Of the many forms of this filter, we may mention the ‘Syphon Filter for Travellers,’ by means of which wholesome water may be drunk from any pond or stream by simply immersing the filter therein and drawing the water through the tube by suction. Of the ‘Silicated Carbon Filter,’ Professor Wanklyn says that it will render river water containing a considerable amount of free and albuminoid ammonia as pure as deep spring water.
3. Bischoff’s Patent Spongy-iron filter.—This differs from one invented many years ago by Dr Medlock, in bringing the water into contact with spongy iron instead of thin iron rods, and thus effecting filtration much more rapidly. Medlock believed that the iron rods brought about the oxidation of the nitrogenous organic matter and its consequent conversion into nitrites and nitrates. Bischoff states that he has experimentally investigated the properties of spongy iron, and finds that it—
a. Decomposes even distilled water, which has been previously boiled.
b. That it reduces nitric acid to ammonia.
c. That the amounts of organic nitrogen and albuminoid ammonia are always much reduced after filtration through spongy iron.
d. That a minute quantity of iron is dissolved by the carbonic acid contained in the water, ferrous bicarbonate being formed. The latter being soon oxidised and precipitated is easily removed by filtration.
e. That the action of spongy iron on impure water is two fold, viz. chemical and mechanical. “The chemical action is clearly indicated by the decomposition of water. The readiest explanation for the decomposition of water, is, the intimate contact between the electro-positive and electro-negative bodies, such as metallic iron and carbon, or even metallic iron and any ferric oxide, which has escaped reduction, or which has been reoxidised by exposure to air or water; and it may well be supposed that, consequent to the galvanic current thus produced, the atmospheric oxygen dissolved in water is ozonised, and caused to act as a powerful oxidising agent in organic matter.”
We extract the tables on the next page from the Sixth Report of the Royal Commission on Rivers’ Pollution. The Commissioners, we may here state, speak in high terms of this filter.
4. The so-called Magnetic carbide of iron filter. In this, the filtering material is said to be prepared by heating hæmatite with sawdust. This filter has a good repute.
⁂ The Royal Commission “on Rivers Pollution” strongly recommend filters of animal charcoal to be recharged every three to six months, “since they found that myriads of minute worms were developed in the animal charcoal, and passed out with the water when these filters were used for Thames water, and when the charcoal was not renewed at sufficiently short intervals.”