JoLY — Some Experiments on Denudation. 23 



•quantity of the solvent within wide limits, so long as the solid is 

 maintained immersed, seriously effects the results. The primary 

 factor appears to be the stability of the solid material, and hence 

 the extent of surface which this exposes to the solvent is the most 

 important quantitative measurement involved. It is too often the 

 practice to state in such experiments the amounts gone into solution 

 as a percentage of the mass of the entire solid. The latter quantity 

 is in itself of little importance. 



In the case of the last described experiment the materials 

 introduced into the U -tubes were sifted through sieves of measured 

 mesh. Thus in each U-tube the following quantities of basalt were 

 inserted : — 



25 grammes, passed 0-55 m.m., stopped by 0-45 mm. mesh. 

 40 „ „ 0-45 „ „ „ 0-35 „ „ 



20 „ „ 0-35 „ „ „ 0-20 „ „ 



To each of these, 103 grammes of coarse fragments having a mean 

 diameter of about 5 mms. were added. A minimum value for the total 

 surface area is arrived at by assuming the particles spherical in form, 

 and having diameters of the mean values of the mesh which admits 

 and the mesh which stops. The assumption is also applied to the 103 

 grammes of larger fragments. Making the requisite calculations, we 

 arrive at the result that the area exposed is not less than 0*509 square 

 metres. The actual area lies above this minimum value. The 

 particles are rarely rounded, more often rectangular or wedge-shaped 

 and rough. The assumption that the particles were cubical would 

 leave the area still below one square metre. We are probably not far 

 from the actual value in assuming, therefore, one square metre as 

 approximately the total surface area exposed within each tube. 



In the case of the ten-gi'amme charges, on the conclusion of the 

 experiments, each was separated by suspension in water into five 

 degrees of coarseness. These parts were carefully weighed, and the 

 mean diameters estimated by micrometric measurements (from ten to 

 twenty measurements being applied to each assortment), and the total 

 areas calculated, on the assumption that the particles are cubic in 

 form. 



The following are the results in square metres: — basalt, 1*209; 

 orthoclase, 1*799; obsidian, 1*163; hornblende, 0*791. 



On the foregoing data the table which follows fiu'thcr on is 

 calculated representing the amounts of material removed in each case 



