THERMAL RELATIONS. 79 
They cost 9 marks each when ordered direct from Berlin, and can be had without 
delay. Good American thermometers are made by Henry Green, New Vork. 
With this open bath it is easy to keep the range of temperature down to 
0.1 to 0.2 of a degree, and the writer has frequently exposed tubes for ten minutes 
without appreciable change in temperature. Temperatures may be read easily to 
0.1 degree by means of a Zeiss aplanat lens magnifying six times (fig. 25), 
and should be recorded for each half minute during the exposure. Under 
no circumstances should exposures be made in water which is not agitated. 
Of course, for accurate reading the eye and the center of the lens must be 
level with the top of the column of mercury. ‘The lens may be supported 
at the proper level on a grooved piece of cork. If possible the thermom- 
eter used should be compared with some standard instrument. If not, it 
should at least be compared with several other good thermometers in the 
same laboratory. The test-tubes are supported by perforated corks thrust 
into holes bored through a rectangular piece of hard, heavy wood. 
The writer formerly made use only of the first four tests. It seemed 
hardly worth while to recommend that all bacteria be tested for the killing 
effect of cold, so long as we had nothing but the inconvenient and more 
or less inexact methods of salt and pounded ice or of ether and frozen 
CO:; but now that liquid air may be obtained at a small price in many 
of the larger cities, can be shipped long distances, and can be used with 
so little inconvenience, there is no good reason why the effect of freezing 
should not be determined in all cases, since in some instances it is likely 
to prove a valuable means of differentiation. ‘The bacteria may be exposed 
in 5 cc. portions of distilled water or bouillon in block-tin test-tubes, or 
preferably in tubes of resistant glass, for standard periods, e. g., one-half 
hour, 1 hour, 6 hours, 12 hours, 24 hours, 48 hours, etc. ‘They may also 
be exposed to alternate freezing and thawing every fifteen minutes or 
thirty minutes until all are dead. ‘To avoid endospores, the depressing 
effect of by-products, etc., young cultures should be used, and, of course, 
all should be of the same age and grown in the same medium, 2 e., 
bouillon cultures 24 hours or 48 hours old. ‘The tests should be quanti- 
tative rather than qualitative. They may be made as follows: Into 5 cc. 
of sterile water or standard bouillon a carefully-measured quantity, 2. e., 
one loop, 5 drops, % cc., etc., of the culture is placed, stirred’ very thor- 
oughly, and allowed some time for diffusion. To avoid zoogloez, which 
form early in some species, and to reach more uniform measurements, 
it is recommended to take the loop from a bouillon culture rather than 
from agar or other solid media. After sufficient time has elapsed for 
uniform diffusion, six Petri-dish poured plates are made from each of the 
inoculated tubes. ‘The plates should be of the same diameter (area of 60 sq. cm.). 
The amount of agar used for each plate should be 10 cc., and the amount of infec- 
*Fic, 65.—Anschiitz normal thermometer with degrees divided into fifths (Centigrade scale). 
For use in thermal death-point tests. About three-fourths actual size. 
