"Whose least part crackt, the whole does fly": early views on Prince Rupert's Drops

Honor is like that glassy Bubble
That finds Philosophers such trouble,
Whose least part crackt, the whole does fly,
And Wits are crack’d to find out why.
Samuel Butler, Hudibras, Part II, Canto II, lines 385-89.

In the second part of Samuel Butler’s satirical poem Hudibras, published in 1664, the four lines quoted here reference a phenomenon that has perplexed material scientists for over 350 years, and is only now being fully understood. It is also the answer to this month’s Crocodile Mystery.

On the surface, a reference to glass that breaks apart doesn’t seem particularly remarkable. That’s what pretty much all glass does, right? However, there are a couple of key phrases in Butler’s verse that make it clear that he isn’t referring to any old glass. The “glassy bubble” denotes a particular shape of glass—probably something spherical—and in the third line implies more than simple shattering.

And indeed, as several of our readers guessed from last week’s image, Butler’s lines are referring to a phenomenon known as Prince Rupert’s Drops.

Prince Rupert’s Drops (image from Wikimedia Commons)

These glass drops are formed by cooling them in water, and when you do so, they exhibit some remarkable physical properties: the head of the drop is so strong as to be nearly impervious to crushing, even with blows from a hammer; however, if the tail is snapped, they explode with great force.

They are known as “Prince Rupert’s Drops” after Prince Rupert of Rhine, cousin to Charles II. They are also known as Prussian, Dutch, Holland, or Bavarian tears, and were known to northern Europe in the early 17th century (and frankly, probably known to glassmakers long before that—they are relatively simple to produce, as you’ll see in the video at the end of the post). Rupert’s name got attached to them because he was instrumental in getting them to Charles II, who in turn handed them over to the Royal Society for study in 1661.

But they weren’t the first Britons to examine these glass drops. Dutch secretary, artist, and all around polyglot, Constantijn Huygens (father of the far more famous astronomer, Christiaan) reportedly sent examples of these drops to Margaret Cavendish for examination in 1657, while she was living in Antwerp. (Cavendish’s thought was that the heads of the drops contained a sulphurous substance that could function as ammunition, causing the observed explosion.)¹

The earliest (English) published reference I was able to find to these glass drops occurs in the 1662 publication The Art of Glass, produced in London by Octavian Pulleyn.

Title page from The Art of Glass. (Folger 141- 042q)

The book itself is a translation of Antonio Neri’s Italian text on glassmaking from 1611 by Christopher Merret, to which is added a discussion of these glass drops, which the Royal Society had been investigating.

Title page detail (Folger 141- 042q)

Sir Robert Moray, a Scots army officer and privy councillor, was one of the founding members of the Royal Society in 1660 (and due to his close connection with Charles II, was chosen to preside over many of the early meetings). He also undertook his own experiments, one of which was an investigation into these curious glass drops.

According to Thomas Birch’s 18th century History of the Royal Society, on March 4, 1660/1, Charles II sent five “little glass bubbles, two with liquor in them, and the other three solid” to the Royal Society for examination. Shortly thereafter, several of their members were sent off to a Woolwich glasshouse to inquire about ways to experiment with them. By July of 1661, the Society had commissioned more of these drops made for experimentation, which Moray was to carry out himself. On August 14, 1661 the Society received Moray’s report.²

That report is what was reproduced in the 1662 Art of Glass publication. The Royal Society had not yet started their own publications, which is why I believe this is the first reference to these drops in English print.

Folger 141- 042q, p. 354 (leaf 2A1v)

While simple, the diagram of the drop is to the point, and serves as a foundation for the rest of Moray’s observations.

Moray’s diagram: simple, yet effective. (Folger 141- 042q, p. 354 (leaf 2A1v))

Moray’s observations included the realization that formation of these drops had something to do with cooling them in water: “If one of them be cooled in the air, hanging at a thread, or on the ground, it becomes like other Glass, in all respects, as solidity, &c.” Moray also tried cooling the drops in other substances. In “Sallet Oyl” (“salad oil”—probably olive oil) he noted that they “do not miscarry so frequently as in cold water” but they also didn’t exhibit the same explosive tendencies when the tail was snapped. Milk, “spirit of wine” (vinegar), turpentine, and even mercury were all tried as a cooling medium, but without the same results as those drops cooled in water.³

Obviously, the fascination with these glass drops did not end with Moray’s report to the Society. They became known as a curiosity, and entered the realm of public awareness, as evidenced by Butler’s use of them as a metaphor in Hudibras only two years after Moray’s report.⁴

And only a year later, in 1665, Robert Hooke’s Micrographia included another discussion of these glass drops.

Although best known for being the first book to describe and show living matter under a microscope, Micrographia also has a number of sections that deal with inorganic matter as well.

Hooke’s Micrographia is probably better known for the up-close-and-personal images of bugs than for the inorganic matter. (Folger 140- 490f)

Micrographia opens with discussions of “a sharp small needle,” the edge of a razor, linen, and silk. On page 10, “Observ. VI” is regarding “small glass canes.” Between pages 10 and 11 Schem. IV is inserted and it is from here that last week’s image was taken (all of these images are taken from the 1667 second edition, but as far as I could tell with a comparison in EEBO, these sections are identical to the first edition):

Folger 140- 490f, Schem. IV, between p. 10-11

The discussion of glass canes begins on page 10 by investigating “whether it were not possible to make an artificial pore as small as any natural I had yet found” (it was) and then moves into a discussion of what we now know is surface tension (i.e. why liquid globules of nearly any substance will form a sphere).

Then, on page 33, Hooke begins “Observ. VII Of some Phaenomena of Glass drops” and spends the next eleven pages discussing these objects.

“…being exceeding hot, and thereby of a kind of sluggish fluid Consistence, are suffered to drop down from thence into a Bucket of cold Water, and in it to lye till they be grown sensibly cold.” (Folger 140- 490f, p. 33)

Once again, the fact that these drops are cooled in water is noteworthy, because that seems to be what creates the unusual properties.

The figure on the left is an example of a whole glass drop, showing the inclusion of a number of small bubbles in the head. The figure in the right of the image was drawn after a bit of Hooke’s ingenuity: he wanted to capture the fracture patterns of one of the drops as it exploded, believing (rightly, as it turned out) that it might help him understand what was going on. So he coated the drop in isinglass, which was often used in making gelatin, but could also be used to form a mostly-transparent glue (it sometimes gets called fish glue). Having thus coated the drop, Hooke could snap the tail and, held in place by the glue, the drop would retain its basic shape, showing the fault lines of the breakage.

Based on these observations, Hooke theorized that the part of the molten glass that came in contact with the water cooled rapidly, while the interior of the drop, insulated and constrained by the hardened outer shell, cooled more slowly. This caused a “springgy tension” that held the drop in balance, much like the keystone of an arch, until the forces were released by snapping the tail.

Hooke’s theory held up to the observations of others, but it was not until 1920 that A. A. Griffith, also working for the Royal Society, was able to put a mathematical formulation to the state of tension found in these glass drops. And even then, the exact physics behind the stresses that create such strength in the bulb end of the drop were not fully understood until the 21st century. Using high speed cameras, a team of physicists were finally able to map out the exact stresses that occur within these drops.⁵

Below is a video from Purdue University College of Engineering explaining these glass drops and showing how they are made:

So the 20th century mathematics and the 21st century camera technology all help support Hooke’s 17th century observations. Not bad for a guy with a microscope and some fish glue.

I will leave you with one final thought: today, many physics students first encounter Robert Hooke when they learn about Hooke’s Law. This law states that the force needed to extend or compress a spring by some distance is proportional to that distance:

F = -kX

where F = the force applied to the spring, k is the spring constant (the stiffness of the spring) and X is length of expansion/compression. (The negative is inserted because it is generally assumed that you are calculating the amount of force generated as the spring returns to its natural state: that is, after the spring has been compressed or expanded, how much force does it generate while “springing” back.)

Hooke officially published this law in 1678, but it is believed that he began formulating his ideas about it as early as 1660. So his experiments with these glass drops in the early 1660s, with their high stress behaviors, may have played a role in the creation of one of the basic laws of physics that we know today. A “springgy tension,” indeed.