6 
C. A. LUDWIG 
some value. The reaction of the agar varied with the different lots 
made up from nearly zero to slightly over + i on Fuller's scale, but 
was usually about + 0.8. 
The exposures of the bacteria and some of the fungi to the gases 
were made in cotton-plugged test tubes confined in airtight chambers. 
This method has the disadvantage that it is practically impossible to 
get quantitative data by its means, such as could be obtained by using 
Petri plates and counting colonies, but the development can be fol- 
lowed better from day to day in tubes than in plates within a larger 
vessel. In most of the work it was a great advantage to be able to 
observe the cultures easily without removing them from the gas. 
The airtight chambers used consisted of four Novy jars and a number 
of bell jars with tubulature at the top which were fitted with per- 
forated rubber stoppers holding tubes for the introduction of gas. 
Each bell jar was placed in a base composed of a heavy crystallizing 
dish with a layer of plaster of Paris about 2 cm. thick, impregnated 
with paraffin, in the bottom. The plaster of Paris was prevented 
from breaking the dish in setting by putting paraffined corrugated 
paper next to the wall of the dish in order to take up the expansion. 
The chambers were sealed by running melted paraffin between the 
case of the bell jar and the wall of the dish. 
During the earlier part of the work, the gases were introduced 
from a Hempel gas burette by means of the pressure of a few centi- 
meters of water. However, in most of the experiments the gas was 
allowed to enter directly and its amount was regulated by means of 
a mercury manometer. The reservoir for the mercury in this case 
was a wide-mouthed bottle closed by a two-hole rubber stopper through 
one hole of which the tube containing the mercury column passed. 
By means of a second glass tube through the other perforation in 
the stopper the apparatus was easily attached to any chamber in 
which the gas pressure was to be measured. When a certain amount 
of gas was to be introduced into a given chamber, the chamber and 
the manometer were connected at the same time to an aspirator and 
exhaustion was carried out, usually to about 15 cm. of mercury. 
The apparatus was then connected to the gas container and gas was 
allowed to enter until the pressure had risen the calculated amount 
on the scale, the calculation being on the basis that the amount of 
gas in a given volume varies directly as the pressure. The apparatus 
was then allowed to finish filling with air, after which it was closed 
