F (109 degrees C) or 16 degrees F (9 degrees C) above the boiling point of water. Horace de Saussure was unsure of how the sun heated the glass boxes. Today we can better explain what happened. Sunshine (photons) penetrated the glass covers. The black inner lining absorbed the sunlight and converted it into heat. Though clear glass allows the rays of the sun to easily enter through it, it prevents heat from doing the same. As the glass trapped the solar heat in the box, it heated up. Its inventor realized that someday the hot box might have important practical applications, as "it is quite small, inexpensive and easy to make." Indeed, the hot box has become the prototype for the solar collectors that have provided sun-heated water to millions since 1892.
In the 1800 people started experimenting with the suns solar power by a metal water tank, painted black, into the sun to absorb as much solar energy as possible. These were the first solar water heaters on record. The downside was that even on clear, hot days it usually took from morning to early afternoon for the water to get hot. And as soon as the sun went down, the tanks rapidly lost their heat because they had no protection from the cool night air.
