180 BACTERIOLOGY. 
A fairly high temperature—but one still below that 
which would injure delicate fabrics—increases not only 
the activity of formaldehyde gas but also its penetra- 
tive power, and for heavy goods it is essential. The 
production of a partial vacuum in the chambers be- 
fore the introduction of the formaldehyde gas still 
further assists its penetration. 
The length of exposure necessary for complete dis- 
infection depends upon the nature of the disease for 
which it is carried out—the penetration required, the 
concentration of the gas used, the amount of moisture 
in the air, the temperature of the air, and the size and 
shape of the room. For surface disinfection in rooms, 
when as much as 12 ounces of formalin are used for 
each 1000 cubic feet, five hours’ exposure is amply 
sufficient, most bacteria being killed within the first 
few minutes. For the destruction of micro-organisms 
protected by even a layer of thin covering, double the 
formalin and double the time of exposure should be 
allowed, and even then the killing of many species of 
non-spore bearing bacteria cannot be counted upon 
in ordinary rooms. When absolutely complete disinfec- 
tion is demanded, where penetration of gas is required, 
the goods must be placed in chambers where moderate 
heat can be added and all leakage of gas prevented. 
Various forms of apparatus can be properly employed 
to liberate formaldehyde gas for purposes of disinfec- 
tion, as each of these is lauded by its maker as the 
best; it may be of interest to give the results obtained 
by us from those in most common use. ‘There are two 
essentials to any good method—namely, that the for- 
maldehyde gas is given off quickly, and that there is — 
no great loss by deterioration of the formalin. 
