CONDITIONS INFLUENCING- PREVALENCE 221 



their destuaction means that of their posterity. Their 

 numbers, moreover, are comparatively small, and the situa- 

 tions in which they are likely to be found can be predicated 

 with tolerable certainty. I do not, however, propose that 

 one should actually search for them, for the insects under- 

 stand too well their business to make any search profitable ; 

 but it may be taken as certain that the nooks and corners 

 of every room and native hut harbour numbers of them, and 

 it is perfectly easy to destroy them by fumigation with 

 sulphur employed in the same way as in ordinary disin- 

 fecting operations. The room to be dealt with should of 

 course be closed up as closely as possible, but no elaborate 

 arrangements for closing all crannies are necessary, as they 

 succumb to a proportion of the vapour in the air that would 

 be perfectly harmless to bacteria. It is very little use to 

 fumigate with pure sulphur, as it burns so uncertainly that 

 it generally goes out before half the material is consumed, 

 and it is even more futile to burn the sulphur by throwing 

 it on a charcoal brasier as is often done, because the result 

 is mainly vaporised sulphur, which is quite useless. A very 

 short exposure to the fumes suffices to kill the stoutest 

 Mosquito that ever buzzed, but as the buildings to be 

 dealt with are mostly extremely pervious, it is of the first 

 importance that the sulphur should be burned very quickly, 

 so as to produce a volume of sulphurous anhydride large 

 enough to be lethal before the product of the portion first 

 burned has time to escape. What is wanted, in fact, is a 

 mild firework which will burn out in a very short time, 

 but yet will not flare up to a dangerous extent. About one 

 part each of nitre and charcoal to eight of sulphur answers 

 well, the mixture being made up into pastilles weighing 4 oz. 

 each, by means of a little gum- water, and dried in the sun. 

 Pastilles of this description were made up for me by Messrs. 

 Waldie, of Cawnpore, and I found that even in thatched 

 buildinfjs not a single insect was left alive. The floor of 

 a bath-room, in which hardly any Mosquitoes could be found 

 by any ordinary search, was found after fumigating in this 

 way covered with dead Mosquitoes, a circumstance which 

 gives a good idea of the effectual way in which they hide 



