A few weeks ago I happened to be near Joe’s espresso café bar on the corner of Medway St. and Horseferry Road, with around twenty minutes to spare. Joe’s is an old-style independent café, very focused on their lunch menu and take away coffees. Nonetheless, there is a decent sized seating area in a room adjacent to the ‘bar’ where you can sit with your coffee and watch the world go by on Horseferry Road. It is always nice to come across a friendly café that allows you to sit quietly and people-watch. As I sat and watched the taxis pass by, I became aware of the fact that it had got quite cold. The people who had just left the cafe had left the door to the room slightly open; the cold was ‘getting in‘. Now I know, heat goes out, cold does not come in but sitting there in that café that is not how it felt. Then it struck me, rather than cause me to grumble, the slightly open door should remind me of the experiments of Carl Wilhelm Scheele (1742-1786).
Scheele was a brilliant chemist but one who performed experiments that would make our university health and safety departments jump up and down spitting blood. Recognised for discovering oxygen in the air (Priestley discovered it a few years later but published first), manganese and chlorine, Scheele also investigated arsenic and cyanide based compounds. It is thought that some of these experiments (he described the taste of cyanide) contributed to his early death in May 1786 at the age of 43. Fortunately, none of this has a connection to Joe’s espresso café. What links Scheele with Joe’s, is Scheele’s discovery of ‘radiant heat’ as he was sitting in front of his stove one day.
Scheele’s house was presumably very cold in winter. He describes how he could sit in front of his stove with the door slightly ajar and feel its heat directly and yet, as he exhaled, the water vapour in his breath condensed into a cloud in the air. The heat from the stove was evidently heating Scheele, but not the air between Scheele and the stove. He additionally noted that this heat travelled in straight lines, horizontally towards him, as if it were light and without producing the refraction of visible light associated with air movement above a hot stove. Nor was a candle flame, placed between Scheele and the stove, affected by the passage of the heat. Clearly this ‘horizontal’ heat was different from the convective heat above the stove. Scheele called this ‘horizontal form’ of heat, ‘radiant heat’.
A few years later, the astronomer and discoverer of Uranus, William Herschel (1738-1822) was investigating glass-filter materials so that he could better observe the Sun. Using a prism to separate white light into its familiar rainbow spectrum, Herschel measured the temperature of the various parts of the spectrum. Surprisingly, the temperature recorded by the thermometer increased as the thermometer was moved from the violet end to the red end of the spectrum and then kept on rising into the invisible region next to the red. We now recognise Herschel’s observation of infra-red light as responsible for the radiant heat seen by Scheele, though a few more experiments were required at the time before this was confirmed.
Further work by William Hyde Wollaston (1766-1828) and, independently Ritter (1776-1810) & Beckmann not only confirmed Herschel’s infra-red/radiant heat observations but also showed that, at the other end of the spectrum was another invisible ‘light’ that produced chemical reactions. Indeed, milk is often sold in semi-opaque plastic containers because of the fact that the taste and nutritional content of the milk are affected by such sunlight induced chemical reactions.
So, it seems to me that, in addition to an interesting story with which to idle away 20 minutes in a café, this set of thoughts offers a variety of experiments that we could try at home. If we are out, we could try to discern the different ways that heat is transferred from one body to another (as Scheele). If we had a prism, we could perhaps repeat Herschel’s experiment very easily with a cheap (but sensitive) thermocouple and, if we were really ambitious hook it up to a Raspberry Pi so that we could map the temperature as a function of wavelength. Finally, we could investigate how light affects chemical reactions by seeing how milk degrades when stored in the dark, direct sunlight or under different wavelengths. If you do any of these experiments please let me know what you discover in the comments section below. In the meanwhile, take time to enjoy your coffee, perhaps noticing how the hot mug is warming your hands.
Books that you may like to read and that were helpful for this piece:
“From Watt to Clausius”, DSL Cardwell, Heinemann Education Books Ltd, 1971
“On Food and Cooking: The science and lore of the kitchen” H McGee, Unwin Hyman Ltd 1986
Apologies to university H&S departments, you guys do a great job (mostly!) in trying to help to prevent us dying from our own experiments too prematurely.