Certain physical qualities which pertain to the malarial poison can also be profitably made points of subjective study. These are very closely connected with the answer to the second question, or "How the malarial poison obtains access to the human system." They will therefore be briefly noticed in relation to the instrumentality of each in conveying malaria into the system.
The first to be mentioned is ponderability, which the following facts prove that malaria possesses:
Those different atmospheric states which affect the range of diffusion of known air-borne yet ponderable substances exert similar influences upon the malarial poison.
Altitude illustrates the ponderability of malaria by powerfully retarding its diffusion.
High readings of the barometer favor its aërial dissemination.
Fogs, smoke, dust, or floating particles presumably more buoyant than this poison may exert greater or less influence in overcoming the obstacle which ponderability attaches to malaria as an air-borne agent.
Currents of air passing continuously and steadily in one direction over the breeding-places of malaria increase the limits and intensity of toxic range.
The atmosphere is undoubtedly the medium by means of which malarial poison is most frequently brought into the human system. Liability to intoxication is increased in direct ratio to the proximity of points of exposure to places of development; to similarity of level; to situation in the line of prevailing winds which have traversed the breeding-ground; and, lastly, to the extent and fertility of the locality of production.
Whether malaria passes through the respiratory apparatus directly into the circulation, or is lodged upon the fauces and absorbed through some other surface, is not clearly ascertainable. It is certainly not deprived of its noxious qualities by stomach digestion, and therefore, sometimes at least, may reach the blood through the alimentary canal.
Malaria is miscible with water. It is capable of being carried by currents of water through distances and periods of time altogether undetermined, without losing either its toxic effects or, perhaps, the faculty of reproduction. It is more than likely that this means of conveyance has effected its distribution to continents and islands too widely separated to justify a belief that it was wind-wafted. No observations need be adduced to establish the water-borne habit of the malarial poison, or the positive liability to its toxic effects when received into the stomach through this medium. These facts have been well understood from the time of Hippocrates.