146 



LECTURES ON 



Fig. 13. 



Closing here our study of the atmosphere considered as a source of 

 the food of plants, we still need to remark somewhat upon the physi- 

 cal properties of gases in relation to vegetable life; so far, at least, as 

 may give some idea of the means by which they gain access into the 

 plant. 



Whenever two or more gases are brought into contact in a confined 

 space, they instantly begin to intermingle, and continue so to do until, 

 in a short time, they are each equally diffused throughout the room 

 they occupy or pass into a condition of osmotic equilibrium. If two 

 vessels, one filled with carbonic acid, the other with hydrogen, be con- 

 nected by a tube no wider than a straw, and be placed so that the heavy 

 carbonic acid is below the fifteen times lighter hy- 

 drogen, we shall find after the lapse of a few hours 

 that the two bodies are in a state of uniform mix- 

 ture. On closer study of this phenomenon it has 

 been discovered that gases diffuse with a rapidity 

 proportioned to their lightness. Hence, by inter- 

 posing a porous membrane between two gases of 

 unequal density, the lighter passing more rapidly 

 into the denser than the latter into the former, the 

 space on one side of the membrane is overfilled 

 while the other side is partially emptied of gas. 

 This fact is taken advantage of for the visible illus- 

 tration of the fact of gaseous diffusion. 



In the accompanying figure 13 is represented a long 

 glass tube b widened above into a funnel, and having 

 cemented upon this an inverted cylindrical cup of 

 unglazed porcelain a. The funnel rests in a round 

 aperture made in the horizontal arm of the support 

 while the tube below dips beneath the surface of 

 some water contained in the wine glass. The porous 

 cup, funnel, and tube being occupied with common 

 air, a glass bell c is filled with hydrogen gas and 

 placed over the cup as shown in the figure. In- 

 stantly bubbles begin to escape rapidly from the 

 bottom of the tube through the water of the wine 

 glass, thus demonstrating that hydrogen passes into 

 the cup faster than air can escape outward through its pores. If the 

 bell be removed, the cup is at once bathed again externally in common 

 air, the light hydrogen floating instantly upward, and now the water 

 begins to rise in the tube in consequence of the return to the outer 

 atmosphere of the hydrogen which before had diffused into the cup. 

 It is the perpetual action of this diffusive, or, as it is scientifically 

 termed, osmotic tendency, which maintains the atmosphere in a state 

 of such uniform mixture that accurate analyses of it give for oxygen 

 and nitrogen almost identical figures, at all times of the day, at all 

 seasons, all altitudes, and all situations, except near the central sur- 

 face of large bodies of still water. Here the fact that oxygen is more 

 largely absorbed by water than nitrogen diminishes by a minute amount 

 the usual proportion of the former gas. 



