The increased use of glass during the seventeenth and eighteenth centuries reawakened the awareness of its ability to trap solar heat. That no one ever thought of finding out just how much solar heat glass could trap surprised the Swiss polymath Horace B. de Saussure. He therefore built in 1767 a rectangular box from wood, insulated with black cork and covered the top with three sheets of glass. When he tilted the box toward the sun, the temperature inside the solar hot box rose above the boiling point of water. Because of the large amount of solar heat the device retained, it became known as a solar hot box. Saussure cooked inside the hot box the first recorded solar meal. The solar hot box became popular among experimental scientists in the eighteenth and nineteenth century.
Samuel Pierpoint Langley, head of the Smithsonian in the latter part of the nineteenth century, built a solar hot box and took it with him on an expedition to Mount Whitney. He melted snow inside the box for drinking water despite freezing temperatures outside. Renowned English astronomer Sir John Herschel amused passersby in South Africa by cooking meals in his solar hot box. “On one occasion,” he wrote, “a very respectable stew was prepared and eaten with no small relish by the entertained bystanders.” Reading Herschel’s account led fellow nineteenth-century astronomer Jacques Babinet wonder why, “In countries in which the atmosphere is always clear, as in Egypt, Arabia and Persia, where fuel is scarce and dear, people have never thought of utilizing the concentrated rays of the sun under glass for tasks where heat is needed, such as cooking.”
The solar hot box design became the prototype for solar thermal collectors used to heat water and homes.
Saussure’s solar hot box also models with amazing precision the dynamics of global-warming. . In the early 19th century, the French physicist Joseph Fourier was the first to notice the similarity between de Saussure’s hot box and what might happen should the atmosphere somehow lose its transparency to heat generated by solar radiation after reaching the earth, as most climate scientists suspect it does as humans emit greater amounts of greenhouse gases.
Fourier suggested that, like the glass covers, our atmosphere allows the short wave radiation of sunlight to easily pass through. But when the sunrays hit the earth, just as they do at the bottom of the hot box, they turn into longer heat waves, which cannot easily escape through the glass or a carbon-saturated sky, causing the heat to accumulate inside the box and on a planetary scale in the lower levels of the atmosphere. The clouds of carbon dioxide surrounding Venus provide us with a living example of the ultimate solar hot box where temperatures at the planet’s surface hover at around 890 degrees Fahrenheit.
This post summarizes the author’s Chapter 6 of Let It Shine: The 6,000-Year Story of Solar Energy.
Photo from Smithsonian Institution Archives