Rest Versus Motion
Table of Contents
Do substances become fluid by being resolved into their infinitely small indivisible components?
I take a hard substance such as stone. Then I reduce it through a hammer into powder which are still:
- finite in size
- have shape
- can be counted
Once heaped up, they remain in a heap.
If an excavation be made within limits the cavity will remain and the surrounding particles will not rush in to fill it; if shaken the particles come to rest immediately after the external disturbing agent is removed;
The same effects are observed in all piles of larger and larger particles, of any shape, even if spherical, as is the case with piles of millet, wheat, lead shot, and every other material.
But if we attempt to discover such properties in water we do not find them; for when once heaped up it immediately flattens out unless held up by some vessel or other external retaining body; when hollowed out it quickly rushes in to fill the cavity; and when disturbed it fluctuates for a long time and sends out its waves through great distances.
Seeing that water has less firmness than the finest of powder, in fact has no consistence whatever, we may, it seems to me, very reasonably conclude that the smallest particles into which it can be resolved are quite different from finite and divisible particles; indeed the only difference I am able to discover is that the former are indivisible.
The exquisite transparency of water also favors this view; for the most transparent crystal when broken and ground and reduced to powder loses its transparency;
The finer the grinding the greater the loss; but in the case of water where the attrition is of the highest degree we have extreme transparency.
Gold and silver when pulverized with acids [acque forti] more finely than is possible with any file still remain powders,* and do not become fluids until the finest particles [gl’ indivisibili] of fire or of the rays of the sun dissolve them, as I think, into their ultimate, indivisible, and infinitely small components.
I have seen lead melted instantly through a concave mirror only 3 hands [palmi] in diameter.
Hence if the mirror were very large, well-polished and of a parabolic figure, it would just as readily and quickly melt any other metal.
Such effects as these render credible to me the marvels accomplished by the mirrors of Archimedes.
Archimedes’ books rendered credible to me all the miracles described by various writers.
And if any doubt had remained the book which Father Buonaventura Cavalieri** [87] has recently published on the subject of the burning glass [specchio ustorio] and which I have read with admiration would have removed the last difficulty.
With solar rays in melting metals, must we believe that such a furious action is devoid of motion or that it is accompanied by the most rapid of motions?
Other combustions and resolutions are accompanied by motion, and that, the most rapid;
Note the action of lightning and of powder as used in mines and petards; note also how the charcoal flame, mixed as it is with heavy and impure vapors, increases its power to liquify metals whenever quickened by a pair of bellows.
Hence I do not understand how the action of light, although very pure, can be devoid of the fastest motion.
But what kind and what is the speed of light? Is it instantaneous or momentary or does it like other motions require time? Can we not decide this by experiment?
Everyday experience shows that the propagation of light is instantaneous.
When artillery is fired at great distance, the flash reaches our eyes instantly. But the sound reaches the ear only after an interval.
The only thing I am able to infer from this familiar bit of experience is that sound, in reaching our ear, travels more slowly than light.
It does not tell me whether that light is instantaneous or not.
Is the speed of light really instantaneous?
This can be tested by experiment.
Two persons take a lantern. Blocking its light by the hand will block the light seen by the other.
Let them stand opposite each other at a distance of a few cubits and practice until they acquire such skill in uncovering and occulting their lights that the instant one sees the light of his companion he will uncover his own.
After a few trials the response will be so prompt that without sensible error [svario] the uncovering of one light is immediately followed by the uncovering of the other, so that as soon as one exposes his light he will instantly see that of the other.
Having acquired skill at this short distance let the 2 experimenters, equipped as before, take up positions separated by a distance of 3 miles.
Let them perform the same experiment at night, noting carefully whether the exposures and occultations occur in the same manner as at short distances.
If they do, then the propagation of light is instantaneous.
But if time is required at a distance of 3 miles which, considering the going of one light and the coming of the other, really amounts to 6, then the delay should be easily observable.
Telescopes may be used for experiments at still greater distances of 10 miles.
Each observer adjusts one for himself at the place where he is to make the experiment at night.
The lights are not large and are therefore invisible to the eye at so great a distance. But this is solved by a telescope.
I have tried the experiment for 1 mile.
I was not able to determine whether light was instantaneous or not.
If it is not instantaneous, I would say it is as fast as the lightning flash between clouds 10 miles from us.
We see the beginning of this light – I might say its head and source – located at a particular place among the clouds;
But it immediately spreads to the surrounding ones, which seems to be an argument that at least some time is required for propagation;
For if the illumination were instantaneous and not gradual, we should not be able to distinguish its origin – its center, so to speak – from its outlying portions.
What a sea we are gradually slipping into without knowing it! With vacua and infinities and indivisibles and instantaneous motions, shall we ever be able, even by means of a thousand discussions, to reach dry land?