Horace de Saussure observed :"It is a known fact, and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass."
He placed 3 boxes of glass which could fit into each other, their bottoms cut out on a dark colored table. After placing this apparauts in sun for several hours he measured the temperature inside the boxes. The inside of the first box was the coolest of the 3. The temperature inside the inner most bax reached 189.5 degrees F . Quite fascinating huh?
This phenomenon can be explained by the fact that light energy was transformed into heat energy.
"Much of this heat was released into the glass boxes as warm air and thermal radiation. But clear glass has a peculiar property: it easily allows sunshine to pass through, but inhibits thermal radiation from doing the same. Therefore this trapped energy heated the air inside the box. The glass walls also blocked the heated air from escaping, but some heat was lost by conduction through the glass."
He carried out more experiments with simple apparatus like a blackened box and som insulation to minimise the heat loss . And he confirmed: "the sun shines with almost equal force at higher and lower elevations. At lower elevations there are greater amounts of carbon dioxide and water vapor in the air. This denser atmosphere holds in the solar heat more effectively, retarding its escape into space; so it gets hotter at these elevations. But the glass covers of a hot box present an equally effective barrier to solar heat trying to escape from the box whether it is located in the mountains or at sea level; so it registers the same temperature in either place."
Sir John Herschel did a similar experiment in 1830 in South Africa. He used a mahogany container blackened on the inside and covered with glass, set into a wooden frame protected by another sheet of glass and by sand that was heaped up along its sides. The outcome of Herschel’s experiments with this hot box was not only scientifically interesting but also pleasing to the palate, as his notes indicate:
"As these temperatures [up to 240ºF] far surpass that of boiling water, some amusing experiments were made by exposing eggs, meat, etc. [to the heat inside the box], all of which, after a moderate length of exposure, were found perfectly cooked. . [On] one occasion a very respectable stew of meat was prepared and eaten with no small relish by the entertained bystanders."
"The following day, using a much simpler hot box, Herschel made an egg,
Which burned Peter’s [his son’s] fingers as if fresh from the pot. It was done as hard as a salad egg and I ate it and gave some to my wife and six small children that they might have it to say they had eaten an egg boiled hard in the sun in South Africa."
Samuel Perpont Langley inspired by Sir Herschel's experiment carried out his own experiment.
In 1881, Langley took a trip to Mt. Whitney to study the effects of solar energy. And he noted : "As we slowly ascended . . . and the surface temperature of the soil fell to the freezing point, the temperature in a copper vessel, over which lay two sheets of plain window glass, rose above the boiling point of water, and it was certain that we could boil water by the solar rays in such a vessel among the snow fields."
De Saussure, Herschel, and Langley all demonstrated that temperatures exceeding the boiling point of water could be produced in a glass-covered box. Its inventor realized that the hot box might have important practical applications. As de Saussure stated almost self-effacingly, "Someday some usefulness might be drawn from this device . . . [for it] is actually quite small, inexpensive, [and] easy to make."
Source : http://solarcooking.org/saussure.htm
Indeed, such a simple device can be a poor man's cooker. If used by everyone it will save atleast 20% of gas/electricity.
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