60 



to, says: "On the whole, the most promisino- method which suggests 

 itself is the employment of some cheap solid material or powder which 

 dissolves slowly, which kills the larva3 without injuring higher animals, 

 and which renders small pools uninhal)itable for the larvre for some 

 months. If, for instance, a cartload of such material would suffice 

 to extirpate the larva3 oyer a square mile of a malarious town, the 

 result would be a large gain to its healthfuiness. Dr. Fielding-Ould 

 has lately reported favorably on tar. "' 



Tar and its comj)ounds. — Again, in the report of the Liverpool 

 School of Tropical Diseases the following words occur: ' ' Perhaps more 

 permanent oil than kerosene wovdd be more permanently effective. 

 Fresh tar dropped in a puddle makes a film like that of oil and has 

 been favorably reported on. Quicklime has been suggested, and all 

 these should certainly be tried." 



The writer is rather at a loss to know exactly what is meant ])y the 

 expressions "fresh tar" and "tar" in the above paragraphs. He has 

 conducted an .experiment, however, with a substance known to the 

 trade as "coal tar," a thick viscid liquid. A few large drops of this 

 substance were dropped into a glass vessel containing approximately 

 2 quarts of water in which were more than 100 full-grown larva? of 

 Culex. All the drops but one sank at on<^e to the bottom, the last one 

 floating upon the surface for some time. No surface film seemed to 

 form from the tar, l)ut after the expiration of forty-eight hours the 

 water was found to ])e more or less impregnated by the tar, having 

 turned somewhat darker in color, while the odor of the tar was per- 

 ceptible. At the expiration of five days nearly all the Culex larva? 

 were dead; 1 had succeeded in transforming to pupa, and 6 or 6 remained 

 at the surface enfeebled and dying. Thus more than 95 per cent had 

 been killed. In the meantime, however, twent3'-four hours after the 

 experiment began, 3 ^.^^ masses were laid on the surface of the water 

 by outside females of Culex. These had hatched in forty-eight hours 

 more, and on the fifth day, although the original full-grown larvae 

 were practically exterminated, many young larva? were swarming- 

 actively about in the tar water. They continued to grow and to 

 remain apparently perfectlj^ active and healthy, although the odor of 

 the tar was distinctly perceptible and the color of the water was dark, 

 and even a thin oily film remained over a portion of the surface. 



From this experiment it was plain that the killing etfect of the tar 

 in the preparation used is comparatively fugitive, and it was next 

 decided to test some of the coal-tar products. The object of this line 

 of experimentation was not only to test the suggestions of the English 

 observers, but also on account of the fact that as almost every com- 

 munity manufactures its own illuminating gas it was considered an 

 easy and probably econominal way of securing a mosquito larvicide, if 

 it should prove to be eflective. Coal tar is distilled into various grades 



