Glass drops or tears coated in glue,
after detonation, (cross section is left)
from Robert Hooke's
Micrographia 1664, between p. 10, 11.
These glass drops became a novelty at royal courts throughout Europe, given a glass furnace they were easy to make, easy to demonstrate, and never failed to amaze observers who had not seen them before. They sparked animated discussion in the many scientific societies that had sprung up; what forces of nature were involved that a piece of glass could resist a hammer yet explode into dust at the loss of its slender tail?
In the late seventeenth century a Roman publisher by the name of Tinassi  regularly issued compilation of noteworthy letters. In his journal’s edition for the year 1672, he published two letters by Geminiano Montanari, a mathematics professor at the university in Bologna, both on the subject of glass drops. In the introduction, he suggests that these curiosities were
Believed to be introduced in Sweden, Holland, then in England, France and Italy; In Paris in the year 1656, many experiments were made at the Academia which met at the home of Mr. Montmor.  Many have written of this, among others Monconys  in his "Journey to England,”  Thomas Hobbes in his Problematica Physica, […] and Christopher Merrett, which in the Latin translation of L’Arte Vetraria of Antonio Neri,  is inserted the experiences of the Royal [Society] of England, [and] Mr. Robert Hooke  in his Micrographia. At a time when the concepts of atoms and molecules were still being debated, the glass drops became a nucleus around which a new science developed of mechanical tension and compression. A simple drip of glass caused sharp minds to puzzle and to take a closer look.
In Micrographia, Robert Hooke wrote about how he “ground away neer two thirds of the ball, yet would it not fly to pieces, but now and then some small rings of it would snap and fly off, not without a brisk noise and quick motion, leaving the Surface of the drop whence it flew very prettily branched or creased, which was easily discoverable by the Microscope. This drop, after I had thus ground it, without at all impairing the remnant that was not ground away, I caused to fly immediately all into sand upon the nipping off of the very tip of its slender end.”
Hooke continues to describe coating drops in fish glue (isinglass) which was tough enough to hold the piece together when the tail was snapped. “The drop gave a crack like the rest, and gave my hand a pretty brisk impulse: but yet the skin and leather was so strong as to keep the parts from flying out of their former posture and, the skin being transparent, I found that the drop retained exactly its former figure and polish, but was grown perfectly opacious and all over flaw’d, all those flaws lying in the manner of rings, from bottom or blunt end, to the very top or small point.” (See illustration above.)
He discovers that heating the glass drop and then allowing it to cool slowly neutralizes the explosive effect. Finally, he puts it all together: rapid cooling of the surface causes the interior to be compressed like a spring. Snipping the tail, where the skin is thinnest releases all the pent-up energy at once and the piece explodes.
 Tinassi 1672, p. 95.
 Niccolò Angelo Tinassi, active 1654-1690.
 Henri-Louis Habert de Montmor (c. 1600–1679), founded the Montmor Academy, which met at his house in Paris from 1657 until its dissolution in 1664.
 Balthasar de Monconys (1611–1665).
 Monconys 1677, for glass drops see pp. 32, 42 (fig. 4). https://books.google.com/books?id=L0b687oZRVQC
 1662. Problematica Physica (translated in English in 1682 as Seven Philosophical Problems)
 This reference is not to Merrett’s 1662 translation of Neri (1612), but to the 1668 or 1669 edition by Frisius in Amsterdam which includes Merret’s annotations.
 Robert Hooke (1635–1703).
 Hooke 1664, pp. 33–44.
*This post first appeared here 2 January 2015
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