HYDROGEN INTO ATOMS 151 



The foregoing results afford satisfactory proof that hydrogen, par- 

 ticularly at low pressures, is readily dissociated into atoms by metallic wires 

 at very high temperatures. 



There is, however, good reason to suspect that the actual values tor the 

 degree of dissociation previously given are considerably too high. The cause 

 of this was thought to be an incorrect assumption as to the diffusion co- 

 efficient of hydrogen atoms through molecular hydrogen. 



The remainder of the present paper deals with the results obtained by 

 two methods which lead to more or less quantitative data on the dissocia- 

 tion, without necessitating any assumptions as to the magnitude of the 

 diffusion coefficient. 



The first method is based on measurements of the total heat losses from 

 tungsten wires at a series of different pressures, ranging from lo mm. up 

 to atmospheric pressure. If the previous results were correct, that is, that 

 hydrogen is 44% dissociated at atmospheric pressure and 3100^ K., then, 

 at a pressure of 10 mm., the dissociation should be 96.5%. A further rise 

 in temperature could then only slightly increase the degree of dissociation, 

 for it is already close to the limit of 100%. We see, then, from equation 

 (3), that the heat loss under these conditions would have only a very small 

 temperature coefficient. It was hoped, at the outset of these experiments, 

 that the actual degree of dissociation could be determined from the de- 

 crease in the temperature coefficient of VJd as the pressure was progres- 

 sively lowered. 



The experiments to be described have shown, however, that even at 

 10 mm. pressure the temperature coefficient of Wd is practically as great 

 as at atmospheric pressure, showing that even at these low pressures the 

 hydrogen around the wire is not nearly completely dissociated. These ex- 

 periments, however, give an upper limit to the degree of dissociation. 



In the second method, measurements were made at very much lower 

 pressures, from o.oi mm. up to 0.2. At the lowest pressures, the conditions 

 should become very simple, for the molecules travel in straight lines directly 

 from filament to bulb. Under such conditions, the hydrogen atoms pro- 

 duced practically never return to the filament without having struck the 

 bulb many times and having had ample opportunity of recombining. The 

 filament is, therefore, struck only by hydrogen molecules, and from the 

 formula . — — — — 



'"'V.^^" (5) 



*^ Here ;;; is the rate (in grams per sq. cm. per second), at which the hydrogen 

 comes into contact with the filament. M is the molecular weight of H2 (i.e., 2), T is 

 the temperature of the bulb, and p is the pressure of the hydrogen in the bulb. The 

 derivation of this equation has been given in a previous paper (Pliy.';. Rcz'. N. S., 3, 

 329 (1913)) and Pliysik. Z.. 14. 1273 (1913)- 



