W. Nicoll 
161 
highly desirable. To add disinfectants to a defective or leaking pail 
would be very useful but it would be very much more expensive than 
having the pail renewed or put in proper order. The application of 
disinfectants to isolated deposits of faecal material would not always 
ensure the destruction of the parasites, and it would be much less 
satisfactory than removing the deposits and having them destroyed or 
placed in a suitable receptacle. In the case of large areas of country, 
such as plantations where larvae may occur in great numbers and 
infection take place, it would be much more expensive and laborious 
to employ disinfectants than to enforce sanitary cleanliness and the 
use of proper footwear, though of course it w'ould be helpful. There 
is the added objection that the continuous use of disinfectants would 
have a deleterious effect upon the soil and the growth of vegetation. 
Influence of Salt. 
Other substances which are not usually regarded as noxious have 
from time to time been suggested. Amongst these is common salt 
(sodium chloride). This suggestion is a result both of natural observa¬ 
tion and of experimental work. It was observed for instance, in 
Cornwall, that while nlany of the mines were heavily infected, others 
escaped altogether. On investigation the most probable explanation 
was found to be that the uninfected mines were partly under the sea 
and that sea-water had percolated into them. As a result the water 
in these mines was found to be heavily charged with salt (up to 2 or 
3 %) while in the other mines the amount of salt in the water was 
comparatively low. Similar observations have been made in mines in 
many other parts of Europe. Later experimental work appeared to 
justify this conclusion. This matter will be referred to in greater detail 
in the following pages. 
The Life-History of the Hook-Worms under Normal Conditions. 
While the literature on the subject of hook-worm disease and its 
many medical aspects is exceedingly extensive, that dealing with the 
minute details of the parasites and their life-history is much more 
circumscribed. A general account of these is to be found in all text¬ 
books of Parasitology and Tropical Medicine, but for the purpose of 
the present paper a much more detailed resume is necessary. The 
most comprehensive work on the subject is that by Looss (1905 and 
