Mitchell on the Penetrativeness of Fluids. 309 



As proved in some experiments, already detailed, many solids are 

 dependent on water for the power of penetrating tissues, or gases, 

 &c., and it appears probable that many odorous solids, in particular, 

 enter the atmosphere, solely by penetrating its hygrometric con- 

 stituent. Thus, in solution, colouring matters readily, in certain 

 cases, pass through membranes impenetrable without such aid, and 

 every one has perceived the singular smell of a dusty road, after a 

 shower, even at a very considerable distance. In a damp day, or 

 immediately after rain, we more distinctly and vividly enjoy the fra- 

 grance of the parterre. Malaria seems to be dependent on the 

 same cause for its penetration into the atmosphere, for every one 

 knows the greater hazard of a residence in low damp situations, and 

 the general unhealthiness of a damp summer, or autumn. As elec- 

 tricity is a great hydragogue, and substances in a negative state 

 forcibly attract moisture, we might expect to find that season most 

 damp and unwholesome in which the atmosphere maintained an 

 electro-negative condition, and that driest and most healthful when 

 it was electro-positive. Facts on this subject are yet to be created ; 

 but this one presents an aspect germain to the subject. Mr. Wil- 

 liam Mason, of Philadelphia, a philosophical instrument-maker, 

 respected both for his ingenuity and correct moral character, in- 

 formed me, that when, in 1820, the yellow fever existed here as an 

 epidemic, he could not excite an electrical machine at his residence, 

 in the infected district, although at his shop, which lay at some 

 distance from it, the operation of the machine was sufficiently 

 powerful*. 



There exists between the lower surface of air, and the upper sur- 

 face of water, a space possessed of powers analogous to those of the 

 interspaces of substances in general. Along this plane, certain sub- 



* Aqueous gas penetrates the air more or less rapidly according to the tem- 

 perature and moisture of the atmosphere. According to our law of progressive 

 diminution, evaporation is slower in a moister atmosphere, and vice versa. The 

 following experiment shows that aqueous gas has also its rate of penetration. A 

 long tube, surmounted by a bladder, held water and mercury ; the former of which 

 being above, was in contact with the membrane. Although the mercury rose gra- 

 dually as water escaped, yet some air found its way through the bladder, and 

 occupying the upper part of the tube, separated the liquid and bladder from each 

 other. Under such circumstances only, air and aqueous gas could reach its lower 

 surface. Notwithstanding this, and the gradual increase of the quantity of air, 

 the mercurial column continued to rise, showing that the rate of the penetration of 

 aqueous gas, is greater than that of atmospheric air, by which it could not be 

 counterbalanced. Curious to see the effect, 1 tied over the summit of the tube, a 

 bag, holding carbonic acid, which thus replaced the atmosphere. Almost imme- 

 diately, the mercury gave intimation of descent, by losing its convex summit. It 

 did fall, and carbonic acid entered through the membrane, faster than the moibture 

 had at any time escaped. 



