SANITARY NOTES ON POTABLE WATER. 569 
what he calles “ dilute ” urine, and finding that, for reasons 
which he did not understand, but which I will presently 
explain, the result was unfavorable to the filter, he finishes 
by using "more concentrated 93 urine, with a view of im¬ 
proving the purification by the filter. It is as difficult to 
understand the logic of such reasoning as the analytical 
fact stated by him that the "dilute” urine which he used 
for his experiments contained before filtration as much as 
8* 1625 grm., and the "more concentrated” urine only 
6*1775 grins, total nitrogen per litre. Equally unintelligible 
is the statement that ammonia could in no instance be dis¬ 
covered in the urine after filtration through spongy iron. I 
cannot find any remark whether or not he was able to 
discover ammonia in the urine before filtration. 
The difficulty of determining ammonia in normal urine 
consists, according to Neubauer and Vogel, # in its liability 
to form carbonate of ammonia by decomposition of the nitro¬ 
genous compounds. They say that ammonia occurs always 
in a distillate of urine obtained at the lowest possible tem¬ 
perature. It would thus be interesting to learn how Lewin 
demonstrated the absence of ammonia. 
Again, according to Gorup-Besanez,t urine contains on 
the average of numerous analyses 04 grms. of ammonia 
per litre. What can possibly have become of such an 
enormous quantity, the more as it is well knownj 
that even ordinary water, after filtration through spongy iron, 
contains frequently considerable quantities of ammonia? I 
will, in concluding, refer to only one more fatal mistake 
which the reporter to the Imperial Sanitary Board made. 
The purifying action of a filtering medium necessarily 
depends upon the time of contact with the water. The 
spongy iron filter is provided with a special arrangement for 
regulating the yield of water with great accurracy, but the 
regulation, at least of the passage of water through the 
spongy iron, does not commence until a filter, as is always 
the case in its practical use, is full of water. Until then 
the water rushes through the spongy iron almost as fast 
as it can be filled in. Now let us see how Lewin managed 
his experiments. 
He drained the filter, allowing the water to run off as 
completely as possible, and then poured in between 50 c.c. 
and two litres of the several liquids at a time. These small 
quantities, therefore, passed through the spongy iron in an 
# ‘ Analyse des Harns,’ p. 61. 
f c Physiologische Chemie,’ p. 530. 
X Chemical News , January 4, 1878, and following numbers. 
