ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
597 
perature, the pieces of brick were completely grown over by thq 
Crenotlirix. 
Tree-stems as Filters.'* — The discovery made by Pfitzer that sea- 
water could be deprived of its salts and rendered drinkable by filtering 
it through tree-stems has been tested by Herr Wilm on tree-stems of 
various kinds of wood of 1-0*5 m. long, and 0*4-0 *5 m. in circum- 
ference. The stems were connected with an air-pump by means of lead 
tubing. The permeability of the different woods was very unequal, and 
only a pressure of 3-5 atmospheres could be used, for with more the bark 
cracked and the water spurted out. The filtered sea water was at first 
yellowish and opalescent, afterwards becoming bright and clear. It 
tasted salty from the beginning. The yellow colour was due to the 
wood juices, which gave the Trommer reaction for sugar. The amount 
of salt in the first litres was somewhat less than in non-filtered water, 
though soon it attained the same concentration. The experiments 
showed that only very large stems could produce salt-free water. As 
the procedure was tedious and expensive, it cannot be used for 
practical purposes, because the stems must be large and their bark 
must be unimpaired, and even then they only deliver salt-free water in 
small quantities for a comparatively short time, soon getting blocked up 
and grown up with bacteria. 
The suitability of tree-stems for filtering off bacteria was also 
examined into. Ordinary tap-water was forced through stems of fixed 
size for 4-5 hours, the water being examined bacteriologically both 
before and after the experiment. For the first 2-3 days the water was 
germ-free, but ultimately it came to contain more than the unfiltered. 
Stems sterilised by forcing boiling water through lost the power of 
filtering altogether. 
Microscopical Plate Counting.! — According to Herr Neisser, plate 
colonies are best counted in the following ways : — (1) Thinly sown 
plates of pure cultures (less than 600 colonies) are better counted with 
an ordinary hand lens than with the Microscope. (2) Thickly sown 
plates (especially water) should be counted with the Microscope. 
(3) Mixed plates should always be counted with the Microscope on 
account of the presence of very small colonies. 
Quick Method of Filtering Blood-Serum. J — Drs. G. Campbell and 
A. D. Ghiselin, while disclaiming any originality for their idea, describe 
a new filter which answers every purpose, and which can be readily 
prepared at a moderate price. The filter proper is on the principle of a 
single bougie water filter sufficiently strengthened to allow the safe use 
of a high pressure, and so arranged that a sterilised flask may be attached 
to the bougie in such a manner that the filtrate undergoes no risk of 
contamination. To the filter is connected a drum filled with liquefied 
carbon dioxide, such as is used in charging soda-water. The drum 
consists of an iron cylinder 4 ft. long by 4 in. diameter. To the upper 
* Hygien. Kundschau, 1895, pp. 445-50. See Centralbl. f. Bakteriol. u. Para- 
sitenk., l te Abt , xviii. (1895) pp. 26-7. 
f Zeitsckr. f. Hygiene, xx. pp. 119-45. See Centralbl. f. Bakteriol. u. Para- 
sitenk., xviii. (1895) p. 25. 
X Bull. Jolms-Hopkins Hospital, vi. (1895) pp. 91-3 (3 figs.). 
1895 2 E 
