What Causes Contraction and Expansion?
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If contraction and expansion consist in contrary motions, one should find for each great expansion a correspondingly large contraction.
But our surprise is increased when, every day, we see enormous expansions taking place almost instantaneously.
Think what a tremendous expansion occurs when a small quantity of gunpowder flares up into a vast volume of fire!
Think too of the almost limitless expansion of the light which it produces! Imagine the contraction which would take place if this fire and this light were to reunite, which, indeed, is not impossible since only a little while ago they were located together in this small space.
There are a thousand such expansions. These are more obvious than contractions since dense matter is more palpable and accessible to our senses.
We can take wood and see it go up in fire and light, but we do not see them recombine to form wood.
We see fruits and flowers and a thousand other solid bodies dissolve largely into odors, but we do not observe these fragrant atoms coming together to form fragrant solids.
But where the senses fail us reason must step in; for it will enable us to understand the motion involved in the condensation of extremely rarefied and tenuous substances just as clearly as that involved in the expansion and dissolution of solids.
Moreover we are trying to find out how it is possible to produce expansion and contraction in bodies which are capable of such changes without introducing vacua and without giving (61) up the impenetrability of matter; but this does not exclude the possibility of there being materials which possess no such properties and do not, therefore, carry with them consequences which you call inconvenient and impossible.
Expansion and contraction can take place without our admitting the penetrability of matter and introducing vacua, properties which you deny and dislike.
I agree with the peripatetic philosophers in denying the penetrability of matter. What do you think of the vacua and Aristotle’s opposition to it?
Aristotle is against the ancient view that:
- a void is a necessary prerequisite for motion
- motion could not occur without the void
Aristotle shows that motion proves the void as an untenable idea.
- He supposes bodies of different weights to move in the same medium
Bodies of different weights move in one and the same medium with different speeds which stand to one another in the same ratio as the weights; so that, for example, a body which is ten times as heavy as another will move ten times as rapidly as the other.
- He supposes one and the same body to move in different media
The speeds of one and the same body moving in different media are in inverse ratio to the densities of these media.
Thus, for instance, if the density of water were ten times that of air, the speed in air would be ten times greater than in water.
From this second supposition, he shows that, since the tenuity of a vacuum differs infinitely from that of any medium filled with matter however rare, any body which moves in a plenum through a certain space in a certain time ought to move through a vacuum instantaneously.
But instantaneous motion is an impossibility; it is therefore impossible that a vacuum should be produced by motion.
The argument is ad hominem.
It is directed against those who thought that the void is a prerequisite for motion.
If I concede also that motion cannot take place in a void then it means that the assumption of a void considered absolutely and not with reference to motion, is still valid.
But to tell you what the ancients might possibly have replied and in order to better understand just how conclusive Aristotle’s demonstration is, we may, in my opinion, deny both of his assumptions. And as to the first, I greatly
I doubt that Aristotle ever tested by experiment whether 2 stones, one weighing 10 times more than the other, fall at the same time at 100 cubits or not.
His language would seem to indicate that he had tried the experiment, because he says: We see the heavier; now the word see shows that he had made the experiment.
Sagr. But I, Simplicio, who have made the test can assure you that a cannon ball weighing one or two hundred pounds, or even more, will not reach the ground by as much as a span ahead of a musket ball weighing only half a pound, provided both are dropped from a height of 200 cubits.
But, even without experiment, we can prove that a heavier body does not move more rapidly than a lighter one provided both bodies are of the same material and in short such as those mentioned by Aristotle.
But does a falling body have a definite speed fixed by nature that is changed only by the use of force or resistance?
One and the same body moving in a single medium has a fixed velocity which is determined by nature and which cannot be increased except by the addition of momentum or diminished except by some resistance which retards it.
If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter.
But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than 8.
But the two stones when tied together make a stone larger than that which before moved with a speed of eight.
Hence, the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than the lighter one, I infer that the heavier body moves more slowly.
I am all at sea because it appears to me that the smaller stone when added to the larger increases its weight and by adding weight I do not see how it can fail to increase its speed or, at least, not to diminish it.
Here again you are in error, Simplicio, because it is not true that the smaller stone adds weight to the larger.