Mitchell on the Penetrativeness of Fluids. 313 



contains only -Y^yu of its volume, perishes instantly. A dog of 

 middle size is destroyed in air that contains -3 J^, and a horse would 

 fall a victim to an atmosphere containing ^J^. 



* Dr. Chaussier proves, that to kill an animal it is sufficient to 

 make the sulphuretted hydrogen gas act on the surface of its body, 

 when it is absorbed by the inhale nts.' 



One of the objections to the belief in aerial poisons most confi- 

 dently urged by antimiasmatists, is the absence of all proof of absorp- 

 tion of gaseous matter, and indeed this was the sole difficulty of any 

 real moment in the way of the triumphant establishment of the theory 

 of miasm. Will it now be going too far to say, that this difficulty 

 is removed, and that we can explain why miasmata affect persons so 

 differently who reside in different apartments of the same house, or 

 who live on opposite sides of the same street. Although being a 

 very little nearer to the source or to the ground may not appear im- 

 portant, yet the difference of a few yards makes in either case a 

 momentous distinction. Very near to its source a gaseous substance 

 occupies a larger portion of the atmospheric space, and presents not 

 only more matter, but matter less restrained by the molecular power 

 of the air with which it is mingled. Not only is a greater quantity 

 presented, but it is withheld from admission into the tissues by a 

 slighter restraint. 



As pressure unquestionably affects the rate of gaseous infiltration, 

 a difference in the amount of atmospheric pressure will perhaps be 

 considered of some importance, and assist in accounting for the ge- 

 neral unhealthiness of low situation and intertropical latitudes. 



Spontaneous evaporation has been long a subject of interest to the 

 philosopher, and has not hitherto admitted of adequate explanation.- 

 Now we perceive, that in elevating moisture into the atmosphere, a 

 very powerful agent is at work, one capable of subverting the cohe- 

 sion even of solids, and of producing the continued infiltration of 

 the atmosphere. Heat being also capable of destroying the attrac- 

 tion of aggregation, augments evaporation and interstitial infiltra- 

 tion. On this (I speak it hesitatingly) depends the power of 

 steam. Caloric penetrates gases as they do each other, and escapes 

 from them in exactly the same manner when substances which con- 

 tain less of it invite its penetrant power in a new direction. Thus, 

 for illustration, carbonic acid penetrates common air, and, so far as 

 we know, will expand it, if constantly supplied, to an amount of 

 power not yet measured. But so soon as another gas or penetrable 

 substance is presented, it begins to withdraw from the air and to 

 penetrate that. The hollow intestine used in one of our experiments 



Y2 



