AMONG CATTLE IN IK ELAND IN 1812. 
577 
our sense of smelling, evading the detection of any addition to its 
usual component? If a quantity of musk is weighed after its hav- 
ing remained for years in an apartment, it will not be found to have 
lost any appreeiable quantity of its original weight, although the 
air of the chamber will have been strongly impregnated with the 
perfume during the entire period of the time which the musk may 
have remained in it. The ablest chemist cannot detect the ex- 
istence of the particles of perfume in the air through the. medium 
of any other analysis than the test with which he is supplied by 
bountiful nature, namely — his nose. This fact is offered as a 
beautiful example of the superiority sometimes evinced by the 
powers of instinctive nature over those resulting from an artificial 
acquirement. In the same manner the miasma or poison of the 
atmosphere is detected by its producing specific effects on differ- 
ent organs and tissues of the living frame. 
Granting the position, that the exciting causes of some epi- 
zootic and epidemic diseases rest in the air, the question na- 
turally suggests itself — how does the vitiated atmosphere pro- 
duce its effects? Is it by coming merely into contact with the 
part affected, or is it through the medium of respiration that the 
blood becomes affected, and then in its circulation disseminates 
the noxious principle it received from the air throughout the en- 
tire system, in order to exert itself on the different parts suscep- 
tible of its action? By those unacquainted with the science of 
medicine it may be asked — what is the reason that every portion 
of the animal is not affected, all parts being supplied with the 
same blood ; every drop of which, according to the explanation 
just given, must have been equally impregnated with the noxious 
principle as it passed through the lungs in the function of respi- 
ration ? 
This question, I apprehend, is one of much easier solution 
than may be possibly imagined. It is fully answered by ad- 
mitting that, for any thing to be acted upon by an agent, it 
requires to possess a certain degree of susceptibility. Thus, if 
half-a-dozen sticks of sealing-wax were placed beside each other, 
one or more of them being heated, and then all the six exposed 
to pressure, by passing a roller, having a pattern cut on it, over 
them, it will be found that those only that have been rendered 
susceptible by heat will receive the impression, although they all 
have been exposed to the same degree of pressure. 
The same principle may be applied to the animal frame, which 
is a most complicated piece of mechanism, composed of a variety 
of parts having different characters and degrees of susceptibility. 
The subject admits of a still more scientific explanation. It is 
universally admitted that the cause of epizootic disease is generally 
an aerial poison. Its being a poison of this character, we can re- 
