72 SCIENTIFIC AMUSEMENTS. 



Solubility of Liquids in Solids. 



It is a matter of everyday observation, that substances of a 

 porous nature will absorb fluids ; thus a sponge, a towel, a piece of 

 unglazed earthenware, or a lump of charcoal, will each absorb a con- 

 siderable quantity of water or other analogous fluid without allow- 

 ing it to drain completely away by gravitation on standing. This 

 phenomenon, however, is not exactly a case of solubility, inasmuch 

 as the action only takes place on the surface of the minute 

 cavities or pores with which such substances are permeated, being 

 in fact simply a variety of the action of minute tubes in taking 

 up fluids termed capillarity, which again is only one result of 

 what is known as surface action (Chapter XVII.). Certain 

 solid substances, however, exist, which although so compact and 

 destitute of porosity that no gas or liquid can be forced through 

 them by simple pressure (unlike such things as a porous earthen- 

 ware jar, which will allow water to pass slowly through it by 

 weight alone), will nevertheless gradually allow certain liquids 

 applied to the outside of a lump of the solid to pass inwards, and 

 become disseminated throughout the whole mass. Thus solid 

 articles of gold and silver allow liquid mercury to creep into their 

 interior should this fluid wet their outsides ; one result of which 

 is that the precious metal becomes considerably brittle. 



Expt. 69. To penetrate Gold by Mercury. Wet a small gold 

 coin with mercury (by well rubbing), and allow it to stand some 

 time ; it will become so brittle that it may easily be broken in 

 half ; and if the fractured surfaces are examined, it will be seen 

 that the mercury has crept inwards, and become distributed 

 throughout the entire mass, the action much resembling the 

 passage of water into a mass of blotting paper, excepting that with 

 the blotting paper the pores are relatively large, and will admit 

 any fluid capable of wetting the paper ; whereas the gold, if porous 

 at all to mercury in the same sense that unglazed pottery is to 

 water, is only porous to that fluid, and will not allow other fluids 

 to pass into the interior of a solid lump, even though these fluids 

 are freely capable of adhering to, or wetting the exterior surfaces 

 of the lump of gold. 



Silver is similarly penetrated by mercury, though not quite so 

 readily as gold. 



Caution. In certain experiments, where fluid mercury is used 

 (e.g., collecting gases, as in Expt. 280), it is well to take off one's 

 watch and chain, rings, &c. ; otherwise these may be seriously 

 damaged by the action of little drops and splashes of mercury 

 accidentally coming in contact with them. 



