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effect, was to divide up the whole area into sections and place a trained Indian
in charge of each section, with orders to go round every day and destroy any
larvæ he might find. The Assistant Surgeon in charge of these men went
round the whole area during the week. The staff was told that, their work
would he inspected very carefully ; and after ten days or a fortnight a surprise
visit was paid to the docks. The careful search of about 200 pools conducted
during this visit showed scanty large larvæ to be present in only 5 pools.
Complete absence of larvæ was not expected as ova were continually hatching
out and many small larvas were being produced in this way every day, but the
comparative absence of full-grown larvæ and pupae showed that few of them
were reaching maturity.
200. As the result of an enquiry regarding the larvacides in use in
Panama, Colonel W. G. Gorgas, U. S. A., the Chief Officer in charge of the
sanitation of the Canal Zone, very kindly supplied the following formula and
directions for preparing a larvacide, which has been used with very great
success at Panama.
" Gorgas' Larvacide—
Carbolic Acid, crude ..
Resin
Solution of Caustic Soda
.. 100 gals.
.. 66 lbs.
.. (20 lbs. of the solid in solution).
" The method of manufacture is to place the carbolic acid in a vat and
" heat to the boiling point by a steam coil. The resin is then powdered and
" added to the acid, is melted by the heat, and goes into solution. During the
" process of adding the resin, the mixture being formed is thoroughly stirred
"by a paddle fixed on an iron rod in the vat and revolved by a crank. The
" Caustic Soda is then added. After heating for two or three minutes a small
" amount of the mixture in the vat is tested to see if it thoroughly emulsifies
" with water. The emulsion formed is of a dirty white or pinkish appearance.
" If emulsification is complete the larvacide is drawn off into barrels, allowed
" to cool, and is then distributed to the line for use. The method of use is, to
" one part of the larvacide add three parts of water, which forms an emulsion.
" This emulsion is then spread on breeding places by the use of the ' Knap-
" sack Spray.' The advantages are that the larvas are readily killed, and
" their presence may be discovered in unsuspected places ; as on spraying the
" mixture on the water, the whitish appearance of the emulsion in contrast
" with water brings the larvæ into relief. By this method if larvæ have not
" been found on inspection, it is known whether or not to continue the
" application of the larvacide by the appearance, or non-appearance, as the case
" may be, of the larvæ in the emulsion formed by the larvacide with the water
" of the suspected breeding place.
" The objection to its use is that even in very high dilution it will kill fish.
"It is also destructive to algae and to a lesser extent to plant life, which is an
" advantage. By the addition of a 1/32 per cent. solution of arsenic the action
" on vegetation is greatly enhanced. A small amount of the larvæide mixed
" with the ordinary crude oil is found to be very efficacious on running water.
" The carbolic acid used is one that should contain 20 per cent. of phenol and
" cresols, and a specific gravity of not greater than 0.968. The amount of
" resin and caustic soda used varies according to the quality of the crude
" carbolic acid, and each shipment of carbolic acid must be testel as to
"the amount of resin and caustic soda used before the entire lot is converted
"into larvacide. As the crude carbolic varies greatly, each shipment must
" be tested. The cost of the larvacide manufactured at Ancon is about 141/2
" cents per gallon, United States Currency."
201. Owing to the fact that it was impossible to obtain crude carbolic
acid in Bombay of the kind specified by Colonel Gorgas, it was found
necessary to modify the formula considerably in order to prepare a suitable
carbolic emulsion for experimental use. The crude carbolic acid obtained had
a specific gravity of 1.040 and was stated to contain 97 per cent. of phenols and
cresols. This acid required five times the amount of resin and soda given in