. 12) What of artificial, and accidental forms?

15. (Pg. 12) What is meant by divisibility?

16. (Pg. 12) What examples can you give, to prove that the particles of a body are minute in the extreme?

17. (Pg. 13) What produces the odour of bodies?

18. (Pg. 13) How do odours exemplify the minuteness of the particles of matter?

19. (Pg. 13) Can matter be in any way annihilated?

20. (Pg. 13) What becomes of the fuel, which disappears in our fires?

21. (Pg. 14) How can that part which evaporates, be still said to possess a substantial form?

22. (Pg. 14) What do we mean by _inertia_?

23. (Pg. 14) Give an example to prove that force is necessary, either to give or to stop motion.

24. (Pg. 14) What general power do the particles of matter exert upon other particles?

25. (Pg. 15) What is that species of attraction called, which keeps bodies in a solid state?

26. (Pg. 15) Does the attraction of cohesion exist in liquids, and how is its existence proved?

27. (Pg. 15) If the particles of air attract each other, why do they not cohere?

28. (Pg. 15) From what then do you infer that they possess attraction?

29. (Pg. 15) How do you account for some bodies being hard and others soft?

30. (Pg. 16) What is meant by the term _density_?

31. (Pg. 16) Do the most dense bodies always cohere the most strongly?

32. (Pg. 16) How do we know that one body is more dense than another?

33. (Pg. 16) What is there which acts in opposition to cohesive attraction, tending to separate the particles of bodies?

34. (Pg. 17) What would be the consequence if the repulsive power of heat were not exerted?

35. (Pg. 17) If we continue to increase the heat, what effects will it produce on bodies?

36. (Pg. 17) What body has its dimensions most sensibly affected by change of temperature?

37. (Pg. 17) What power restores vapours to the liquid form?

38. (Pg. 17) What examples can you give?

39. (Pg. 17) How are drops of rain and of dew said to be formed?

40. (Pg. 18) What is meant by a capillary tube?

41. (Pg. 18) What effect does attraction produce when these are immersed in water?

42. (Pg. 18) What is the reason that the water rises to a certain height only?

43. (Pg. 18) Give some familiar examples of capillary attraction.

44. (Pg. 18) In what does _gravitation_ differ from cohesive attraction?

45. (Pg. 18) What causes bodies near the earth's surface, to have a tendency to fall towards it?

46. (Pg. 19) What remarkable difference is there between the attraction of gravitation, and that of cohesion?

47. (Pg. 19) In what instances does the power of cohesion counteract that of gravitation?

48. (Pg. 19) Why will water rise to a less height, if the size of the tube is increased?

49. (Pg. 20) Why do not two bodies cohere, when laid upon each other?

50. (Pg. 20) Can two bodies be made sufficiently flat to cohere with considerable force?

51. (Pg. 20) What is the reason that the adhesion is greater when oil is interposed?

52. (Pg. 21) What other modifications of attraction are there, besides those of cohesion and of gravitation?

CONVERSATION II.

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.