up, forcing air out through a one-way valve, then pull the piston down, leaving a vacuum in the tube (there is a replica of Hooke’s pump in the Science Museum in London). When a glass vessel was attached to the pump via another one-way valve, the piston could be pumped up and down repeatedly, sucking more and more air out of the glass vessel.
At about the time Hooke was developing his pump, in the late 1650s, another Englishman, Richard Towneley, was repeating the experiments carried out by Florin Périer, using a Torricelli barometer, on Pendle Hill in Lancashire. He surmised that the lower pressure of air at higher altitude is because the air is thinner (less dense) there, and mentioned this idea, which became known as Towneley’s Hypothesis, to Boyle. Boyle was intrigued, and gave Hooke the task of carrying out experiments to test the hypothesis.
The simplest of these experiments did not involve the air pump. Hooke took a glass tube shaped like the letter J, with the top open and the short end closed. He poured mercury into the tube to fill the U-bend at the bottom (just like the U-bend in a kitchen sink), sealing off the air trapped in the short arm of the J. When the mercury was at the same level on both sides of the U-bend, it meant that the trapped air was at atmospheric pressure. But when more mercury was poured in to the tube, because of its extra weight the pressure increased and forced the air in the closed end into a smaller space. Boyle was not a great one for calculations, but Hooke was, and he made careful measurements of the amount of mercury being added and the amount by which the trapped air was squeezed, which showed that the volume of the trapped air was inversely proportional to the pressure. In other words, if the pressure doubles, the volume is halved; if the pressure triples, the air is squeezed into a third of its original volume, and so on.
Other experiments carried out by Boyle and Hooke did use the air pump, and showed, for example, that water boils at a lower temperature when the air pressure is reduced (which explains why it is hard to make a good cup of tea on top of a mountain). This was a very tricky experiment, as it involved placing a mercury barometer inside a sealed glass vessel where the water was being heated, to monitor the pressure as air was pumped out.
The experimental results were first announced to the world in Boyle’s book, New Experiments Physico-Mechanical Touching the Spring of the Air, published in 1660. But at that time he did not explicitly spell out the inverse law relating volume and pressure. That appeared in the second edition of his book, published in 1662, and as a result it became known as Boyle’s Law, even though Hooke had done the experiments and made the calculations on which the law was based.
All of this was important to scientific thinking, because it supported the idea that the air is made of atoms and molecules, flying around and colliding with one another. It was also important in practical terms, because the realization that air has weight, and that it can be extracted using pistons to leave a vacuum, fed directly into the idea of the steam engine (see here).
| No. 10 | REVEALING THE MICROSCOPIC WORLD |
Robert Hooke may have missed out on getting his name attached to ‘Boyle’s’ Law, but he soon achieved an even greater experimental success as a pioneer of the use of the microscope. In the second half of the seventeenth century other experimenters also studied the world of the very small using lenses to magnify tiny objects, but it was Hooke who did the most thorough job, and explained his discoveries in a book, Micrographia, which was published in 1665. It was written in English, unusually for the time (most learned tomes were written in Latin), and easy for any educated person to understand. Samuel Pepys called it ‘the most ingenious book that ever I read in my life’.
© Natural History Museum, London/Science Photo Library
Drawing of the head of a fly, from Micrographia.
There were actually two ways to achieve the kind of magnification needed to make microscopy worthwhile. One was pioneered by Hooke’s contemporary, the Dutch draper and amateur scientist, Antoni van Leeuwenhoek. His ‘microscopes’ were single tiny lenses, some no bigger than a pinhead, mounted in strips of metal. They had to be held very close to the eye, and acted as powerful magnifying glasses – so powerful that they could enlarge images 200 or 300 times. These were very difficult to work with, but van Leeuwenhoek made many important discoveries, including tiny living creatures in droplets of water. Hooke used this kind of lens when he needed to pick out the tiniest details in the objects he studied, but he also used a different experimental setup, the forerunner of modern microscopes. These microscopes were made from combinations of lenses, mounted in tubes 6 or 7 inches long (approximately 15–18 centimetres), similar to the way in which telescopes are made. They were easier to work with, but did not give such powerful magnification as the tiny single lenses.
© British Library/Science Photo Library
Seventeenth-century drawing of a flea observed through a microscope by Robert Hooke (1635–1703).
But ‘easier’ does not mean ‘easy’. Anyone who has used a microscope knows that it is important to have a bright light shining on the object being studied at the focus of the instrument. But there was no electric light (or even gaslight) in the 1660s. Candles were not bright enough to do the job. So Hooke used an ingenious arrangement of glass lenses and round containers filled with water to act as spherical lenses to focus light from the Sun (or sometimes a candle) on the object of interest. This worked well for inanimate objects. But he also wanted to study living things, such as ants. These would soon crawl out of the focus of the microscope, but if he killed them then their bodies shrivelled up. He tried sticking them down with wax or glue, but they still wriggled about too much to be studied. Then he hit on the idea of dosing them with brandy to make them unconscious. The ant, he said, was soon ‘dead drunk, so that he became moveless’.
Of course, there was no way to photograph the objects under the microscope, and Hooke’s book is full of beautiful, detailed drawings of what he saw. A seventeenth-century experimenter had to be an artist as well as a scientist. Hooke showed his readers how much irregularity there is in something as seemingly perfect as the point of a needle or the edge of a razor, and he also showed them how much regularity there is in crystals. This, he said, must result from a regular arrangement of the particles making up crystals – an early hint at the existence of atoms.
But perhaps his most spectacular realization was that fossils are the remains of once-living creatures. In the middle of the seventeenth century, it was widely accepted that these peculiar stones, resembling living things, are just pieces of rock that have been distorted by some unknown process to look like living things. But Hooke, with the evidence of his microscopic studies in front of him, said that fossils are not simply contorted pieces of rock. The details matched the patterns of living things too precisely for that to be true. He said that the fossils we now call ammonites must be ‘the shells of certain Shellfishes, which, either by some Deluge, Inundation, earthquake, or some such other means, came to be thrown to that place, and there to be filled with some kind of mud or clay, or petrifying water’. And he realized that because such remains are now found far from the sea, there must have been major changes to the Earth in the past. The reference to a ‘Deluge’ seems to have been a sop to those who believed in the literal truth of the story of the Biblical Flood. Hooke made his own views clear in a lecture at Gresham College in London, where he said that ‘parts which have been sea are now land’, and ‘mountains have been turned into plains, and plains into mountains, and the like.’ A profound inference to draw from looking at tiny objects through a microscope.
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