50 THE LIFE OF THE PLANT 



which can be penetrated by them ; each of them will 

 strive to permeate the other. Obviously if the two 

 gases tend to spread, and their particles move at 

 different velocities, a temporary change of volume will 

 result in the apparatus ; the volume will increase or 

 decrease according as the gas without enters more 

 quickly or more slowly than the gas within escapes. 

 A similar phenomenon will take place in a few minutes 

 on the threshold of this hall. Let us suppose that 

 there are three hundred persons at present in the 

 hall ; and, further, that one hundred of them are 

 bored with this lecture (all too prolonged), and are 

 impatient to hear the end of it and to leave the hall ; 

 while another hundred persons are standing outside 

 waiting to enter for the next lecture. If the former 

 leave the hall in the same hurry as the latter enter it 

 the number of people in the hall will remain the same. 

 But if those outside, not feeling so weary after an hour's 

 mental effort, should prove more energetic, the number 

 of people in the hall in the first instance will increase, 

 the hall will fill up, and only later, when those wishing 

 to leave actually do so, will the number remaining 

 decrease to the original three hundred. The same thing 

 happens here. If I surround this porous vessel with gas, 

 the particles of which enter more quickly than the 

 particles of air contained in it pass out, the vessel will 

 for a short time contain more particles of gas than it can 

 actually hold, and the superfluity of gas will escape in 

 bubbles from the end of the tube. I take a glass bell 

 full of hydrogen. Since this gas is lighter than air, it 

 can be kept for a certain time in a vessel with its opening 

 turned down. I lower the bell (c) over the porous 

 vessel (a). The inside of the vessel contains ordinary 

 air ; outside, under the bell, is hydrogen. If particles 

 of hydrogen are endowed with more rapid motion than 

 particles of air, the inner volume of gas must increase, 

 and you hear and see the bubbles of gas bubbling through 



