13. Those bodies are called _diaphanous_, upon which, whilst the beams of a lucid body do work, the action of every one of those beams is propagated in them in such manner, as that they still retain the same order amongst themselves, or the inversion of that order; and therefore bodies, which are perfectly diaphanous, are also perfectly homogeneous. On the contrary, an _opacous_ body is that, which, by reason of its heterogeneous nature, doth by innumerable reflections and refractions in particles of different figures and unequal hardness, weaken the beams that fall upon it before they reach the eye. And of diaphanous bodies, some are made such by nature from the beginning; as the substance of the air, and of the water, and perhaps also some parts of stones, unless these also be water that has been long congealed. Others are made so by the power of heat, which congregates homogeneous bodies. But such, as are made diaphanous in this manner, consist of parts which were formerly diaphanous. [Sidenote: The cause of lightning and thunder.] 14. In what manner clouds are made by the motion of the sun, elevating the particles of water from the sea and other moist places, hath been explained in chapter XXVI. Also how clouds come to be frozen, hath been shown above at the 7th article. Now from this, that air may be enclosed as it were in caverns, and pent together more and more by the meeting of ascending and descending clouds, may be deduced a possible cause of _thunder_ and _lightning_. For seeing the air consists of two parts, the one ethereal, which has no proper motion of its own, as being a thing divisible into the least parts; the other hard, namely, consisting of many hard atoms, which have every one of them a very swift simple motion of its own: whilst the clouds by their meeting do more and more straiten such cavities as they intercept, the ethereal parts will penetrate and pass through their watery substance; but the hard parts will in the meantime be the more thrust together, and press one another; and consequently, by reason of their vehement motions, they will have an endeavour to rebound from each other. Whensoever, therefore, the compression is great enough, and the concave parts of the clouds are, for the cause I have already given, congealed into ice, the cloud will necessarily be broken; and this breaking of the cloud produceth the first clap of thunder. Afterwards the air, which was pent in, having now broken through, makes a concussion of the air without, and from hence proceeds the roaring and murmur which follows; and both the first clap and the murmur that follows it make that noise which is called _thunder_. Also, from the same air breaking through the clouds and with concussion falling upon the eye, proceeds that action upon our eye, which causeth in us a perception of that light, which we call _lightning_. Wherefore I have given a possible cause of thunder and lightning.