and the Cause of Epidemic Disorders. 121 



sphere and wholesome or normal ah', being recognised. We 

 shall dwell, for an instant, on these two cases severally. 



1*^ case. — If we have recognised a compound of carbon and 

 hydrogen in an atmosphere which is alleged to be vitiated, by 

 means of some absorbing reagent, — or, if Ave have concluded 

 that we had found a poisonous miasma, because the water, 

 precipitated by some means or other from this atmosphere, pre- 

 sented phenomena which result from the spontaneous decom- 

 position of organic substances, — we have not established the 

 conclusion in question by experiment, which should have con- 

 sisted of a demonstration of the deleterious properties of the 

 two substances. This demonstration is absolutely necessary ; for 

 we have only to remember — that oils and empyreumatic acids 

 are unceasingly mixing with the atmosphere in consequence 

 of incomplete combustion, — that cai'bureted hydrogen is ever 

 developing itself from morasses, — and that volatile organic 

 substances, such as essences, aromas, &c. proceed from vege- 

 tables and animals, — to be convinced, that, by submitting a 

 sufficient volume of air to the processes of precipitation, we 

 may manifest in ordinai'y wholesome air, the existence both 

 of organic matter and of carbureted hydrogen. 



2d case. — On the other hand, in the case where the pre- 

 sence of a miasma or some deleterious substance of animal 

 origin in the atmosphere is denied, because, by eudiometric 

 methods, we cannot shew it contains any substance which is 

 foreign to the wholesome or normal state of the atmosphere, 

 our conclusion is far too rash. For there may be in an at- 

 mosphere some deleterious substance, which shall escape the 

 acumen of the chemist, inasmuch as it is in too small a propor- 

 tion to the normal air to be recognised by means of reagents, 

 precisely as the presence of a metal, which had not yet been 

 described, would escape the notice of the most able analysts, if 

 this metal was contained in an alloy, but in extremely small 

 quantity. We ought here also to remark, that if chemical analysis 

 should not demonstrate the existence of this foreign body in the 

 air, which accidentally contains a substance sensible to one of our 

 senses, this is not always a reason for our concluding that it is 

 incapable of doing so, and even by means of our present me- 

 thods ; for it may be possible in two ways, 1st, by the help of 



