De cometis

in the tenth house, then Mars dominates, dissolves the power of Saturn fixed in the earth, and makes the gathered matter fly out into the air, especially when it has afflicted the Moon in an airy place; then the appearances of comets occur. This is the opinion of Albumasar in the eighth book of Great Conjunctions and Hali in the eighth part of Revolutions of Years.

In many generations of these materials, when they are not constrained to remain, one part after another is directed, flies out, and is mixed with the air like an odor as it multiplies through the medium. It is possible in the generations of comets that all parts disposed by the stars are gathered together at once, but that one part is exhaled first, while another remains afterward. Much of this material is also fixed in the earth. Hence, as if from a mineral, through a long process of time, a very subtle respiration and exhalation of its spirits occurs, which are mixed with the air. Hence, the beginnings of such principles are continuously renewed.

An introduction of some things by whose similarity the substances of comets receive their natures and properties from the heavens.

3 It must certainly be believed that in all living things a seed is produced, in which the greatest virtue is born, by the wondrous mixture of the providence of the Supreme Creator, as in Genesis 1: "Let the earth bring forth the green herb and that which bears seed." In such a way that one grain produces a hundredfold, as light is eighth. And from the smallest nucleus, a tree grows a thousand times larger. In these also, no person of sound mind would doubt that infinite succeeding generative spirits are contained, just as in the sperm of animals.
Gems also grow by the virtue of the stars acting upon the earth, altering its parts, Gems by virtue dissolving, segregating, and purifying them, preparing them with their virtues by introducing a spirit similar to the seminal force. From this they are coagulated and fixed, and the virtue and nature is inhibited in what is being fixed, and a great power is born, as appeared to the philosophers and would still appear, if their constellations were duly wished for. As the virtue of the topaz descends from elfeta an Arabic term for a celestial influence or point, that of the sapphire from alhayoth an Arabic term for a star or celestial influence, as Hermes hands down in his On Fifteen Stones. Thus, virtues are also granted to metal ores by celestial forms through many natural operations succeeding one another, in the likeness of which philosophers have ordered alchemy, in their operations being able to change nature, by introducing those who first calcine bodies, then dissolve them, and segregate the elements, so that they can receive a sublimated spirit almost absolute from elementary properties. By fixing this, they constitute the elixir, as Geber hands down, for which, similar to the seeds of living things, there is the greatest virtue in purifying metals, altering their matter, and transmuting them into another kind according to its preparation.

How celestial virtue similar to the seminal force is acquired by cometary substances and similar things in the likeness of those mentioned.

X I have made a commemoration of these things, because a similar process occurs in the generations of comets and their properties. When the dominating stars effect alterations in the depths of the earth, disposing the oily and exhalable parts for the reception of their forms, and imbuing them with their own proper virtues by which...