The cause of light in glow-worms, rotten wood, and the Bolognan stone.
6. The shining of glow-worms, some kinds of rotten wood, and of a kind of stone made at Bologna, may have one common cause, namely, the exposing of them to the hot sun. We find by experience that the Bologna stone shines not, unless it be so exposed; and after it has been exposed it shines but for a little time, namely, as long as it retains a certain degree of heat. And the cause may be that the parts, of which it is made, may together with heat have simple motion imprinted in them by the sun. Which if it be so, it is necessary that it shine in the dark, as long as there is sufficient heat in it; but this ceasing, it will shine no longer. Also we find by experience that in the glow-worm there is a certain thick humour, like the crystalline humour of the eye; which if it be taken out and held long enough in one's fingers, and then be carried into the dark, it will shine by reason of the warmth it received from the fingers; but as soon as it is cold it will cease shining. From whence, therefore, can these creatures have their light, but from lying all day in the sunshine in the hottest time of summer? In the same manner, rotten wood, except it grow rotten in the sunshine, or be afterwards long enough exposed to the sun, will not shine. That this doth not happen in every worm, nor in all kinds of rotten wood, nor in all calcined stones, the cause may be that the parts, of which the bodies are made, are different both in motion and figure from the parts of bodies of other kinds.
The cause of light in the concussion of sea water.
7. Also the sea water shineth when it is either dashed with the strokes of oars, or when a ship in its course breaks strongly through it; but more or less, according as the wind blows from different points. The cause whereof may be this, that the particles of salt, though they never shine in the salt-pits, where they are but slowly drawn up by the sun, being here beaten up into the air in greater quantities and with more force, are withal made to turn round, and consequently to shine, though weakly. I have, therefore, given a possible cause of this phenomenon.
The cause of flame, sparks, & colliquation.
8. If such matter as is compounded of hard little bodies be set on fire, it must needs be, that, as they fly out in greater or less quantities, the flame which is made by them will be greater or less. And if the ethereal or fluid part of that matter fly out together with them, their motion will be the swifter, as it is in wood and other things which flame with a manifest mixture of wind. When, therefore, these hard particles by their flying out move the eye strongly, they shine bright; and a great quantity of them flying out together, they make a great shining body. For flame being nothing but an aggregate of shining particles, the greater the aggregate is, the greater and more manifest will be the flame. I have, therefore, shown a possible cause of flame. And from hence the cause appears evidently, why glass is so easily and quickly melted by the small flame of a candle blown, which will not be melted without blowing but by a very strong fire.
Now, if from the same matter there be a part broken off, namely, such a part as consisteth of many of the small particles, of this is made a spark. For from the breaking off it hath a violent turning round, and from hence it shines. But though from this matter there fly neither flame nor sparks, yet some of the smallest parts of it may be carried out as far as to the superficies, and remain there as ashes; the parts whereof are so extremely small, that it cannot any longer be doubted how far nature may proceed in dividing.
Lastly, though by the application of fire to this matter there fly little or nothing from it, yet there will be in the parts an endeavour to simple motion; by which the whole body will either be melted, or, which is a degree of melting, softened. For all motion has some effect upon all matter whatsoever, as has been shown at [art. 3], chap. XV. Now if it be softened to such a degree, as that the stubbornness of the parts be exceeded by their gravity, then we say it is melted; otherwise, softened and made pliant and ductile.
Again, the matter having in it some particles hard, others ethereal or watery; if, by the application of fire, these latter be called out, the former will thereby come to a more full contact with one another; and, consequently, will not be so easily separated; that is to say, the whole body will be made harder. And this may be the cause why the same fire makes some things soft, others hard.
The cause why wet hay sometimes burns of its own accord; also the cause of lightning.