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This 'philosophical letter to the Grand Duke of Tuscany' describes several experiments on the making of glass droplets and glass 'worms' that, when they break, become pulverised glass immediately. The experiments vary the temperature in the oven, the weight of the glass, its thickness etc.The letter was written after the carnival of 1669. This is most likely a copy of the letter by Geminiano Montanari to the Grand Duke of Tuscany, sent to the Royal Society.

These glass drops and worms became known as 'Prince Rupert's drops'. The ‘glass bubble’ in the shape of a teardrop had the property that its thicker part could not be broken with a hammer, but breaking off the tip of its tapered end would shatter it completely. It was a well-known phenomenon on the continent by the 1650s, and the glass bubbles were called ‘Dutch tears’ (lacrymae Bataviae or gocciole olandesi), possibly indicating their origin (likely Mecklenberg). These drops were introduced to the English Court by Prince Rupert on his arrival in September 1660. In March 1661, Charles II sent some ‘glass bubbles’ to the Royal Society for examination, and Sir Robert Moray reported on their properties and manufacture to the meeting on 14 August 1661.

Geminiano Montanari (1633-1687), professor of mathematics at Bologna and a close colleague of Malpighi's, sent to the Royal Society/Oldenburg in April 1670 his manuscript outlining his experiments with the glass drops and the cause of their properties. The Italian manuscript was eventually translated by Thomas Henshaw, and partially read at the meetings in April. This manuscript was probably a draft of Montanari’s Speculazione Fisiche (Bologna: Manolesi, 1671), in which he showed a long spiralled tail made at Murano.

Subject: Chemistry

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Paper, 'A philosophical letter to the Grand Duke of Tuscany' by unknown author, 1669, CLP/4i/37, The Royal Society Archives, London,, accessed on 22 June 2024

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  • Classified Papers

    Dates: 1592-1741

    The 'Classified Papers' of the Royal Society are papers from British and international natural philosophers and scholars categorised according to subject areas.

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