Prof. Graham on the Diffusion of Liquids. 215 



a salt takes place. Thus when the diffusion of nitrate of pot- 

 ash was interrupted every two days, as in a former experiment 

 with chloride of sodium, the progress of the diffusion for eight 

 days was found to be as follows in a 4 per cent, solution, with 

 a mean temperature of 66°. 



Nitrate of Potash. 



Diffused in first two days . . 4*54 grs. 



Diffused in second two days . 4 # 13 grs. 



Diffused in third two days . . 4*06 grs. 



Diffused in fourth two days . 3*18 grs. 



15*91 



The absence of uniformity in this progression is no doubt 

 chiefly due to the want of geometrical regularity in the form 

 of the neck and shoulder of the solution phial. A plain cy- 

 linder, as the solution cell, might give a more uniform pro- 

 gression, but would increase greatly the difficulties of mani- 

 pulation. 



The diffusion of carbonate of potash will no doubt follow a 

 diminishing progression also ; but there is this difference, that 

 the latter salt will not advance so far in its progression, owing 

 to its smaller diffusibility, in the seven days of the experiment, 

 as the more diffusible nitrate does. The diffusion of the car- 

 bonate will thus be given in excess, and as it is the smaller 

 diffusion, the difference of the diffusion of the two salts will 

 not be fully brought out. 



The only way in which the comparison of the two salts can 

 be made with perfect fairness, is to allow the diffusion of the 

 slower salt to proceed for a longer time, till in fact the quan- 

 tity diffused is the same for this as for the other salt, and the 

 same point in the progression has therefore been obtained in 

 both ; and to note the time required. The problem takes the 

 form of determining the times of equal diffusion of the two 

 salts. This procedure is the more necessary from the inap- 

 plicability of calculation to the diffusion progression. 



Further, allowing the Times of Equal Diffusion to be 

 found, it is not to be expected that they will present a simple 

 relation. Recurring to the analogy of gaseous diffusion, the 

 times in which equal volumes or equal weights of two gases 

 diffuse are as the square roots of the densities of the gases. 

 The times, for instance, in which equal quantities of oxygen 

 and hydrogen escape out of a vessel into the air, in similar 

 circumstances, are as 4 to 1 ; the densities of these two gases 

 as 16 to 1. Or, the times of equal diffusion of oxygen and 

 protocarburetted hydrogen are as 1*4142 to 1, that is as the 



T2 



