periodic Precipitates. 73$ 



of silver arsenate constituted. But this line will not thicken 

 itself far outwards, since the silver arsenate forming a little 

 beyond, as the diffusion progresses, will prefer to diffuse 

 back and deposit itself upon the nucleus already in existence. 

 In this way the space just outside the nucleus becomes 

 denuded of the weaker ingredient (sodium arsenate). This 

 process goes on for a time, bat ultimately when the stronger 

 solution has penetrated to a place where a sufficiency of the 

 weaker still remains, a condition of things arises where a 

 new precipitation becomes possible. But between these 

 lines of precipitation there is a clear space. The process 

 then recurs and, as it appears, with much regularity. This 

 view harmonizes with the observed diminution of the linear 

 period as the concentration increases. 



We may perhaps carry the matter a little further, con- 

 sidering for simplicity the case where the original boundary 

 is a straight line, the strong solution occupying the whole of 

 the region on one side where x (say) is negative. For each 

 line of precipitation x is constant, and the linear period may 

 be called dx. According to the view taken, the data of 

 the problem involve three concentration? — the two con- 

 centrations of the original solutions and that of arsenate 

 of silver at which precipitation occurs without a nucleus. 

 The three concentrations may be reckoned chemically. 

 There are also three corresponding coefficients of diffusion. 

 Let us inquire how the period dx may be expected to depend 

 on these quantities and on the distance x from the boundary 

 at which it occurs. Now dx, being a purely linear quantity, 

 can involve the concentrations only as ratios ; otherwise the 

 element of mass would enter into the result uncompensated. 

 In like manner the diffusibilities can be involved only as 

 ratios, or the element of time would enter. And since these 

 ratios are all pure numbers, dx must be proportional to x. 

 In words, the linear period at any place is proportional, 

 cceteris paribus, to the distance from the original boundary. 

 In this argument the thickness of: the film — another linear 

 quantity — is omitted, as is probably for the most part 

 legitimate. In imagination we may suppose the film to 

 be infinitely thin or, if it be of finite thickness, that the 

 diffusion takes place strictly in one dimension. 



The specimens that I have prepared, though inferior to 

 M. Leduc's, show the leading features sufficiently well. 

 I have used the arsenate of silver procedure, and the 

 broadening of the intervals in passing outwards is very 

 evident when the plate is viewed through a Coddington lens. 



