GASES IN METALS 189 



solution.'-" These data support their assumption that tlie process of 

 solution consists first of saturation of a surface layer of the silver 

 with oxygen and then diffusion into the metal. If diffusion is the 

 limiting factor in rate of solution, the data obtained should be ex- 

 pressed by the equation : 



A'=l'°g7^. (5) 



where i^ is a constant, / is time, s is the concentration of a saturated 

 solution of gas in the metal, and x is the average concentration of gas 

 dissolved in the metal at time /. Steacie and Johnson found that 

 their data did fit this equation, if the first few points were neglected, 

 and they plotted curves showing the change in rate of solution with 

 temperature. 



The Analysis and Measurement of Gases in Metals 



Theory 



The most obvious method of determining the quantity and compo- 

 sition of gases in metals consists of melting a sample in vacuum and 

 collecting, measuring, and analyzing the liberated gases. The experi- 

 mental procedure is difficult, however, and the inherent errors are of 

 such magnitude that most results are at best only qualitative. 



One of the principal sources of error in these experiments is the 

 evolution of gases from furnace walls and hot refractories. When the 

 metal is heated by induced high frequency electric currents, however, 

 this error can be reduced, and it can be minimized further by main- 

 taining a large metal to refractory ratio. Another source of error, of 

 equal importance, is introduced when metal vapor condenses on the 

 comparatively cool parts of the apparatus and reabsorbs some of the 

 gas previously liberated. Although this effect results usually from 

 heating the metal in a high vacuum to too high a temperature, it is 

 not easy to eliminate, because, when the temperature is reduced, the 

 evolution of gas becomes too slow and the recovery of gas incomplete. 

 Errors also are introduced by gaseous products often formed by 

 reactions betw'een impurities in the metal melted and the refractory 

 oxides of the crucible. This occurs, for instance, when melting steel 

 in a refractory oxide crucible. The carbon reacts with the oxides 

 to form carbon monoxide, carbon dioxide, or both. 



Apparatus and Method 

 Despite difficulties and errors in the determination of gases evolved 

 from metals melted in vacuum, some special vacuum melting pro- 

 20 Loc. cit. 



