DISINFECTION AND STERILIZATION. 179 
that its destructive action on micro-organisms depends 
upon a number of factors, the chief of which are its 
concentration in the surrounding atmosphere, the length 
of the contact, the existing temperature, the accompany. 
ing moisture, and the nature of the organism. 
The necessary concentration of the gas in the sur- 
rounding atmosphere to kill the micro-organisms varies 
with each species, for some resist chemical agents much 
more than others, and also with-the freedom of access 
of the gas to the bacteria, for if they are under cover 
or within fabrics a greater amount of gas must be gen- 
erated than if they are freely exposed. 
For purely surface disinfection, when the less resistant 
bacteria or other micro-organisms are to be destroyed, 
there will be required, according to the method used, 
6 to 10 ounces of formalin of full strength, or its equiv- 
alent, to 1000 cubic feet. 
For the destruction of the more resistant, but non- 
spore bearing forms, such as typhoid fever or tubercle 
bacilli, at least twelve ounces of formalin should be used. 
The gas penetrates through fabrics with difficulty, and 
to pass through heavy goods the concentration of the 
gas must be doubled and heat added. 
The Value of Moisture. At first it was thought that 
formaldehyde gas acted more effectually in a dry atmos- 
phere, but further investigation has proved that although 
it does destroy bacteria with the amount of moisture 
usually present in the air, and contained in their own 
substance, yet it acts much more powerfully and cer- 
tainly when additional moisture is present, and best 
when present up to the point of saturation. The 
actual spraying of walls and goods to be disinfected 
with water is even more efficacious. 
