126 ON FALLOW. 



tegration of a solid body must be in proportion to its surface ; 

 for the more points which we expose to the action of the destruc 

 live agencies, the more rapidly will their effects be produced. 



When a chemist subjects a mineral to analysis, in order to 

 break up the compound, that is, to give solubility to its constitu- 

 ents, he is obliged to perform the very tedious and difficult task 

 of reducing it to an impalpable powder. He separates the fine 

 dust from the grosser particles by means of a fine sieve, or by 

 elutriation, and exerts his utmost patience to obtain a fine pow- 

 der ; because he is aware that the solution of the mineral will 

 be incomplete, and that all his operations will prove ineffectual, 

 if he be at all careless in this preliminary operation. 



The influence of an increased surface upon the weathering of 

 a stone, or, in other words, on the changes which it suffers by the 

 action of the constituents of the atmosphere, and by water, is 

 very well pointed out in the interesting description given by Dar- 

 win of the gold mines at Yaquil, in Chili. The gold ores, after 

 being reduced to a very fine powder in mills, are subjected to a 

 process by which the particles of metal are separated from the 

 lighter parts of the ore. The particles of stone are carried away 

 by a stream of water ; while those of gold fall to the bottom. 

 The former are conducted into a tank, where they are permitted 

 to deposit. As the tank fills gradually, the fine mud is removed 

 from it, and is left in heaps to itself, that is, it is exposed to the 

 action of the air and of moisture. From the nature of the elutria- 

 tion to which it was subjected, the finely-divided ore can no longer 

 contain any salts, or soluble ingredients. Whilst it lay at the 

 bottom of the tank covered with water, and therefore excluded 

 from air, it suffered no change ; but when exposed to air, a pow- 

 erful chemical action ensues in the heaps, and this action is 

 recognised by the abundant efflorescence of salts, which cover 

 their surface, from the effects of disintegration . After the finely 

 divided ore has been exposed to the action of the weather for two 

 or three years, during which time it hardens, it is again elutri- 

 ated, and the processes of exposure and elutriation are repeated 

 six or seven times, new quantities of gold being obtained each 

 time, although in smaller proportions ; this gold is liberated by 

 the chemical process of weathering or of disintegration. 



