72 
containing the calcium chloride was then removed. A thermometer 
was then introduced and the temperature in the bottle determined. 
Having ascertained this, the quantity of water necesssary to give a 
certain percentage of relative humidity was easily estimated by 
referring to the pscychro metric tables for obtaining the vapor pres- 
sure, relative humidity, and temperature of the dew-point, prepared 
by the weather Bureau, United States Department of Agriculture. 
The desired quantity of water was measured in a graduated pipette 
and placed in a test tube, the open end of which had been drawn out 
into a small tube about 0.5 centimeter in diameter; to be more accu- 
rate, the tube with its contents was then carefully weighed. The 
small end of the test tube was introduced into the bottle and the other 
end gently heated. This caused the water to pass into the bottle in 
the form of steam. After vaporizing the water, the tube was left in 
place until it had cooled; this caused some of the moisture to recon- 
dense on the sides of the tube. The tube was removed and again care- 
fully weighed, the difference in the two weighings giving the quantity 
of water actually injected in the form of aqueous vapor. 
This factor, and the temperature, being known, the percentage of 
relative humidity was easily computed from the tables referred to 
above (p. 72) . The exact degree of relative humidity desired was often 
not easily obtained, as it was difficult to inject the precise quantity 
of water wanted, and, furthermore, the temperature was subject to 
change. The slightest variation in either 1 of these 2 factors gives 
a different percentage of relative humidity. 
For making the bacteriological exposures, 24-hour old agar cultures 
of B. coli communis and spores of B. subtilis, grown about 40 days 
on agar slants, were used. An emulsion of the organisms in distilled 
water was made, small slips of filter paper about 0.5 cm. square were 
moistened with the emulsion and placed in the incubating room for 
about 1 hour in order to thoroughly dry them. 
The slips of filter paper were then placed in a small copper-wire 
tray, previously sterilized, and exposed to the action of the formal- 
dehyde in the bottle by introducing through a hole in the large cork 
stopper. This tray was provided with a tight-fitting cork on the near 
end, so that when the tray was in place in the bottle the cork would 
close the hole through which it was introduced. This arrangement 
made it possible to withdraw the tray wholly or partly when desired. 
The slips of filter paper were taken out at definite intervals and 
planted in tubes of nutrient bouillon. The tubes were then placed in 
the incubating room at a temperature of 37° C. and the results re- 
corded after 10 days’ incubation. 
