The floriated initial M in the above text comes from the 16th century alchemical book Della tramutatione metallica sogni tre by Gio. Battista Nazari Bresciano.
Floriated, or decorative, initials are common in antiquarian books. Typically new sections of text were marked with a decorative initial letter. In the early years of printing, these letter were typically added by hand, so the printed text would leave a space big enough for the letter. Sometimes, you might open a book where the decorative initial was never included, there’s just a blank space.