TENSION OF GASES. 167 



did under a pressure of one atmosphere. And so on. It 

 may be more convenient to have the barometer tube open 

 at both ends, the upper end being closed with the thumb 

 or finger before lifting. 



287. A Summing: Up. From the foregoing experi- 

 ments we have a right to conclude that the density and 

 tension of a given quantity of gas are directly, and 

 that its volume is inversely, as the pressure ex- 

 erted upon it. Representing the volumes of the same 

 quantity of gas by V and v, and the corresponding pres- 

 sures and densities by P and p, D and d, our conclusion 

 may be algebraically expressed as follows : 



H- P- ^L 

 v ~~ P " Z>' 



EXERCISES. 



1. Under ordinary conditions, a certain quantity of air measures 

 one liter. Under what conditions can it be made to occupy (.) 500 

 cu. cm. ? (&.) 2000 cu. cm. ? 



2. Under what circumstances would 10 cu. inches of air at the 

 ordinary temperature weigh 31 grains ? 



3. Into what space must we compress (a.) a liter of air to double 

 its tension ? (6.) A liter of hydrogen ? 



4. A barometer standing at 30 inches is placed in a closed vessel. 

 How much of the air in the vessel must be removed that the mer- 

 cury may fall to 15 inches ? 



5. A vertical tube, closed at the lower end, has at its upper end 

 a frictionless piston which has an area of one sq. inch. The weight 

 of this piston is five pounds, (a.) What is the tension of the air 

 in the tube? (&.) If the piston be loaded with a weight of ten 

 pounds, what will be the tension ? 



6. When the barometer stands at 28^ inches, the mercury is at 

 the same level in both arms of a Mariotte's tube. The barometer 

 rises and the difference in the two mercurial surfaces of the Ma- 

 riotte's tube is half an inch, (a.) In which arm is it the higher? 

 (6.) Why? 



