1901) MICROSCOPICAL JOURNAL. 271 
was for a long time used as a disinfecting agent against 
yellow fever, and experience found it to be trustworthy. 
But later it was disparaged because laboratory tests 
showed that it lacked the power of killing spores and has 
little penetrating power through fabrics.. But now that 
we know it is the mosquito which carries the infection, 
the usefulness of this agent is revived. 
Formaldehyd gas is a feeble insecticide. Mosquitoes 
may live ina very weak atmosphere of the gas over-night. 
It will kill them, however, if it is brought in direct con- ~ 
tact in the strength and time prescribed for bacterial 
disinfection. For this purpose any of the accepted me- 
thods for evolving the gas is applicable, but the methods 
which liberate a large volume in a short time are more 
certain than the slower ones. 
Direct contact between the insects and the gas is much 
more difficult to obtain in ordinary room disinfection 
against mosquitoes than against germs, because the sense 
of self-protection helps the former to escape from the ef- 
fects of the irritating gas. They hide in the folds of 
towels, bedding, clothing, hangings, fabrics, and out-of- 
the-way places where the formaldehyd gas does not pene- 
trate in sufficient strength to kill them. The gas is poly- 
merized aud deposited as paraform in the meshes of fa- 
brics, which prevents its penetration, and large quanti- 
ties are lost by being absorbed by the organic matter of 
fabrics, especially woolens. In our tests, whenever the 
insects were given favorable hiding places, such as in 3 
crumbled paper or in toweling, they quickly took advant- 
age of the best place for themselves and thus escaped 
destruction. , 
There is a striking analogy between the strength of 
the gas and the time of exposure necessary to penetrate 
the fabrics in order to kill mosquitoes, and the strength 
and time necessary to penetrate in order to kill the spores 
of bacteria, 
