MISCELLANEA. 55 
from thence at different angles ; and the rock itself, which is 
quite porous, is abundantly charged with the same material. 
The nitre itself is very pure, containing mere traces of nitrates 
of lime and magnesia. The sandstone in which it occurs is 
siliceous, containing a little carbonate of lime arid a notable 
quantity of silicate of potash. 
NEW SOURCE OE GUM ARABIC. 
It is asserted that in the north of Texas, towards Arkansas, 
as well as in the State of New Mexico and the adjacent 
Indian territory, gum has been found in inexhaustible 
quantities, and of a character scarcely, if at all, inferior to 
that imported from the East. It is gathered from the 
Merquite tree, — a kind of acacia, abundant in that section 
of the country, especially in elevated and dry localities, and 
exudes spontaneously in a semi-fluid state from the bark of 
the trunk and branches, soon hardening and becoming nearly 
colourless by exposure. July, August, and September, are 
the months for collecting it, and the quantity obtained from 
each tree varies from an ounce to three pounds, which may 
be greatly increased by incisions. Even as it is, a good 
hand, it is said, would probably be able to collect from 10 
to 20 lb. a day; thus affording employment to thousands of 
the wild Indians of the plains. Ai.d should it command 
one half the price paid for gum arabic, it will constitute a 
most valuable article of traffic. 
WHY EPIDEMICS RAGE AT NIGHT. 
It was in one night that four thousand persons perished 
of the plague in London. It was by night that the army of 
Sennacharib was destroyed. Both in England and on the 
continent, a large proportion of cholera cases, in its several 
forms, have been observed to have occurred between one and 
two o’clock in the morning. The danger of exposure to the 
night air has been a theme of physicians from time immemo- 
rial; but it is remarkable that they have never yet called in 
the aid of chemistry to account for the fact. It is at night 
that the stratum of air nearest the ground must always be 
the most charged with the particles of animalized matter 
given out from the skin, and deleterious gases, such as car- 
