PROFESSOR GRAHAM ON THE DIFFUSION OF LIQUIDS. 
35 
in excess, and as it is the smaller diffusion, the difference of the diffusion of the two 
salts will not be fully broug-ht 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 quantity 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 inapplicability of cal- 
culation 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 diffu- 
sion, 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 square root of 2 to the square root of 1 ; the densities of these gases 
being 16 and 8, which are as 2 to 1. The densities are the squares of the equal-dif- 
fusion times. It is not therefore the times themselves of equal diffusion of two salts, 
but the squares of those times which are likely to exhibit a simple relation. 
(1.) While the 4 per cent, solution of nitrate of potash was diffused as usual for 
seven days, the corresponding solution of carbonate of potash was now allowed to 
diffuse for 9’90 days ; times which are as 1 to T4142, or as 1 to the square root of 2. 
The results were as follows : diffused of — 
Nitrate of potash at 64°T, in seven days, 13’81 grs. . . 100 
Carbonate of potash at 64°*3, in 9'9 days, 13’92 grs. . . 100'8 
The three experiments on the nitrate of potash, of which 13'81 grs. is the mean, 
were 13’98, 13’86 and ]3'60 grs., as already detailed. The three experiments on the 
carbonate were 14*00, 13*97 and 13*78 grs. The difference in the means of the two 
salts is only 0*1 1 gr. The results appear to be as near to equality as could be reason- 
ably expected from the method of experimenting. Seven and 9*90 may therefore be 
considered as the times of equal diffusion indicated for nitrate and carbonate of 
potash. The times of equal diffusion, or the diffusibilities of nitrate and carbonate 
of potash, would appear therefore to be in the proportion of the square root of 1 to 
the square root of 2. 
The explanation of such a relation suggested by gaseous diffusion has been antici- 
pated. It is that the two salts have different densities in solution, that of nitrate of 
potash being 1, and that of carbonate of potash 2. We are thus led to ascribe, what 
may be called Solution Densities, to the salts. The two salts in question are related 
exactly like protocarburetted hydrogen gas, of density 1, to oxygen gas of density 2. 
The parallel would be completed by supposing that the single volume of oxygen to 
F 2 
