The title page is shown in Fig. 4.2. This is followed by an 11-page preface in which Boyle gave some of his reasons for the style of the book. An interesting feature is the extent to which he emphasized that he did all the experiments himself (albeit with the help of Hooke). The experiments were described in considerable detail so that readers could reproduce them if they wished. For example, on the second page of the preface, which is headed “To the Reader,” Boyle stated, “On my being somewhat prolix in many of my Experiments, I have these Reasons to render, That some of them being altogether new, seem to need the being circumstantially related, to keep the Reader from distrusting them.” It might seem strange to the present-day reader that Boyle put so much emphasis on the fact that all the experiments were actually carried out. However, a number of books before this contained a mixture of actual and “thought” or imaginary experiments. As an example, Galileo in his great book Dialogues Concerning Two New Sciences [14] discussed how the force or resistance of a vacuum could be measured. He described how a piston could be made to fit perfectly in an inverted cylinder filled with water with all the air excluded. When weights were added to the piston until it fell, the force of the vacuum could be derived. However, this is not an experiment in the sense we now use the term, but an imaginary situation that Galileo developed to explain a concept. Boyle was at great pains to emphasize that his experiments were actually done, and in fact some of the detail is rather tedious.

Another issue briefly referred to in the preface is why the book was written in the form of a letter to Boyle’s nephew. In fact, throughout the book, there are numerous allusions to the recipient of the letter, which allowed Boyle to emphasize important points. For example, on page 17, after describing the construction of the pump, Boyle writes “Your Lordship will, perhaps, think that I have been unnecessarily prolix in this first part of my Discourse: But if you had seen how many unexpected difficulties we found to keep out the externall Air, even for a little while, when some considerable part of the internal had been suckt out; You would peradventure allow, that I might have set down more circumstances than I have.” This little conceit of imagining that he is talking directly to his nephew is somewhat like the format Galileo used in his book referred to above [14] in which the whole of the scientific thesis was given in the form of a discussion between three people.

After Boyle’s preface titled “To the Reader,” there is another brief introduction headed “Friendly Reader,” but this was written by the editor of the book, Robert Sharrock (1630–1684), who took Boyle’s presumably handwritten manuscript and prepared it for publication. At the same time, he prepared a Latin edition because, as he said, “Since the Mountain cannot come to Mohamet, Mohamet will go to the Mountain.” By this, he meant that many scientists outside England wanted to read the book but could not understand English. Latin was still the lingua franca of the intelligentsia of the civilized world.

The next section of the book is an expanded table of contents under the heading “A Summary of the chief Matters treated of in this Epistolical Discourse.” Boyle explained that the book begins with a “Praemium,” an old term for an introduction, that is devoted to a description of the pump. This is followed by brief descriptions of the 43 experiments in the book and a short conclusion at the end.¹

It should be added here that this 1660 edition does not include what we now know as Boyle’s Law, that is the inverse relationship between the volume of a gas and its pressure [20]. This was added in the second edition of 1662, as described later.

The first 19 pages of the book describe the air pump in considerable detail. The description in the text is made much clearer by the fine engraving of the pump shown in Fig. 4.3. A modern reconstruction of the pump is shown in Fig. 4.4.

Boyle divided the description into two parts. The first is the glass “receiver” on the top in which the partial vacuum was developed and in which the experiments were carried out. The most striking feature of this is its great size. Boyle stated that it contained ~ 30 wine quarts, each of them containing “near 2 pound of water.” In modern units, this is a volume of ~ 28 l, which, if the receiver were spherical, would mean a diameter of ~ 38 cm. This is the size of the receiver in two modern reconstructions.² In fact, Boyle wanted a larger receiver, stating “We should have been better pleas’d with a more capacious Vessel, but the Glass-men professed themselves unable to blow a larger, of such a thickness and shape as was requisite to our purpose.” Boyle apparently recognized the enormous compressing force that would be developed on the receiver when the air pressure was reduced and of course an implosion could have been disastrous. In fact, in one of the experiments, a glass vial containing air at normal pressure exploded and cracked the receiver.

At the top of the receiver, there was a round hole ~ 4 in. in diameter with a lip of glass almost 1 in. high. This allowed relatively large objects to be introduced into the receiver. The orifice was closed with a brass ring that was cemented in place. The brass ring had a smaller hole in it ~ 0.5 in. in diameter through which smaller objects could be introduced into the receiver. This hole was closed with a ground brass stopper, which could be rotated so that a string attached to the bottom could control equipment in the receiver. In addition, this stopper could be easily removed and replaced. By contrast, the 4-in. brass ring had to be re-cemented every time it was removed. Incidentally, Boyle makes occasional comments that the design could be improved, and at this point he suggests that a better design for the 4-in. hole would be a ground glass taper “in case your Lordship should have such another Engine made for you.”

At the bottom of the receiver was a brass stopcock similar to designs used today. The connection between the stopcock and the receiver was a challenge. The solution was to form a piece of “tin” with a conical shape and fill the space between the tin and the receiver with cement made of pitch, resin, and wood ashes “well incorporated.” To prevent the cement from plugging the hole in the receiver during this process, a cork was placed in the hole and withdrawn through the top of the receiver with a string after the cement had set. Modern glassmakers are able to form the glass to fit the stopcock, and in fact this is what was done in the modern reconstruction at the Museum of the History of Science, Oxford, UK (Fig. 4.4).

The second part of the pump was the hollow cylinder together with the piston, which was driven by a rack and pinion. The cylinder was a piece of cast brass about 14 in. long with a hole 3 in. in diameter bored within it. Machining a hole of accurate constant diameter would have been a challenge, but boring large holes in metal was a well-known skill in making cannons.

One of the most critical parts of the pump was the piston or “sucker” as Boyle referred to it. Unfortunately, the description is less detailed than we would like. Boyle simply says that it consisted of two parts, one (marked 44 just above the rack in Fig. 4.3) “somewhat less in Diameter than the cavity of the cylindre, upon which is nail’d a good thick piece of tan’d shoe Leather, which will go so close to the Cylindre, that it will need to be very forcibly knock’d and ram’d in, if at any time it be taken out.” It is not clear from this description exactly how the piston is constructed. Presumably, the first part [44] is made of wood to accept the nails, but where the thick piece of leather was placed is uncertain. Conant [12] in his discussion of the pump shows the leather seal as nailed to the upper side of the wooden piece, but this would not work well to form an airtight seal as the piston was pulled down because the leather would rise up at the periphery. Much better would be to place the leather on the bottom of the wooden piston. This is the arrangement used in reverse in modern bicycle pumps. The reconstruction of the pump in the Science Museum in London interpreted Boyle’s description differently, and there the strip of leather is nailed around the periphery of the wooden piston. It is uncharacteristic of Boyle to be so vague about this critically important element of the pump because, of course, the ability of the pump to reduce the pressure in the receiver depended critically on the fit of the piston in the cylinder.

The piston was pulled down using a rack and pinion, which is clear from the engraving. Finally, the cylinder had a small valve at the top consisting of a tapered hole with a well-fitting brass plug that could be easily removed. To operate the pump, the stopcock at the base of the receiver was closed, the plug valve was opened, and the piston was cranked to the top of the cylinder. Then the plug valve was closed, the stopcock was opened, and the piston was cranked down. This process was then repeated as necessary. In some experiments, for example experiment 37, the piston was drawn down before the stopcock was opened.

At the end of this description of the pump, Boyle apologized to his nephew, stating that the description may seem to be unnecessarily prolix. However, he then went on to say that he included many details because of the great difficulty of getting the pump to perform satisfactorily. He had many problems with air leaks through the cement at the top and bottom of the receiver, and we can assume that the fit of the piston in the cylinder was always a serious problem. To reduce leaks, he poured into the top of the receiver a little “sallad oyl” to make the stopcock more airtight. Also a “pretty store of oyl” was poured into the cylinder to lubricate the piston. Interesting (and surprising to Boyle) was the fact that adding some water to the oil improved the operation of the pump.

Because of the problem of leaks and the resulting loss of the vacuum in a relatively short time, Boyle divided possible experiments into two types. The first was those that could be carried out in a short time, and the book concentrates on this group. The second type of experiment, such as studies of the preservation of animal or other bodies in a vacuum, or the germination and growth of vegetables, required a sustained partial vacuum over a long period of time, and Boyle conceded that his pump could not provide this.

Despite these imperfections, the pump was a major technological and engineering advance. As such, it owed much to the ingenuity of Hooke. The result was that, for the first time, it was possible to subject various materials and processes to a partial vacuum while setting up an identical control experiment in the air alongside the pump. Alternatively, the experiment could be performed in the receiver at normal atmospheric pressure and again when the air was removed. Boyle himself was quite aware of how innovative all this was, and the modern-day reader of the 1660 book senses the excitement and novelty of it.

How successful was the pump in developing a partial vacuum? As we shall see in experiments 17–19, Boyle reported that the level of mercury in a Torricellian barometer could be brought down to 1 in. above the surface of the mercury trough, indicating that the pressure had been reduced to ~ 3 % of its normal value (corresponding to an altitude of ~ 23 km). Again, in experiment 33, Boyle pumped down to a good vacuum and found that it then required ~ 150 lbs. of weight on the piston to pull it down. If we take the diameter of the piston as 3 in., this represents essentially full atmospheric pressure.

As indicated above, Boyle reported 43 separate experiments [13, 16]. However, some of these are clearly related, and it is convenient for us to divide them into seven groups. Because the work was done nearly 350 years ago, it is not surprising that some experiments are more interesting than others. In particular, physiology was in its infancy in 1660, and the experiments reported here precede the important studies of Boyle himself and those of Hooke, Lower, and Mayow later in the century. It should be remembered that Boyle’s book appeared only 32 years after William Harvey’s groundbreaking De Motu Cordis [15], which ushered in a revolution in physiology. Interestingly, Boyle was one of Harvey’s patients because of his weak eyes.