Part 7

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?