REPORT OF THE MALARIA EXPEDITION. 
41 
(i.) The whole of the area of operations must be carefully dealt with by draining away or 
filling up every collection of water which is found to harbour Anopheles larvae at 
any time. 
(2.) Some cheap culicicide which will have the effect of rendering the pools pcrmanenth' 
uninhabitable for the larvae must be employed. 
The first will be an engineering work, the details of which need not be considered b\- us. 
Its cost will obviously depend on tlie number of pools existing in tlie locality of operations, and on 
tlie natural difficulties in the way of draining tiiem or filling them with cartli. 
The second measure will depend on the discovery of the substance whicii will liave the 
effect required. It must be some cheap solid substance which kills larvas without injuring higher 
animals, and which, when sprinkled on depressions in the ground, will render the pools which form 
there uninhabitable for the larvas for a long time — say for months or longer. Such a substance 
has yet to be found by proper experiments ; but we may suggest tar and lime as being possibly 
suitable. 
When placed in water, tar constantly gives off a film like that produced by kerosene oil, 
but more permanent. Judging from Dr. Fielding-Ould's experiments \_Adden(htm II.], it would 
seem to destroy larva? in a more effective manner tlian oil docs ; but it remains to be seen how 
long its effects will last. 
Lime is suggested by a paper by Grellet [28], of which we have seen only a review. Dr. 
Grellet gives instances to show that this substance is inimical to malaria ; and cites particularly 
the example of Chatillon-sur-Loing, where malaria is said to have disappeared as tlie result of an 
extensive application of lime to the soil for agricultural purposes. As it could scarcely have acted 
otherwise than by rendering surface pools uninhabitable for Anophelei larvae, we may surmise that 
lime may be a substance such as we require. 
The fact that Anopheles do not exist in many localities where suitable pools must be 
present, suggests that the soil there contains constituents which are inimical to the larvne ; and it 
is at least probable that something like the required culicicide actually exists. If it be found, we 
may possess a very practicable mode of eradicating the insects from areas whicli we elect for 
operations. 
In conclusion, there can be little doubt that we may often be able largely to reduce the 
number of Anopheles in some localities by these measures. If we may assume - as it seems we 
may — that the malariousness of a locality depends ceteris parihus on the number of Anopheles 
present in it, this would mean that these measures may often enable us largely to improve the 
health of certain places. 
28. Under what Conditions do Operations against Anopheles promise to be 
most Successful? — It will, however, be at once apparent that the practicability of employing 
these measures in any given locality must depend on two factors, namely, (i) the number of 
Anopheles pools present, and (2) the feasibility of attacking the pools. 
Judging from the careful study of the question which wc made in Freetown, we 
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