234 
POPULAR SCIEKCE REVIEW. 
The influence of surface is discovered again in the ordinary 
process of flltration. It was shown by Dr. Hofmann and 
Mr. Witt, in their report on •the water supply in London, 
that the water which passed through the filter beds of the 
water companies’ reservoirs, was robbed of some portion of 
the salts held in solution. The late Dr. Normandy, when 
engaged in his experiments on the production of drinkable 
water from the sea, discovered that sea water was rendered 
free from salt, or nearly so, by being filtered through about 
thirty feet of siliceous sand or powdered glass. The re- 
moval of organic colouring matter from water, by passing 
through a few feet of earth, is another example of the same 
power in action. These phenomena are shown, yet more 
strikingly, by charcoal. Hence its employment for purifying 
water, and its use for removing the annoyances arising from 
putrefactive fermentation. Experiments have shown that char- 
coal possesses the power, by virtue of its porosity, of condensing 
within itself many times its own volume of certain gases and 
vapours. This property is not peculiar to charcoal — all porous 
bodies exhibit it to a greater or less degree — but the power is 
strikingly manifested b}^ this substance. It maybe incidentally 
mentioned here, that Dr. Stenhouse has, by connecting a piece 
of charcoal with a voltaic battery, and plunging it into a solution 
of platinum, succeeded in coating all its interstitial spaces with 
a film of that metal. This is, in itself, another example of the 
surface action to which it is desired to draw attention. This 
platinized charcoal possesses all the powers of ordinary charcoal 
greatly exalted. It acts, indeed, as spongy platinum does, and 
not only condenses the gases escaping from putrid matter, but 
combines them with oxygen and slowly burns them away. 
An instantaneous light lamp was common enough some years 
since. Hydrogen gas was produced, by a simple arrangement, 
by the oxidation of zinc in water, and stored in a bottle for use. 
When, by turning a stop-cock, a jet of hydrogen gas was pro- 
jected upon a piece of spongy platinum, it was rapidly condensed 
and, at the same time, forced into combination with oxygen. 
The result of this was the production of heat sufficient to ignite 
the jet of hydrogen gas. Faraday showed how directly this 
depended on surface action. Taking a piece of perfectly clean 
platinum, he plunged it into a mixture of oxygen and hydrogen 
gases. These united to form water on the surface of the metal, 
and by the heat evolved, in this process, the metal became red hot. 
It may appear too much to sa}^ that the solution of sugar or 
of salt in water is an analogous process to those which have 
been thus liastily and popularly described. A little attentive 
consideration will, however, carry conviction to the mind, that in 
the solution of a lump of sugar in water, we see the diffusion of 
