io THE POPULAR SCIENCE MONTHLY. 



The nauseous smelling gas we call sulphide of hydrogen has a 

 density only a little greater than that of air, and its molecules must 

 therefore move with very nearly as great velocity as the average air- 

 molecule that is to say, about 1,480 feet a second ; and we might 

 therefore expect that, on opening a jar of the gas, its molecules would 

 spread instantly through the surrounding atmosphere. But so far from 

 this, if the air is quiet, so that the gas is not transported by currents, a 

 very considerable time will elapse before the characteristic odor is per- 

 ceived on the opposite side of an ordinary room. The reason is obvious 

 the molecules must elbow their way through the crowd of air-mole- 

 cules which already occupy the space, and can therefore advance only 

 slowly ; and it is obvious that, the oftener they come into collision with 

 their neighbors, the slower their progress must be. Knowing, then, 

 the mean velocity of the molecular motion, and being able to measure 

 by appropriate means the rate of diffusion, as it is called, we have the 

 data from which we can calculate both the number of collisions in a 

 second and also the mean path between two successive collisions. The 

 results, as we must expect, are of the same order as the other mo- 

 lecular magnitudes. But inconceivably short as the free ' path of a 

 molecule certainly is, it is still, in the case of hydrogen gas, 136 times 

 the diameter of the moving body, which would certainly be regarded 

 among men as quite ample elbow-room. 



Although, in this lecture, I have as yet had no occasion to mention 

 the radiometer, I have by no means forgotten my main subject, and 

 everything which has been said has had a direct bearing on the theory 

 of this remarkable instrument ; and still, before you can understand 

 the great interest with which it is regarded, we must follow out another 

 line of thought, converging on the same point. 



One of the most remarkable results of modern science is the discov- 

 ery that all energy at work on the surface of this planet comes from 

 the sun. Most of you probably saw, at our Centennial Exhibition, that 

 great artificial cascade in Machinery Hall, and were impressed with the 

 power of the steam-pump which could keep flowing such a mass of 

 water. But, also, when you stood before the falls at Niagara, did you 

 realize the fact that the enormous floods of water, which you saw 

 surging over those cliffs, were in like manner supplied by an all-power- 

 ful pump, and that pump the sun ? And not only is this true, but it is 

 equally true that every drop of water that falls, every wave that beats, 

 every wind that blows, every creature that moves on the surface of the 

 earth, one and all, are animated by that mysterious effluence we call 

 the sunbeam. I say mysterious effluence ; for how that power is trans- 



1 There is an obvious distinction between the free and the disturbed path of a mole- 

 cule, and we cannot overlook in our calculations the perturbations which the collisions 

 necessarily entail. Such considerations greatly complicate the problem, which is far 

 more difficult than would appear from the superficial view of the subject that can alone 

 be given in a popular lecture. 



