184 THE ARTS AND INDUSTRIES 



and the excess allowed to drip off, or it may be applied 

 by means of a swab of cotton upon a little stick. The 

 paper must be allowed to dry in the dark before being 

 used. It is best not to prepare any more paper than will 

 be used within a few days, as it spoils quite quickly. 



It is not necessary to cover the whole of a piece of 

 paper if the print is to be a small one upon a large sheet 

 of paper. Merely apply the mixture where it is needed. 



71. Solar Heaters. 



In Experiment 22 it was noticed that black and rough 

 materials become much warmer in the sun than do other 

 objects. Man has made use of this knowledge by making 

 the sun warm water for him. If many feet of water pipe 

 are painted black and exposed to the heat of the sun 

 the water within them will become quite hot. This is 

 a simple solar heater. The longer the pipe the more 

 water can be heated at one time. Man has learned 

 something else about the heat from the sun. If a box is 

 made of wood, painted black inside, and covered with 

 glass, the heat which is received from the sun can enter 

 the box but cannot get out again as rapidly. Thus the 

 temperature inside the box rises much higher than the 

 temperature outside, if the sun is shining. Now if such 

 a box is filled with many feet of water pipe, the water 

 which is inside the pipe will become very hot. Such a 

 heater on a sunny day can raise the temperature of water 

 high enough for a hot bath. The water from a solar 

 heater does not have to be heated very much in order to 

 make it boil, so for cooking purposes it causes a saving 

 in fuel. 



