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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 withdrawn, is 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 



