Bartholomew says: "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” (fulgurare) is the same as “to strike” (ferire). 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."
Our debt to Ben Franklin
Watching a thunder and lightning storm is often enjoyable - when you’re in a safe place. We know how lightning works now, and we distinguish readily among the many kinds:
- cloud-to-ground lightning (when negative particles in storm clouds short out with positive particles on the ground)
- cloud-to-cloud lightning (when negative particles in the clouds short out with positive particles higher up in the clouds)
- ball lightning, which purportedly persists for several seconds
- intracloud or sheet lightning that lights up clouds
- heat lightning is lightning seen on the horizon, often in a storm just far enough away not to be really seen
- red sprites and blue jets: high altitude electrical discharges that go up from the clouds toward the stratosphere
- St Elmo’s fire: a bright blue glow appearing at the ends of tall, sharply pointed objects such as ship masts
In lightning, the ionized particles form an electric surge of plasma – electricity is invisible, but when it passes through things it affects them. So the electric surge heats the air along its way and the air glows, then it gets so hot it explodes, giving us the bluish-white flashes we see and the crack or thunder we hear.
We know much about this thanks in part to Benjamin Franklin’s kite experiment.
Bartholomew's debt to Aristotle
Bartholomew lived long before Franklin, so he relies on Aristotle, whose thinking about vapors seems remarkably close to our knowledge of the charged particles and plasma. Don’t let any such similarity fool you – the underpinnings are still quite different.
Bartholomew’s terms alone show that he did not think about lightning as we do. For example, the difference between fulmen and fulgur is that between a hit and a destructive hit. There is no cloud-to-cloud lightning here, nor sheet lightning, and he could not have known about red sprites and blue jets at all. His “bright lightning” (fulmen clarum) comes from Pliny the Elder, not Bede, and may refer to ball lightning, but we really aren’t sure.