208 PHENOMENA, ATOMS, AND MOLECULES 



body, being a real continuation of the space lattice of the solid. This 

 layer of atoms (or molecules) on the surface may be said to be adsorbed. 



The surface of the metal is thus looked upon as a sort of checkerboard 

 containing a definite number, No of spaces per square cm. Each elementary 

 space is capable of holding an atom or a definite part of a molecule of 

 adsorbed gas. The number of elementary spaces, No, is probably usually 

 equal to the number of metal atoms on the surface. But this is not 

 essential, for we can imagine cases in which each metal atom holds, for 

 example, two adsorbed atoms or molecules, so that we should then have 

 twice as many elementary spaces as metal atoms on the surface. 



Let us now apply this theory to the dissociation of hydrogen in contact 

 with a heated tungsten wire. We will first calculate the rate at which 

 atomic hydrogen condenses on the bare surface when atomic hydrogen at 

 a pressure p surrounds the wire. 



The number of grams of gas which strikes a sq. cm. of surface per 

 second is 



m 



= Vi^^- (■) 



Let [X represent the number of gram molecules of gas striking a sq. cm. 

 per second. Then [i = w/M or 



_ P • 



Expressing p in bars,''' and placing R = 83.15 X lO^ ergs per degree, 

 this reduces to 



M •= 43.75 X lo-" j^. (3) 



By applying this equation, we can readily calculate the rate at which 

 atomic hydrogen (M = i) comes into contact with each square centimeter 

 of surface. The rate at which the gas condenses to form a layer (one 

 atom deep) on the surface will be less than the rate at which it strikes 

 the surface for two reasons. In the first place, only a fraction a of the 

 atoms which strike the bare surface condenses, while the fraction i — a 

 is reflected. Secondly, as the surface becomes covered with hydrogen 

 atoms, many atoms will strike portions of the surface already covered.^ 



' The bar is the C. G. S. unit of pressure, one dyne per sq. cm. One million bars 

 or one megabar is equal to 750 mm. of mercury, which is more nearly average atmos- 

 pheric pressure than the 760 mm. usually used. 



* There is good reason to believe that even the hydrogen atoms which strike a 

 surface already covered condense. But the rate of evaporation of the atoms from such 

 a surface is so much higher than that from a bare surface that the number of atoms 

 in the second layer is probably negligible in comparison with that in the first. 



