Benjamin Franklin

Radiation of energy from the sun.

Franklin's earliest experiment | Franklin's experiment rewritten | Questions.

Image of Ben Franklin.

An extraordinary man who started his working career as a printer. He retired at about thirty five to devote his life to improving the lives of the people around him. He was world famous as a scientist, inventor, and diplomat. The "Franklin stove" that he invented worked by improving the flow of radiation from the stove throughout the room.

Benjamin Franklin became interested in what people wore and if they were comfortable in their clothes. In his autobiography, Franklin describes an experiment using different colored pieces of cloth that he placed on top of snow in the sunlight. Franklin observed which colors penetrated into the snow more quickly than other colors.

Benjamin Franklin did not have the technology that we have today. Here are a few things he did not have:

Franklin's Earliest Experiment

". . . AS to our other subject, the different degrees of heat imbibed from the sun's rays by cloths of different colors, since I cannot find the notes of my experiment to send you I must give it as well as I can from memory.

But first let me mention an experiment you may easily make yourself. Walk but a quarter of an hour in your garden when the sun shines, with a part of your dress white and a part black; then apply your hand to them alternately, and you will find a very great difference in their warmth. The black will be quite hot to the touch, the white still cool.

Another. Try to fire paper with a burning-glass. If it is white, you will not easily burn it; but if you bring the focus to a black spot, or upon letters, written or printed, the paper will immediately be on fire under the letters.

Thus fullers and dyers find black cloths, of equal thickness with white ones and hung out equally wet, dry in the sun much sooner than the white, being more readily heated by the sun's rays. It is the same before a fire; the heat of which sooner penetrates black stockings than white ones, and so is apt sooner to burn a man's shins. Also beer much sooner warms in a black mug set before the fire than in a white one or in a bright silver tankard.

My experiment was this. I took a number of little square pieces of broad cloth from a tailor's pattern-card, of various colors. There were black, deep blue, lighter blue, green, purple, red, yellow, white, and other colors, or shades of colors. I laid them all out upon the snow in a bright sunshiny morning. In a few hours (I cannot now be exact as to the time) the black, being warmed most by the sun, was sunk so low as to be below the stroke of the sun's rays; the dark blue almost as low, the lighter blue not quite so much as the dark, the other colors less as they were lighter; and the quite white remained on the surface of the snow, not having entered it at all.

What signifies philosophy that does not apply to some use? May we not learn from hence that black clothes are not so fit to wear in a hot sunny climate or season as white ones; because in such clothes the body is more heated by the sun when we walk abroad and are at the same time heated by the exercise, which double beat is apt to bring on putrid dangerous fevers? That soldiers and seamen, who must march and labour in the sun, should in the East or West Indies have an uniform of white? That summer hats, for men or women, should be white, as repelling that heat which gives headaches to many, and to some the fatal Stroke that the French call the coup de soleil? That the ladies' summer hats, however, should be lined with black, as not reverberating on their faces those rays which are reflected upwards from the earth or water? That the putting a white cap of paper or linen within the crown of a black hat, as some do, will not keep out the heat, though it would if placed without? That fruit-walls, being blacked, may receive so much heat from the sun in the daytime as to continue warm in some degree through the night, and thereby preserve the fruit from frosts or forward its growth? -- with sundry other particulars of less or greater importance that will occur from time to time to attentive minds? . . ."

[Franklin's earliest scientific experiment seems to belong to the year 1729 or thereabouts, soon after the establishment of the Junto. His own notes are lost, but his Junto friend, Joseph Breintnal, investigated the subject then and again in the winter of 1736-37, and in August 1737 wrote out a memorandum and notes:

"That the heat of the sun penetrates such things as are colored more than such as are white will appear by the following experiments, which I was induced to make about seven years ago, and lately to repeat, from some hints given me by Benjamin Franklin, and from observing that people who come among as from the warm islands do most of them wear white clothes, in which I suppose they find themselves cooler than in others though few of them may know the reason of it; from observing also that a small glass will not burn white paper, though it easily does it if the paper be stained; and from taking notice of a young woman's complaining that her black gloves had burnt her hands, etc."

There is a full account of the Breintnal experiment by I. Bernard Cohen in Franklin's Experiments on Heat Absorption as a Function of Color, in Isis, XXXIV, 404-407, 1943. The only account by Franklin is to be found in a letter to Mary Stevenson, September 20, 1761.]

Van Doren, Carl, 1945: Benjamin Franklin's Autobiography, Selected and Edited by Carl Van Doren, Viking Press, New York, NY, 810 pp., pp. 30-31.

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A Rewritten Account of Franklin's Experiment

I found a book of cloth samples. Tailors use these books when they want to show people what kind of cloth they could have clothes made from. There were many little cloth patches in this book. Some of the patches were black. Other patches were deep blue or lighter blue. There were green patches and purple ones. There were red and yellow and white patches.

One morning after a snowstorm, I took the cloth patches outside and laid each of them on the snow. It was a bright, sunny morning. I'm not sure how many hours later I came back to see what had happened. I think it was a few hours, though.

When I looked at the patches, the black one had melted a lot of snow. The black cloth had sunk so far into the snow that the edges of the hole made a shadow on the cloth. The dark blue patch had sunk almost as far as the black one. The lighter blue patch was in a hole that wasn't a deep as the dark blue patch. Each piece of cloth (except the white one) had melted a hole. The lighter the cloth, the less the cloth patch had sunk into the snow.

Some Questions about Mr. Franklin's Experiment and Similar Experiments:

  1. Tailors are hard to find now. Where could you get some cloth samples that have different colors?

  2. Do you think it would make any difference if you used construction paper instead of cloth?

  3. If you didn't have snow, what could you do that might be similar to Mr. Franklin's experiments? [A suggestion: get a thermometer and wrap it in construction paper; set the paper in direct sunlight for a little time; measure the temperature for each different piece of paper; write a letter about your experiment and its results to someone else in class or as an e-mail to someone on the www.]

  4. Mr. Franklin's original letter contains other observations. What observations like his are related to the effect of sunlight.

  5. Mr. Franklin seems to think that people should wear light colored clothing in tropical countries, where the sunlight is very strong. Why do Bedouins in the Arabian desert wear black clothing? Is Mr. Franklin wrong, or is there something else going on?

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