AIR AND WATER AS SERVANTS OF MAN in 



gas, because the air and illuminating gas, which the term gas 

 usually suggests, are both invisible and not readily handled in a 

 way that leaves much impression on the mind of the child. Some 

 experiments with such a visible gas as chlorine, for instance, is 

 therefore worth while to render his conceptions more definite and 

 exact. Directions for making chlorine gas are given in the Field 

 and Laboratory Guide in Physical Nature-Study, page 54. It may 

 be readily seen; it is heavier than air, therefore it may be poured 

 from one bottle to another as water might be poured. Iodine 

 gas may also be readily formed by heating crystals of iodine. 

 This also is a colored gas and heavier than air. If the child can 

 see some experiments of this sort he readily gains the notion that 

 gases are somewhat similar in their properties and behavior to 

 water, and he will more readily believe that the laws of fluids 

 apply both to liquids and gases. 



Some experiments can be readily performed to demonstrate 

 air pressure. One of the classic experiments, historically, was 

 an experiment performed at Magdeburg. Two large metal 

 hemispheres were placed together so as to form a sphere, their 

 edges being ground smooth so as to fit together quite perfectly. 

 The air was then pumped out from the sphere, and when two 

 horses pulling in opposite directions, one on each hemisphere, 

 were unable to separate them, it was a striking demonstration of 

 the pressure of the air on the outside of the two hemispheres. 

 This apparatus is still known as the Magdeburg sphere and prob- 

 ably may be borrowed from the physics department together 

 with an air pump to make the experiment for the children of the 

 grades. 



Light tin cans can now be obtained with a tin cover that 

 presses into the opening of the can. Such cans are used in home- 

 canning processes. They are also commonly used as containers 

 of paint and molasses. Barely cover the bottom of such a can 

 with water and then set the can on a stove or over a Bunsen 

 burner and bring the water to a boil. The cover of the can may 

 be laid on the opening but not forced on tightly. The can now 



