86 the; diffusion of gases through 



the diver above its horizontal circular mouth, the area of which, a, is given 

 in the eighth column, while //" is the difference of level of the free surface 

 in the diver and the free surface outside of and above it, during the occur- 

 rence of diffusion. The coefficient of diffusion, i. e., the number of cubic 

 centimeters at standard temperature and pressure which diffuse across an 

 orthogonal square centimeter per unit pressure gradient, and the per- 

 centage of solute in solution (grams of salt per loo grams of solution) are 

 contained in the last two columns. 



The cases of air and of hydrogen have already been adequately discussed 

 above and the various exceptional values, particularly the case of vessel A , 

 interpreted. The mean rate for air may be put k X io^'^ = 0.9 and for hydro- 

 gen (the value in the present chapter is probably preferable because of 

 the greater care taken to exclude air), kX 10^^ = 3.4. 



So far as this ratio is trustworthy, it is not out of proportion with the 

 ratios of mean molecular velocities for these gases. It is unfortunate that 

 the experiments above had to be made with a compound gas like air; but 

 the special difficulties involved in endeavoring to obtain similar results 

 with any simple gas, i. e., the provision of an artificial atmosphere in the 

 latter case, etc., seemed, at the outset at least, to more than counterbalance 

 the advantages of a single gas. Whether this adoption was actually a 

 wise step or not will appear in the future. It would not be so difficult to 

 work with hydrogen if a region of constant temperature sufficiently large 

 to contain all apparatus, including the air pump and the observer, were 

 available ; but this has not been the case. A thermostat for such a purpose 

 would not only have to be large but would have to be free from breakdown 

 for years. At the beginning of the experiments much time (/. e., several 

 weeks) must be allowed before a definite rate of diffusion can be said to 

 appear; but a steady condition eventually presents itself, beginning, as a 

 rule, abruptly, and it is not impossible that different liquids select differently 

 constituted gases for final diffusion. Such a gas may be richer or poorer 

 in oxygen than ordinary air. 



The use of distilled water, which is usually inadequately aerated, as well 

 as the use of tap water otherwise pure, are in this respect objectionable; 

 for the former will contain a deficiency and the latter an excess of air. 

 Any change of the gases in the room, as produced, for instance, by gas- 

 burners or by hydrocarbon vapors or even by decay, is to be looked at with 

 apprehension. In this presence the partial pressure of the exceptional gas 

 is zero within the diver and the gradient is at once brought to bear at its 

 maximum value. When gas has been dissolved in a liquid under pressure, 

 the growth of bubbles on rough objects may be noticed long after a ten- 

 dency to effervesce has completely vanished, so that in all cases fresh solu- 

 tions seem to require a long time to reach a normal content of gas. The 

 composition of a mixture is usually different in solution and out of it. In 

 this respect also the temperature variation and the solubility of a gas are 

 menacing; for if the gas were merely added to or deducted from the gas- 



