Nail polish remover!

Leonardo distinguished chemistry from alchemy, which was centered on the belief that mercury, sulphur and salt are the basic elements, and which sought to turn base metals into gold. “The false interpreters of nature declare that quicksilver is the common seed of all metals, not remembering that nature varies the seed according to the diversity of things she desires to produce in the world” (Codex Atlanticus folio 76va). (Quicksilver is  the metallic element mercury, also called argentum vivum.)

His manuscripts include mention of 300 different chemical compounds, and also equipment such as a stove for distilling acqua forte (nitric acid). When we say "distilled" we mean subjected to the process of vaporization and condensation, typically to extract and purify a substance.

Codex Atlanticus 335 rb (new 912r), c.1479-1480. The text reads "fornello da stillare acque forti" (stove for distilling aqua forte).

He went further (of course), introducing a continuously cooled still-head that allowed more efficient condensation of distilled vapor.

Codex Atlanticus folio 400 vc (c. 1479-1480).

He didn't stop there, providing detailed descriptions of a self-feeding chemical furnace, and another still with continuous refrigeration. We should always keep in mind that when he draws a machine there are several quite different interpretations:

• he may have invented it, built it, then sketched it
• he may have invented it (conceived it), but never built it
• he may have seen and sketched someone else's invention
• he may have read about the invention from another source and simply copied the idea

Therefore, regardless of whether Leonardo deserves credit for a particular invention or discovery, we can at least see his thousands of pages of manuscripts as a gateway to the world of Europe 500 to 550 years ago, and we can appreciate what could be known in that time period. And we know there was a vigorous exchange of technological ideas throughout Europe, and beyond.

And what sorts of studies did he do? Leonardo distilled salts from human feces. “Salt may be made from human stool, burnt and calcined and made into lees and dried slowly by fire. And all stools produce salt in a similar way, and these salts when distilled are very penetrating” (Royal Library 12351v). (Calcined means roasted [from calcinare, to heat]; lees refers to sediment,  or dregs; this is insoluble matter.) We know that he prepared nitric acid, copper nitrate, and red ferrous oxide.

He also he prepared acetone--the stuff of nail polish remover--and this is thought to be one of his original discoveries. He writes:

“(1) Stilla aceto e la stillatura sua fa passare per la sua feccia nuovamente (2) bruciata e calcinata o (3) falla passare per il tartaro calcinato e poi (4) ristilla e ricalcina il tartaro e fia acqua risolutiva” (Ms. K3, folio 114r, c.1509-1512).

I've added numbers to break down this passage. The source of this analysis is Ladislao Reti (Le arti chimiche di Leonardo da Vinci, La Chimica e l'Industria, Soc. An. Editrice di Chimica, Milan, 1952, pp. 13-14).
(1) Leonardo frequently mentions distilled vinegar. This was done to produce concentrated acetic acid, separated from other constituents of wine (e.g. salts, acids, colors).
(2) Drip vinegar and the dripped material is made to pass through its dregs newly burned to pass through the tartaro calcinato (bitartrate of potassium, tartrate of calcium). Potassium carbonate and some calcium carbonate will form.
(3) The acetic acid will react with the carbon compounds to form a mixture containing potassium acetate, with some calcium acetate.
(4) The potassium acetate and potassium carbonate, heated to a high temperature in a closed vessel, decompose forming acetone (“acqua risolutiva”).

At least, this is my understanding of Ladislao Reti's analysis. (Of course, this is a modern chemical interpretation, and is based on the period table of the elements as introduced in the 19th century--chemistry as we know it today would have been foreign to anyone in earlier centuries.) Nail polish remover in 1509? Apparently so. I wonder if he invented nail polish too--Lomazzo credits Leonardo with the discovery of pastels, so why not.

A riddle

Leonardo often used the first and/or last page of a notebook to write down miscellaneous thoughts such as lists of things to do. On the last page of the Codex Madrid I (folio 191v) he leaves us a riddle:

I am he who was born before the father
I killed one third of humanity
and then returned to the womb of my mother

To solve the riddle it may help to look at the Italian:

I' son colui che nacqui
inanzi al padre,
la terza parte delli omini ucisi,
po' tornai nel ventre alla
mia madre.

So the middle line has been translated "I killed one third of the human race." While he doesn't leave the answer, scholars think they've figured it out. Before you look at the answer (below) here's a clue:

Source: Woodcut from Anonimo fiorentino c.1500 (Incisioni Fiorentine del Quattrocento a cura di Lamberto Donati, Istituto Italiano d'Arti Grafiche editore Bergamo, 1944, pl.58).

The answer is Cain, whose father Adam was not technically born. When he killed his brother Abel he killed one third of all men. Then he died and returned to the womb of his mother, the earth.

Why there can't be ghosts (or if there are ghosts they're pretty harmless)

Ghosts and percussion.

"There cannot be any sound or voice where there is no movement or percussion of the air. There cannot be any percussion of the air where there is no instrument. There cannot be any instrument without a body. This being so a spirit cannot have either sound or form or force, and if it should assume a body it cannot penetrate or enter where the doors are shut. And if any should say that through air being collected together and compressed a spirit may assume bodies of various shapes, and by such instrument may speak and move with force, my reply to this would be that where there are neither nerves nor bones there cannot be any force exerted in any movement made by imaginary spirits. Shun the precepts of those speculators whose arguments are not confirmed by experience."

Source: Manuscript B 4v (translation by Edward MacCurdy, The Notebooks of Leonardo da Vinci (New York: George Braziller, 1956), Philosophy chapter.

Percussion: the strongest of the four powers

Ms. A 8r

Leonardo became interested in the "four powers" of movement, weight, force, and percussion. Of these he considered percussion to be the strongest. Here he systematically studies the blow of an object (a ball) against a surface such as a wall, a bell, or another ball. He notes that the angle of incidence equals the angle of reflection. Leonardo's ideas about the physical laws of nature extended to his work as an artist. You can see this most of all in the Deluge drawings (at Windsor Castle; see below), leading to cataclysmic destruction. Percussion, he wrote, exceeds any other natural power (Codex Atlanticus 11vb). 

W912380. Source: https://www.royalcollection.org.uk/collection/

This is one of Leonardo's amazing drawings of the deluge. This image, like the others in the series, shows a cataclysmic storm overwhelming a landscape. His writing, near the top of the drawing, says "Of rain. You will show the degrees of falling rain at various distances and of varying degrees of obscurity, and let the darkest part be closest to the middle of its thickness." Here in the midst of this powerful, mysterious, emotionally-laden work of art he leaves this note reflecting his dispassionate, scientific approach. If someone asked him about this work he'd likely explain that he is maximizing the display of the force of percussion. 

Calculating specific gravity

Codex Madrid I, folio 33r

After studying Archimedes, Leonardo invented a device to measure specific gravity by water displacement. To make his apparatus he would have visited a slaughterhouse to obtain ox intestine, and then he provided support using iron wire. He measured the specific gravity of lead as 12 (actual value: 11.4). Today we define specific gravity as the ratio of the density (i.e. mass of a unit volume) of a substance to the density of water or some other reference substance. When Leonardo did this experiment we believe he was the first person to ever measure the specific gravity of a substance. (The ancient Roman architect Vitrivius tells us the famous story of how Archimedes yelled "Eureka!" after figuring out that a crown had silver in it rather than being pure gold, but this is quite different than Leonardo's calculation of a particular value for the specific gravity of a substance.)