Experiments on Mr. Dahlke’s Filter. 301 
lowed strong hydrochloric acid it never came in contact with 
it. After washing for some time the hydrochloric acid made 
its appearance. 
Similar experiments were repeated several times with like 
results, and it was not till I added a fluid containing no 
water and which would not mix with water that I overcame 
the power of the filter to separate soluble salts from solution. 
It is almost impossible to over-estimate the value of this 
discovery, especially as Mr. Dahlke is prepared to fit up 
filters capable of purifying from 10 to 20,000 gallons, which 
without doubt will be of great service in many parts of the 
country where nearly all the water is so highly charged 
with salt as to be unfit for either human beings or cattle. 
To prove what its value may be at sea, I may add that I 
filtered some water collected from the end of St. Kilda pier 
and that the filtrate contained very little more salt than 
Yan Yean. 
It has been known for some years that most porous sub- 
stances have the peculiar power of retaining a portion 
of the soluble salts contained in water, when used as filtering 
media. In 1856 it was shown by the late Mr. H. M. Witt, 
that when water containing soluble salts was passed through 
sand-beds from fivetofifteen percent. of thesalts wereremoved. 
It has also been proved that ordinary agricultural soil has the 
power of purifying sewage water from all its soluble salts and 
organic matter, if it is allowed to flow over a larg earea. 
Various other substances, such as ordinary charcoal, animal 
charcoal, magnetic oxide of iron, and the new silico-carbon 
filter have been described as possessing this peculiar physico- 
mechanical power, but. none to the same degree as the sub- 
stance contained in Mr. Dahlke’s filter. 
