Science: A History in 100 Experiments

There is no clear distinction between experiment and invention. Inventors have to experiment to find out what works, and experimenters often have to be inventors, as the example of the development of the vacuum pump for use in scientific investigation highlights (see here). Although this book concentrates on the more obviously experimental end of this spectrum, there is one beautiful example of the synergy between experiment and invention that is so important historically that it simply cannot be overlooked. This is the way in which investigations of the relationship between heat and temperature led to the development of the steam engine, which in turn powered the Industrial Revolution.

In 1763, James Watt was working as an instrument maker in Glasgow, where he became familiar with Black’s work, but not, at first, with all of his discoveries concerning latent and specific heat. Watt was asked to repair a scale model of a kind of steam engine developed by Thomas Newcomen, often referred to as an ‘atmospheric’ engine, because air pressure was just as important in its operation as steam. Such engines had a vertical cylinder, made of metal, containing a metal piston attached at the top (which was open to the air) by a beam to a counterweight. When the space beneath the piston was filled with steam, pressure would increase and the piston would rise. Then, cold water was sprayed into the cylinder, making the steam condense and reducing the pressure so that atmospheric pressure would push the piston down. By repeating this process over and over again, the resulting rocking motion of the beam could be used to drive a pump sucking water out of a mine.

By experimenting with the scale model of a Newcomen engine and applying his understanding of Black’s discoveries, Watt realized that this kind of engine is not very efficient. On every stroke of the engine, the whole cylinder and piston combination has to be heated up to more than the boiling point of water, in order to allow it to fill with steam. Then, it has to be cooled sufficiently for the steam to condense, even though the steam itself (as he later appreciated) gives up latent heat as it condenses. The heat required to raise the temperature of the cylinder and piston was thrown away with every stroke of the engine.

Watt realized that it would be much more efficient to have an engine that used two cylinders, one of which was kept hot all the time and contained the moving piston, while the other, without a piston, was kept cold all the time. (He wrote in his journal that this idea came to him on a Sunday afternoon in May 1765, as he walked across the Glasgow Green.) In his early models, the cold cylinder, without a piston, was simply immersed in a tank of water. The two cylinders were connected to each other, but at first outside air was still used to push the piston down. Steam pushed the piston up as before, but when the piston reached the top of its stroke a valve opened automatically to let the steam flow into the cold chamber, where it condensed, reducing the pressure and allowing the piston to fall. At the bottom of the stroke, another atomatic valve opened to let fresh steam into the cylinder. Soon, this setup was improved by sealing off the piston’s cylinder from the atmosphere and using hot steam to push the piston down as well as to push it up. But the key concept was the ‘separate condenser’. Watt’s steam engine design was patented in1769.

Because Black had not published all of his discoveries, and Watt had a fairly lowly position in Glasgow, at first Watt did not know about Black’s work on latent heat, and independently made the same discovery. Specifically, in one series of experiments he noticed that when one part of boiling water is added to thirty parts of cold water, the rise in temperature of the cold water can hardly be measured. However, when the equivalent amount of steam at the temperature of boiling water is bubbled through the cold water, it can raise the temperature of the water to boiling point. This discovery by Watt led to discussions with Black, and Black’s understanding of heat helped Watt to make improvements to his steam engine design. Black even helped to fund the development of Watt’s idea into a practical machine. But it was in partnership with the venture capitalist Matthew Boulton in the 1770s that Watt developed the engines that drove the Industrial Revolution.

© Science Photo Library
Steam engines under construction at Boulton and Watt’s Soho Foundry, at Soho, near Birmingham, UK.

Watt went on to apply science in many other areas of practical importance, including developing a process for bleaching cloth, and a successful early method for copying handwritten letters, a forerunner of the photocopier. In all of this work he provided the archetypal example of an experimenter/inventor, blurring the line between ‘pure’ science and ‘practical’ science. As Humphry Davy wrote of him: ‘Those who consider James Watt only as a great practical mechanic form a very erroneous idea of his character; he was equally distinguished as a natural philosopher and a chemist, and his inventions demonstrate his profound knowledge of those sciences, and that peculiar characteristic of genius, the union of them for practical application.’⁸