464 REPORT — 1886. 



Surface Action of Solids. 



The absorption and condensation of gases upon solid surfaces, and more 

 especially in porous substances, is well known. The action of the same 

 substances upon liquids has not been studied to the same extent, but some 

 facts are known which bear upon the question under discussion. 



Graham, in 1830, published ('Pogg.' xix. 139) some experiments by 

 which he showed that animal charcoal, purified by acids, and containing 

 only a small quantity of silica, was capable of removing, from solution 

 in water, not only colouring matters, a fact long previously known, but 

 various metallic salts. Common salt was not precipitated, but solutions 

 of nitrate of lead, acetate of lead, tartar emetic, ammonia-sulphate of 

 copper, were completely deprived of their metal. In some cases the salt 

 was taken up again when heat was applied. In some other cases, as 

 solution of silver nitrate in ammonia and lead oxide in caustic potash, the 

 metal was more or less reduced, being first precipitated as oxide. The 

 quantity of lead oxide precipitated was so great as to be recognisable by 

 its white colour in the charcoal. 



In 1845 Wari'ington (' Phil. Mag.,' xxvii. 269) drew attention to the 

 power possessed by charcoal of removing bitter substances and alkaloids 

 from aqueous solutions. 



Many other solid substances, when in a state of fine subdivision, exert 

 a similar action. Precipitated sulphide of lead, oxide of iron, alumina, 

 clay, &c., possess this power as well as platinum in the state of sponge or 

 deposited upon asbestos (Stenhouse). 



Cotton immersed in solution of alum was observed many years ago by 

 Chevreul to be capable of withdrawing a liquid containing less alum than 

 the original solution, and it has long been known to possess the power of 

 abstracting oxide of lead, tannin, and various soluble colouring matters 

 from their respective solutions (see Crum on the manner in which 

 cotton combines with colouring matter, 'Phil. Mag.' April 1844; and 

 ' J. Chem. Soc' 1862). 



Other porous insoluble substances are said to possess similar powers. 

 Thus sand is said to be capable of removing acetic acid from the first 

 portions of vinegar filtered through it (Gmelin, i. 114), and similarly to 

 remove salt from sea-water (' Ure's Diet.' 1878, Art. Water, Sea). I 

 confess to have tried this experiment without success. Solutions of 

 common salt, and of alum of diflFerent strengths, were filtered through 

 about twelve feet of dry white sand. The first portions of liquid running 

 through were collected separately, but the quantity of salt present was 

 not appreciably less than in the original solution. 



However, it is probable that by varying the form of the experiment 

 the result might have been somewhat different. J. Thoulet (' Compt. 

 Rend.' xcix. 1072, c. 1002) finds that the attraction between the surface 

 of a solid and a dissolved salt can be observed when marble, kaolin, or 

 quartz is immersed in solutions of sodium or barium chloride, and that 

 the action is proportional to the surface of the dissolved solid. 



The action of filter paper upon saline solutions has been examined by 

 Mr. Bayley ('Jour. Chem. Soc' 1878). When a drop of a solution of a 

 metallic salt is placed upon filter paper, the water spreads away into the 

 paper, leaving a more concentrated solution in the centre of the spot. A 

 great difference is, however, observed in the behaviour of the salts of 

 various metals. Silver, lead, and mercuric salts give a wide water ring, 



