TJie Dissolution of an Isotropic Solid. 177 



the velocity of dissolution is therefore in each case a simple 

 algebraic function of the variable, and the determination of 

 the integral will present no difficulties when the kind of 

 regular polyhedron is specified. In the previous examples 

 the mass of the solvent has been supposed to be finite ; but 

 we may suppose that we have a solvent consisting of an 

 infinite amount of anhydrous acid mixed with an infinite 

 amount of water. If in such a mixture a solid of finite 

 dimensions be dissolved, and the medium be kept in a con- 

 stant state of disturbance, the diminution in strength of the 

 acid due to neutralisation by the solid will be so small as to 

 be negligible, and the acid may be considered to be always 

 of its initial strength ; this will be approximately the case 

 when a small mass is dissolved in a large mass of the 

 solvent. If the solvent be an acid solution the strength of 

 the acid will depend on the ratio of the mass of the anhy- 

 dride to the mass of the water ; if this ratio be denoted by 

 q, and this letter be substituted for ^{a) in (24), the dif- 

 ferential equation of solution becomes 



dv = - 7iqs(Jt, 

 from which by substituting — i-rt'^ for dv, we obtain the 

 equation dc=-7iqdt, 



and by integration c=ngt. 



Under these circumstances the time required for the 

 complete dissolution of some of the familiar forms of solids 

 becomes a simple function of some linear dimension of 

 the solid ; for instance the times required to dissolve spheres 

 are as their radii, the times required to dissolve cubes are 

 as their edges ; this last remark also applies to the remain- 

 ing regular polyhedra. By substituting for c in the instan- 

 taneous equation, we may also determine its form and 

 dimensions at any time, and by substituting in (23) we may 

 determine the mass of the shell removed from a solid in 

 time /. 



The most complete series of experiments which I have 



iVI 



