36 



Professor J, A. Fleming 



[Feb. 14, 



point blow out a spray of inky water. Consider the ink spray to 

 represent the carbon atoms shot off from the overheated spot. We 

 see that the cardboard is bespattered on all points except along one 

 line where it is sheltered by the opposite side of the loop. We have 

 thus produced a " spray shadow " on the board (Fig. 2). The 



Fig. 1. 



Fig. 2. 



r^ 



Glow-lamp, having the glass bulb 

 blackened by deposit of carbon, show- 

 ing the molecular scattering which 

 has taken place from the point a on 

 the filament, and the shadow or line 

 of no deposit produced at h. 



" Spray shadow " of a rod thrown 

 on cardboard screen to illustrate 

 formation of molecular shadow 

 in glow lamps. 



existence of these molecular shadows in incandescent lamps leads us 

 therefore to recognise that the carbon atoms must be shot off in 

 straight lines, or else obviously no such sharp shadow could thus be 

 formed. This phenomenon confirms in a very beautiful manner the 

 deductions of the Kinetic theory of gases. I may remind you that at 

 the ordinary temperature and pressure the mean free path of a mole- 

 cule of air is deduced to be about four one-milliouths of an inch. 

 This is the average distance which such a gaseous molecule moves 

 over before meeting with a collision against a neighbour which 

 changes the direction of its path. Let the air be rarefied, as in these 

 bulbs, to something like a millionth of the ordinary atmospheric 

 pressure, and the mean free path is increased to several inches. The 

 space within the bulb — though from one point of view densely popu- 

 lated with molecules of residual air — is yet, as a fact, in such a con- 

 dition of rarefaction that a carbon molecule projected from the 

 conductor can move over a distance of three or four inches on an 



