EVAPORATION AND CONDENSATION 321 



In many chemical and physical problems it is important to know how 

 closely the reflectivity can approach lOO per cent. Thus in calculating the 

 vapor pressure from the rate of evaporation, we made use of (i — r). 

 (See equation (i).) In this case, where y = i, we have seen that we may 

 always place r = o. Similarly in estimating the velocity of a chemical re- 

 action we may use an equation similar to ( i ) in which we use 8 in place of 

 (i-r). 



In studying the mechanism of such reactions we need to know the order 

 of magnitude of i — r. Thus suppose in a given case we find that only 

 one molecule reacts out of every thousand striking the surfaces, that is 

 6 = 0.00 1. In interpreting this result we need to know whether it is possible 

 that 99.9 per cent, of the molecules are actually reflected. 



From the calculations given above, it appears that the reflectivity can 

 in no case exceed 99 per cent., and must in all probability be much less 

 than this. Therefore a value of e as small as .001 must be accounted for 

 in some other way than by reflectivity, as for example by assuming that 

 the greater part of the surface is covered with a material on which the 

 given reaction does not take place. 



The smallest accommodation coefficient that has been observed is 0.19 

 (hydrogen). This corresponds according to Baule's theory (equation 8), 

 to a reflectivity of about 83 per cent., a value which is in reasonable agree- 

 ment with the upper limit of 95-99 per cent, calculated above. 



It thus seems extremely probable that when gas molecules strike solid 

 bodies in no case are more than 90 per cent, of the molecules reflected. 



