WATER-GILDING. 73 



Compact india-rubber, although not porous to gases or liquitls 

 in the ordinary sense, is capable of absorbing or dissolving small 

 quantities of certain fluids (e.g., benzene), without in any way 

 losing its solidity, the fluids penetrating the entire mass in much 

 the same way as the mercury does the gold in the above experi- 

 ment. When somewhat larger proportions of the same fluids are 

 employed, the rubber becomes more or less softened and disinte- 

 grated, and with pretty large quantities entirely dissolved; the 

 solutions thus prepared are employed for rendering textile fabrics 

 waterproof, by impregnating them with the liquid, and allowing 

 the solvent to evaporate, thus leaving the fabrics coated over with 

 a film of rubber. Such goods are often said to be Macintoshed, 

 from the name of the inventor of the process. 



Gold treated with large quantities of mercury is acted on in 

 just the same way, a semisolid or liquid amalgam (or solution of 

 gold in mercury) being formed ; such a solution is sometimes used 

 for gilding steel and other metals, the amalgam being applied to 

 the surface of the metal to be gilded, and the mercury then 

 volatilised by heat, leaving a film of solid gold on the surface of 

 the other metal, which is then brightened or "burnished" by 

 rubbing with a hard substance. This method of superficially 

 coating with gold is termed water-gilding (not because water 

 is employed, but because the gold is virtually rendered fluid by 

 the mercury), and is quite a different process from electro- 

 gilding, which has of late years largely superseded the mercurial 

 process, partly because it is cheaper, and partly because the water- 

 gilding process is a somewhat dangerous and unhealthy trade, 

 owing to the poisonous action of mercury vapours on the work- 

 men when inhaled. 



Expt. 70. To absorb Water by a Solid. Place a piece 

 of hard glue (impure gelatine) in a cupful of cold water, and 

 allow it to stand for some time ; the water will gradually pass into 

 the solid glue, causing it to swell considerably and increase in 

 weight; in time, especially with hot water, the glue will be 

 entirely disintegrated and apparently dissolved ; on allowing the 

 hot liquid to cool and stand, the water will gradually evaporate, 

 and the mass will first thicken and set to a jelly (gelatinise), 

 and then the jelly will slowly indurate or dry up. 



Lumps of gum and many other substances possess the same 

 property ; animal membranes, such as bladders, intestines, sausage 

 skins, and the like, are constituted of substances closely akin to 

 gelatine, and behave in much the same way. Substances of glue- 

 like characters are often spoken of as colloids, and exhibit cer- 

 tain remarkable differences from crystallisable solids (vide Expt. 93). 



