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 



