64 
On Malaria . 
or to the mode in which the moisture exists : since, generally, the plants are simi- 
lar, while all vegetables appear indiscriminately to produce the poison in question. 
In the next place, a marsh, in the popular sense of the term, possesses a certain 
extent ; and in the popular view also, this extent is judged necessary to the pro- 
duction of disease. Facts without number prove, that space, bulk, or extent is 
not essential ; but that the smallest spot of the same character, or of the proper 
character is sufficient to produce disease, though the extent of its influence must 
he expected to vary in consequence : and even without these facts, this would be 
judged true from the following argument. 
It is ascertained that a marshy spot can influence to disease a place situated 
at a considerable distance from it ; and this extent, in Italy, has been ascertained to 
amount to at least three miles. Now* it is plain, that whatever the original body 
of malaria was, it must have been diluted by the atmosphere during such 
a course to a very great degree, or that the quantity reaching any individual 
must be very minute It must be indifferent, therefore, whether that minute 
quantity which thus acts, is a portion of a great body of the same substance, or 
whether it is the whole which was generated ; and this must, in reality, be indif- 
ferent in any case, even without transportation. It is ascertained, that a single 
inspiration will produce the disease ; aniT therefore, whether in this case or 
in that of dilution under transportation, the exposed person receives the whole 
produce of a square yard, or any other given space, or the same quantity 
out of a mass produced by twenty thousand or million of square yards, the conse- 
quence will lie the same. It is a mere question of arithmetic ; and moreover, as 
the whole malaria of any marsh is the collective sum of the portions produced by 
each plant or fragment, whatever can act on the exposed individual will be suffici- 
ent to the effect, though it were the entire quantity in existence, or the single 
part will act as effectually as if it was accompanied by a thousand similar ones. 
Thus it is of no consequence how small the marshy or pernicious spot is, provid- 
ed it can act; and the only difference is, that a smaller spot will act on fewer per- 
sons ; and also, that from the greater dilution which it must undergo on transpor- 
tation, it must act at shorter distances, or may require absolute proximity or contact. 
Having premised these two necessary considerations, it is sufficient now to inquire 
what are the modifications and forms of soil or water, or of certain spots and places 
generally, which contain the elements of a marsh, or are distinguished by a vegeta- 
tion accompanied by water. Whatever these may he, let their names be what they 
may, or their appearances as deceptive, they are at least suspicious, and in very 
many cases they will be found to be real causes of the diseases of malaria, wherever 
the temperature, or the season and climate, are favourable, while that these are so 
with us I have just shown. 
In a general enumeration, I may name the following places or soils, before pro- 
ceeding to such particulars as may be necessary iu explanation. Besides proper 
marshes, fresh and salt, meadows, or wet pasture-lands generally, however situated, 
there are woods, coppices, and thickets, plashv and limited spots of grounds, sea 
walls and river embankments; and, as falling better under the brad of water, 
lakes, rivers, and ponds, including mill-dams, ornamental waters, canals, and the 
pools ot insignificant dimensions which occupy commons, gravel-pits, or other 
similar places, together with agricultural ditches and drams of all kinds. To 
these, I must also add, as coming under the heads of vegetable matter without 
lramir'irmrt GMirors: mill tnnrn .. -Kin • i i , D . 
land, that salt marshes are not insalubrious, and, very particularly, that security 
is obtained whenever they are daily washed by the tide : an ample experience 
shows that they are as pernicious as fresh-water ones, and that a daily tide is no 
security, borne ot the most poisonous tracts iu Europe, indeed, in France Snain. 
Italy and Greece, are salt marshes ; and of how little Iralue or’ protecXu the ea 
is, IS fully proved by the tide-rivers of the tropical climates, most notoriously the 
"atsm-e' 'found fnZT fT' T" tho8e d f trU “ ive Te ^‘ wfiuch 
well known ‘ gUud 1 nee,i uot say > as the > r "e always both obvious aud 
tioned “in mea f ow . s can , Produce malaria : this is to be ques- 
ence and secondW i i can i? g6ne ? and from «her European experi- 
th^^uses Severe w), pr0Ve ? by do . m ? tlc experience, that they actually are 
* with us ; and even mdependently of tlieir ditches or drains, 
