302 PHENOMENA, ATOMS, AND MOLECULES 



covered with an adsorbed layer of hydrogen molecules, while the co- 

 efificient 0.68 applies to hydrogen molecules striking an absolutely clean 

 tungsten surface. 



Some interesting data on the reflectivity of molecules from a platinum 

 surface have been obtained in connection with a study of the combustion 

 of hydrogen and carbon monoxide at low pressures in contact with a 

 heated platinum wire.-^ In these experiments a platinvmi filament was 

 mounted in the center of a spherical bulb of four liters' capacity, provided 

 with an appendix (10 cm. long and 3 cm. in diameter) which was cooled 

 by liquid air. This bulb was connected through a liquid air trap to a 

 McLeod gage and a system for the quantitative analysis of small quantities 

 of gas (of the order of one cubic mm.). Low pressures of mixtures of 

 oxygen with either hydrogen or carbon monoxide were introduced into the 

 bulb and the filament heated to a definite temperature. The products of 

 the reaction, water vapor or carbon dioxide, were condensed in the appendix 

 and thus prevented from returning to the filament. The pressures employed 

 ranged from o. i up to 30 bars, so that the mean free path of the molecules 

 was many times greater than the diameter of the filament. The velocity of 

 the reaction was determined by taking readings of the gage at regular 

 'intervals. 



With filament temperatures up to about 500-700° K., it was found that 

 the reaction velocity increased rapidly with temperature. The rate was 

 proportional to the partial pressure of oxygen, and inversely proportional 

 to the pressure of hydrogen or carbon monoxide. Thus with an excess of 

 oxygen the reaction started slowly, and as the proportion of combustible 

 gas decreased, the rate increased until it became practically infinite (too 

 rapid to measure) . From this it may be concluded that hydrogen and carbon 

 monoxide are negative catalyzers or catalytic poisons. Evidently these sub- 

 stances cannot prevent the reaction on the surface unless they are present 

 on the surface. It thus follows that a surface of platinum at temperatures 

 up to 500-700° K. in pressures of hydrogen or carbon monoxide, as low 

 as one bar, is practically completely covered with an adsorbed film of these 

 gases. 



When the temperature of the platinum was raised, the reaction became 

 so rapid that with a filament having a surface of 0.31 sq. cm., the pressure 

 in the four liter bulb in some cases fell to half value in less than 6 seconds. 

 To lower this rate a new filament (length 0.7 cm., diameter .003 cm.), 

 having a surface of only .0067 sq. cm., was substituted for the old one. 



^' A preliminary account of these experiments, giving only qualitative results, 

 was published in the Jotir. Amcr. Chcni. Soc, 37, 1162, 1915. The complete data on 

 these experiments, together with that on several other similar reactions, will prob- 

 ably be published in the same journal. 



