The thunderbolt is a fiery vapor, compact and solid, falling violently, with greater force than lightning, for it cuts what it touches, it penetrates and burns or melts it, it divides and cuts, and nothing corporeal withstands it. Accordingly Isidore says [Etymologies 13.9], “to strike with a thunderbolt” is the same as “to strike”. As he says, it is composed of the more subtle parts of the elements, from which it inherits its great penetrating strength.  Therefore Isidore [Etymologies 13.9] calls it a blow from a heavenly javelin. It is produced from thick vapor, composed of diverse, contrary [particles] lifted, ignited and inflamed by powerful heat, driven by the collision of the winds and the friction between clouds.  Like a fiery stone it is forced downwards violently from the hollow of the clouds, moving like a javelin.

Therefore, we detect thunderbolts that crash together as they rush through the air with the glittering power of their burning. And although their nature is fiery, nevertheless they are forced by the violence of the motion to descend downward, contrary to the nature of fire, as Bede says. When it  flames and burns in its descent,  is called lightning, as Isidore says [Etymologies 13.9]; when it penetrates and divides, it is called a thunderbolt, as he also says.

Original Latin

Item capito 53 dicit idem fulminum plura genera traduntur, nam quae sicca veniunt non adurunt, sed dissipant, quae humida non urunt sed infuscant, tertium est quod clarum vocant, quod mirae est naturae, nam vinum exhaurit in doliis, intactibus vasis. Aurum et argentum liquat sacculis non combustis.

Quid autem fit coruscatio et unde proveniat, diversi diversimode tradiderunt. Dixit enim Empedocles quod coruscatio est ignis occultatus in nubibus ex radiis solis. Et hoc, dicit Aristoteles, esse falsum, quia si hoc esset, ex omni nube coruscatio accideret, quia in omni nube radii solares occultantur. Anaxagoras autem dixit quod coruscatio est ex aere descendente in nubem vel ad nubem, et se in ea occultante, cuius apparitio est coruscatio, et quando ille ignis extinguitur in nubibus aquosis, stridor illius extinctionis vocatur tonitruum.

Alii dixerunt coruscationem causari ex ventis calidis et siccis, in nubibus comprensis, quibus inflammantur et aduruntur, et ignis in eis egrediens est coruscatio, cuius splendor prius pervenit ad visum quam sonitus ad auditum. Alii dixerunt coruscationem non fieri per ignem, sed per aquae splendentis imaginationem, quae per stellarum de nocte lucentium illustrationem in nubibus videtur.

Sed dicit Aristoteles quod iste sermo error est, quia coruscatio de die videtur sub radiis solis sicut videtur de nocte. Secundum Aristotelem autem vapor aggregatus in nubibus eorum vehementi collisione attritus, ignitur, et fit materia coruscationis, et quia habet aliquas partes terrestres, earum gravitate movetur inferius. Et quia illa coruscatio est vehementis subtilitatis, et non vehementis inflammationis et adustionis, ideo apparet alba, et ideo corpora, quae pertingit, non inficit nec corrumpit. Huc usque Aristoteles, libro 2 Meteorum.