Page:Popular Science Monthly Volume 88.djvu/572

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��Popular Science Monthly

���How Blotting Paper Absorbs Ink

EVERY student of physics knows that water will run up a narrow tube by capillary attrac- tion. Anything immersed in water has a sim- ilar attraction for the water; that is, the ob- ject becomes wet by the wa- ter that clings to it. The amount is lim- ited by the weight of the liquid itself. Place your hand in water, and your hand, when withdrawn, is wet. The lim- ited attraction between the hand and the water is gaged by the weight of the water that clings to the hand. Imagine several hands placed close together in water, but not touching one another. If this composite hand were formed of ten single hands, it would attract ten times as much water as the one hand would attract and hold on its surface. So, a wisp of hay, composed of a hundred spears of dried grass, placed in water, will remove a hundred times as much of the fluid as would cling to one spear. Bushes in a marsh will remove a certain amount of water which will, by capillary attraction, cling to their submerged parts.

Under the microscope, fibrous blotting paper, when absorbing ink, resembles, on a small scale, a marsh matted with shrubs and sticks and twigs, around which water is flowing as ink runs about and among the fibers that together form the spongy paper. There is a limit to the amount of liquid which a "blotter" will absorb, as there is a limit to the amount of water that a marsh will absorb with- out overflowing. That limit, in the "blotter," is the combined capillary at- traction of the fibrous shrubs and sticks and twigs that together form the paper.

��Balsa, Lightest of Woods

EXPERIMENTS made by the Mis- souri Botanical Garden of St. Louis • show that the wood called bal- sa, native to the West Indies and Central Amer- ica, is nearly twice as light as cork.

In the photo- graph a piece of balsa-wood (B) is balancing a piece of Aus- tralian ironbark (A). The two blocks have the same width and thickness, but B is ten times the length of A.

��Blotting paper absorbs ink on the same principle as a handful of hay will absorb a liquid

��Balsa is very soft. It is easily cut with tools, and is imported into the United States from Costa Rica to make the floating parts of life-preservers and life- rafts. The government uses it for buoys and water signals. It has several ad vantages over cork.

Balsa, on the right, is

a wood ten times as

light as ironbark, on

the left

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