170 Dr. J. BoTTOMLEV on 



lution of heat during the solution, this will have some effect 

 on the rate of solution. Also, if there be an evolution of 

 gas, the adhesion of bubbles of gas may interfere with the 

 contact of the solid and solvent. Also the solid may be 

 partially soluble and like cast iron, contain carbon or silicon 

 in forms not soluble, .so that a crust of insoluble matter 

 may accumulate and impede the rate of dissolution. 



In what follows, the following conditions are supposed 

 to hold : 1st, that the mass of the solvent is kept in such a 

 state of agitation, that at any time it may be considered 

 homogeneous ; in such a case the strength of the acid in 

 contact with the solid will be proportional to the total 

 quantity of anhydrous acid remaining uncombined. 2nd, 

 that the temperature remains unaltered during solution ; if 

 it be desired to keep the temperature from rising we may 

 suppose ice or some appropriate refrigerating agent applied 

 to the exterior of the vessel in which the operation takes 

 place, so that the flux of heat outwards shall neutralize the 

 rise of temperature due to chemical action, or by regulating 

 the external application of heat any constant temperature 

 compatible with the circumstances of the experiment may 

 be maintained within the vessel. 3rd, that the successive 

 surfaces exposed to the action of the .solvent are homogene- 

 ous, in which case the action of the solvent along every 

 normal to the surface is the same, so that the thin shell 

 removed in an element of time has everywhere the same 

 normal thickness. With these suppositions, it seems evident 

 that the rate of dissolution will be proportional to the extent 

 of surface exposed to the solvent, it will also be some 

 function of the unsaturated acid. Let v denote volume of 

 solid at time /, let a denote the mass of unsaturated acid 

 (anhydride), s surface of the solid at time t ; then we have 



the relation 



dv^ - ns(\^{a)dt (24) 



where n denotes a constant depending on the temperature 



