ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
681 
the phosphate and carbonate, and the two solutions mixed after 
cooling. 
The next thing is to evaporate down to about one-half the silica 
solution in a flask until 2-3 drops set within five minutes when a drop of 
the salt solution is added. Ten to fifteen minutes suffice to render it firm 
enough to stand being scratched across. When this degree of concen- 
tration is reached the evaporation is suspended and the silica solution is 
pipetted into glass capsules. It is then set by adding to it one-half or 
one-third its volume of the salt solution, according to the degree of con- 
sistence required. The two constituents must be well mixed, and in a 
few minutes a slight opalescence will denote that coagulation has set in. 
The material to be tested may be inoculated by mixing it with the 
salt solution or scratching it over the medium when solidified. For 
sodium carbonate magnesium carbonate may be substituted ; although 
this impairs the transparency, it renders the colonies more evident, since 
this carbonate is dissolved from round about the colonies. The deep- 
lying colonies of the nitrobacteria are very small, while the superficial 
ones form a pretty thick crust along the course of the inoculation track. 
Nitrobacteria may be obtained by direct inoculation of the earth, but 
it is better to set up nitrification in a watery saline solution by means of 
a bit of earth and then to 
transfer some of this to the 
solid medium. In this way 
are developed colonies con- 
sisting almost exclusively 
of nitro-bacteria, and that 
they do form nitrate is 
easily ascertainable by the 
nitric acid reaction with 
diphenylamin. 
A Colony-counter. * — 
Mr. J. E. Line writes : — 
“ In the study of the com- 
parative biology of water- 
supplies, sewage, infusions, 
secretions, &c., it is neces- 
sary to fix the organisms in 
a nutrient medium, culti- 
vate them to a given limit, 
and make a count. To do 
this neatly and effectively 
two pieces of apparatus are 
requisite — an Esmarch tube 
and a colony-counter. Glass 
plates and a linen-prover 
have been made use of, but 
for the more accurate results other and better means are called for. 
The Esmarch tube is simply a test-tube evenly coated internally with a 
solid sterilized nutrient medium — agar-agar, gelatin, combinations of 
* The Microscope, xi. (1891) pp. 179-80. 
