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(X 



and these values substituted in the primitive equation give 

 the equation 



representing two spheres, one cutting the normals to the 

 primitive surface internally, and the other externally, a 

 result which might have been expected. In the case of 

 solution we must take the radius r—c. It might be asked, 

 what is the interpretation of the equation with the ex- 

 pression r+cfor the radius? The physical interpretation 

 is this ; chemical solutions can not only dissolve but also 

 deposit and the external surface corresponds to the case of 

 deposition ; this remark will also apply to the external 

 surface included in the general equation to the instantane- 

 ous surface ; hence, dissolution and deposition are included 

 in the same mathematical investigation. In the present 

 enquiry, dissolution alone is considered. In the case of any 

 isotropic solid dissolution will proceed as follows : in the 

 element of time dt there will be removed a shell having 

 everywhere the same infinitesimal normal thickness ; lines 

 normal to the original surface will be normal to the new- 

 surface ; along these lines again measure off the elementary 

 length dc^ the locus of the extremities will be the new 

 surface bounding the undissolved portion ; this process, 

 continued until we exhaust all the normals, will exhibit 

 the process of solution from its commencement until its 

 completion. 



The relation between the area of the primitive and of 

 the instantaneous surface may be obtained as follows : 



