17 
and the slips of filter paper and pieces of glass placed in Petri dishes 
and saturated with the emulsion. The dishes w r ere then placed in the 
incubator room at a temperature of 37° C. for one hour, the slips of 
filter paper and pieces of glass being small, and as they w 7 ere moved 
about in the Petri dishes from time to time during the hour this 
seemed ample time to thoroughly dry them. 
The other method used for making the exposures in these experi- 
ments is one first used by Doctor Wilson, 0 and which will be called 
in this work the “ Wilson method.” As used in these experiments 
it consisted of a heavy piece of cardboard about 4 by 15 cm. folded 
in the middle like the cover of a small book and having a small 
piece of filter paper about 0.5 by 4 cm. pasted by one end inside of 
it something like the leaf of a book. This left the other end of the 
filter paper unattached. This end w~as contaminated with the organ- 
ism to be exposed. 
A small nick in the filter paper about 0.5 cm. from this end facili- 
tated tearing off the end with forceps wdien so desired. A number 
of these little devices were prepared in this way and sterilized in a 
hot-air sterilizer. The free ends of the filter paper w-ere moistened 
with a w^atery suspension of the organism and dried in the incubator 
room for one hour as with the other methods above described. 
The organisms were then ready to be exposed to the action of 
the formaldehyde gas. 
The exposures were made in the wire trays in the room wdiich 
have been described. 
The trays, being immovable, wnre easily sterilized in a gas flame. 
The slips of filter paper and pieces of glass were carefully placed in 
the trays by means of sterile forceps, care being taken not to have 
any two pieces in contact with each other. 
The exposures w;ith the Wilson method were also made on these 
trays. 
As no appreciable difference in the germicidal action of the formal- 
dehyde on exposures made at different heights in the room was 
observed, most of the exposures, for convenience, were made through 
the middle series of zinc tubes. 
After the organism had been exposed to the action of the formal- 
dehyde the desired time, the rubber stopper was removed and the 
wooden frame carrying the wire tray momentarily withdrawn far 
enough to procure one of the exposures. 
The slips of filter paper and pieces of glass wnre removed with 
sterile forceps and planted in tubes of nutrient bouillon. With the 
Wilson method the slips of filter paper were torn off with sterile 
forceps and planted in the same way. 
« Robert J. Wilson, M. D., assistant director of the Research Laboratory, New York 
City Department of Health. 
