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They are concretions in the same way as thunder and lightning are

discretions, so that if it is true of either that they are not

generated but pre-exist, the same must be true of the other. Again,

how can any distinction be made about the intercepting between this

case and that of interception in denser substances such as water?

Water, too, is heated by the sun and by fire: yet when it contracts

again and grows cold and freezes no such ejection as they describe

occurs, though it ought on their the. to take place on a proportionate

scale. Boiling is due to the exhalation generated by fire: but it is

impossible for it to exist in the water beforehand; and besides they

call the noise 'hissing', not 'boiling'. But hissing is really boiling

on a small scale: for when that which is brought into contact with

moisture and is in process of being extinguished gets the better of

it, then it boils and makes the noise in question. Some-Cleidemus is

one of them-say that lightning is nothing objective but merely an

appearance. They compare it to what happens when you strike the sea

with a rod by night and the water is seen to shine. They say that

the moisture in the cloud is beaten about in the same way, and that

lightning is the appearance of brightness that ensues.

This theory is due to ignorance of the theory of reflection, which

is the real cause of that phenomenon. The water appears to shine

when struck because our sight is reflected from it to some bright

object: hence the phenomenon occurs mainly by night: the appearance is

not seen by day because the daylight is too in, tense and obscures it.

These are the theories of others about thunder and lightning: some

maintaining that lightning is a reflection, the others that

lightning is fire shining through the cloud and thunder its

extinction, the fire not being generated in each case but existing

beforehand. We say that the same stuff is wind on the earth, and

earthquake under it, and in the clouds thunder. The essential

constituent of all these phenomena is the same: namely, the dry

exhalation. If it flows in one direction it is wind, in another it

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