Book 32: fire moving through boiling water; the sun's heat
Millet grains trace bubbles as a model of vapours rising from the earth; molten bronze likens the sun
Under the heading 'Book 32', Leonardo proposes an experiment on the motion fire makes as it penetrates water through the bottom of a cauldron and rises in bubbles to the surface, and on the motions the water makes when struck by that fire. He advises a square glass vessel watched through one wall, with grains of millet dropped into gently boiling water so their movement reveals the water's currents; by this he hopes to study the hot vapours that exhale from the earth and pass up through water and air. A second passage, 'On the sun', answers those who deny the sun is hot because it is whiter and clearer than fire: molten bronze, he replies, is whitest when hottest and more fire-coloured when cooler. A marginal note adds that fire makes molten bronze much hotter than the fire itself.
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Fire penetrating boiling water, traced by millet grains
In a square glass vessel, watched through one wall, fire penetrates the water through the bottom and rises in bubbles; grains of millet dropped in the slow-boiling water reveal the water's motion as it carries them. The experiment is meant to disclose many motions that occur when one element penetrates another.
Hot vapours exhaling from the earth through water
The same experiment is offered as a way to investigate the hot vapours that exhale from the earth and pass through water, twisting because the water impedes their motion, then penetrating the air with a straighter motion.
'On the sun': whiter than fire, like the hottest bronze
Against those who say the sun is not hot because it is whiter and clearer than fire, Leonardo answers that molten bronze at its hottest is more like the sun's colour, and at its coolest more like the colour of fire.
Molten bronze hotter than the fire
A marginal note states that fire makes molten bronze much hotter than the fire itself.