114 



ELEMENTARY GENERAL SCIENCE 



Measure the length of the air column which is now subjected 

 to a pressure equal to two atmospheres, and notice that it is one- 

 half of the original volume. That is, by doubling the pressure 

 we have halved the volume. 



Repeat the experiment with the bottle at any height, and 

 when the apparatus has cooled down measure the difference in 

 the levels of the mercury in the two tubes and also the length of 

 the air column. 



Boyle's Law. If we multiply the total pressure (obtained by 

 adding the height of the barometer to the difference in level of 

 the mercury in the burette and the open tube) by the volume 

 of the air we shall notice that the result is always the same. 

 This relation was discovered by Boyle, and is known as Boyle's 

 Law. It can be expressed by saying that when the tempera- 

 ture remains the same, the volume of a gas varies inversely 

 as its pressure. Or, what is the same thing, the temperature 

 remaining the same, the product of the pressure into the 

 volume is constant. 



EXPT. 111. Perform several experiments with the Boyle's 

 Law apparatus, observing in each case (a) the volume occupied 

 by the air in the burette, (6) the difference in level between 

 the mercury in the closed burette and the open tube, or 

 "head " of mercury. Tabulate your results thus : 



When the air occupies less volume than it did before the stop- 

 cock was closed it is evidently under a pressure greater than 

 that of the atmosphere ; when it occupies a greater volume it is 

 under a pressure less than that of the atmosphere. In either 

 case, however, the pressure to which the air is subjected multi- 

 plied by the volume it occupies gives a constant product. But in 

 performing the above experiment at pressures less than that 

 of the atmosphere, the difference between the heights of the 



