INTERESTING THINGS ABOUT THE AIR 



7. Weight of air. Not only is air like other substances 

 in occupying space but, like them, it has weight. The best 

 way to show that air has weight is to weigh a quantity of it, 

 as may be done in the following manner: The air (almost 

 all of it) may be pumped from a brass globe or glass flask 

 which is so constructed that it may be closed after the air has 

 been removed. The closed vessel may then be placed upon 

 a balance (fig. 6) and weighed. If the vessel is opened (thus 

 allowing air to en- 

 ter it) and again 

 weighed, any dif- 

 ference in weight 

 that is found will 

 t>e due to the air 

 which has come 

 into the vessel. If 

 we suppose all the 

 air was pumped 

 out in the first 

 place, the increase 

 in weight would 

 be- precisely the 

 weight of the air 

 the vessel contains. 



Therefore we may measure the vessel used and calculate the 

 cubic contents, and thus find out the weight of the air per 

 cubic foot or meter, as shown by the experiment. This result 

 will not be quite correct, since not all the air can be removed 

 from the vessel by the pump, and therefore we include part 

 of the air in both weights. Is our result greater or less than 

 the true weight of air? 



By attaching a barometer (sect. 9) to the air pump it is 

 possible to find out what fraction of the air was left in the 

 vessel, and this enables us to tell exactly what relation the 

 amount of air removed by the pump bears to the whole 



FIG. 6. Weight of air 



The globe, which was balanced after part of the air had 



been pumped from it, was found to be heavier when the 



air was again allowed to enter it 



