206 
wfthe' decomposed watery principles, 
‘united *with those of the organic’ re- 
mains «mixed therewith while the water 
is in a stagnant state. And although 
these!two fliids (to wit, aqueous vapour 
and miasma) are produced by the influ- 
ence of the same cause, it would, how- 
ever, appear, that the formation of 
‘aqueous vapour is less immediately the 
cffect of a continued high ‘temperature, 
than of such a state of temperature as is 
necessary for the evolution of the 
marsh miasma. 
The specific qualities of the principles 
composing the marsh miasma remain 
yet to be discovered.—This always rises 
from its source mixed with a consider- 
able quantity of vapour; and seems to 
possess the same specific gravity with it. 
It is here to be observed, that during 
the hot season, the column of heated 
air ascends much ‘higher in the atmo- 
sphere than at any other time of the 
year: whence, every exhalation that 
would be otherwise pernicious to the 
health of those exposed to it, is thereby 
more rapidly raised far above where it 
could produce its noxious effects. 
On the other hand, in ‘the spring, 
when the temperature of the air is 
lower, and the term-of congelation of 
the atmosphere is much nearer the sur- 
face of the earth, every exhalation is 
thereby confined much nearer to its 
source ; and in this way, the peculiar 
fevers caused by marsh miasma in that 
season become more severe, as the 
calorific: power of the sun increases, 
till the hot ‘season sets in; when, in 
consequence of the more rapid evapo- 
ration, the drier state of the surface of 
the earth, and the higher elevation of 
the term of congelation in the atmo- 
sphere, these fevers abate. 
Again, in the autumn, every where a 
more ‘moist season, the temperature of 
the atmosphere and soil being then more 
equal—circumstances the most favour- 
able for the evolution of marsh ‘effluvia, 
and for propagating their effects—it is 
found, that the remittent fever ap- 
pears in its worst form: and, on the 
coasts of the Mediterranean, it is often 
seen with the yellow colour, and every 
ether symptom of fevers arising from 
similar causes within the tropics. In 
autunm, the term of congelation gradu- 
ally descends in the atmosphere, in: 
proportion to the declining power of 
the sun’s rays; ‘and the temperature of: 
the soil being then more permanently 
high than at any other: season, every’ 
exhalation is .clevated; for a certain: 
Dr. Robertson on Remittent Fever. 
7 
[Oet. I, 
way, more rapidly, till it ‘arrives: at a 
colder stratum of the atmosphere, where 
it necessarily sinks: down, either!:to- 
wards its source, or moves alongy with 
the current of the air. elias 
It seems to be owing to this. cause, 
and to the lower temperature of the 
air at every season on high grounds, 
that we find the marsh vapour produc- 
ing its noxious effects, even in elevated 
situations, while those living in places 
on a level with the sources from which 
these exhalations originate remain free 
from disease, 
These occurrences have fallen within 
my observation at Alicant, as also in 
the castle of St. Giorgio, in the island of 
Cefalonia, which is situated at no great 
distance from a principal source of the 
marsh vapour, both many hundred feet 
elevated above the sea-shore. 
It is probably owing to ‘the lower 
temperature of the atmosphere, when 
the sun is off the horizon—thereby re- 
pressing the ascent of vapours from the 
soil, that the miasma most commonly 
produces its effects during the night ; 
and upon the same idea we understand 
why it is most dangerous to health to 
be exposed to the air during the period 
intervening from a short time before 
sunset, and till after the air begins to 
be warmed by the power of his rays in 
the morning. 
Besides the influence of an high or 
low temperature in evolving marsh mi- 
asma, a very tumid state of the atmo- 
sphere has likewise a powerful effect 
of repressing its formation; or, if it is 
produced in such circumstances, it is 
then so much diluted with watery va- 
pour, as to be incapable of shewing its 
peculiar effects on living bodies. In 
this manner may be explained how no 
bad consequences result from the exha- 
lations arising from running water, nor 
are experienced by those living in houses 
upon the sea-shore, whose walls ‘are! 
washed by the sea, so that no dry space 
is occasionally left by it. 
The shores of the Mediterranean 
afford an abundant source for the gene- 
ration of marsh miasma ; and probably it 
is more powerful and continued in: the 
extrication of noxious vapour, than if 
its shores were washed by the return’of 
hich tides, as in the ocean. ~ For, on’ 
these coasts, all matters thrown up by 
the sea remain putrifying on the shore; 
which process is increased and kept ‘tip! 
by the continual dashing of the “spray, 
and the rain that o¢casionally ‘falls over: 
it :_this process. is/always more! nth 
an 
