66 
rapidly in the tropics. From 31 experiments it appears 
that the mean quantity of carbonic acid contained in the air 
of tropical Brazils during the months of April and May 
1866 was 3'28 in 10,000 volumes of air— a decided difference 
from the mean proportion (4*0 vols. in lODOO of air) contained, 
in the atmosphere of Europe, and probably due in great 
measure to the joint action of tropical rain and tropical vege- 
tation in withdrawing this gas from the air. The influence 
of rain on the amount of atmospheric carbonic acid is well 
known from the experiments of Saussure and Boussingault. 
The annual rainfall at Para is very heavy, amounting, 
according to observations kindly furnished to the author 
by Drs. Bruno- Cabral and Jose Abreu to nearly 3 metres 
(118 inches), of which about one-third falls during the 
months of March, April, and May. These determinations 
are consequently interesting as showing the proportion of 
carbonic acid in the atmosphere of the tropics in the middle 
of the rainy season of the year. 
In conclusion, the author begs to tender his thanks to 
Professor Boscoe for the valuable advice and assistance he 
has given him during the above investigations. 
