42 PRACTICAL LESSONS IN SCIENCE. 



upper barrel, near its lower end, allows the downward stroke of 

 the piston to force the water into an air chamber, compressing 

 the air, and the elasticity of the air forces the water forward in a 

 steady stream. 



The volume of a given weight of air or other gas varies in- 

 versely as the pressure upon it ; that is, the more pressure the 

 less the volume and vice versa. We have already learned that 

 air can be compressed, but careful experiments show that if we 

 double the pressure on a given body of air or other gas its vol- 

 ume will be reduced one-half, and if we lessen the pressure by 

 one-half the volume will be doubled. This is called Boyle's or 

 Mariotte's law, from the names of its discoverers. If the volume 

 of gas is decreased by pressure the density is increased, so that 

 the density of air or other gases varies directly as the pressure. 

 The density of the air is greatest at the sea level, while on high 

 mountains the density is much less. 



We have already learned that gases expand and contract as 

 they are heated or cooled, but careful experiments show that this 

 expansion and contraction is somewhat uniform. If we have 491 

 cubic inches of air at the temperature of 32 F. it will have a 

 volume of 492 cubic inches when heated to a temperature of 33 

 F.; that is, it expands - 4 -i- T of the volume at 32 F. for each addi- 

 tional degree of heat. If the law holds good for lower tempera- 

 tures we should find absolute zero at about 495 F. below the 

 present Fahrenheit zero. 



