REPORT ON THE LAWS OF CONTAGION. 75 



XIV. There is only one form in which ponderable matter is ca- 

 pable of being transmitted invisibly through the atmosphere, viz. 

 in that of elastic fluids, either permanent at common tempera- 

 tures, or existing as such veithin a certain range of temperature 

 and pressure. The former are called gases, the latter vajwurs', 

 but the distinction is one of convenience only, and is not marked 

 by any well defined boundaries. Contagious poisons, when dif- 

 fused through the atmosphere environing an animal body by 

 which they are generated, can exist only in the form of vapour. 

 Like all other vapours they must be governed, as respects their 

 degree of concentration in a given space, chiefly by the existing 

 atmospheric temperature. 



XV. Of the chemical constitution of contagious emanations, 

 we are equally ignorant as of that of the liquid poisons. VTe 

 may conclude, however, that they consist of the commonly 

 known elements of animal matter, and that their diversities de- 

 pend, as in several well known instances of gaseous compounds, 

 on modifications of the proportions, or even of the molecular ar- 

 rangement of like proportions, of those elements. Thus, the very 

 same proportions of carbon and hydrogen are known to consti- 

 tute no less than three elastic fluids, each distinguished by pe- 

 culiar mechanical and chemical properties. From the little sta- 

 bility of composition of contagious poisons, evinced by their 

 being decomposed by temperatures not above 212°rahr., as well 

 as, perhaps, by weak chemical agents, it appears that their ele- 

 ments are held together by very fegble affinities. 



The notion, which appears to have originated with Kircher, 

 that contagious emanations are at all connected with the diffu- 

 sion of animalcula or acari through the atmosphere, is purely 

 hypothetical. It has been defended, with a singular want of 

 sound argument, by Nyander, in a dissertation which Linnaeus, 

 with equal want of judgment, has admitted into the fifth volume 

 of the Amoenitates Academiccs. All that can be conceded in 

 favour of such an hypothesis, is, that the assigned cause is not 

 impossible ; but not a single valid analogy has hitherto been 

 advanced to confirm it. On the contrary, the opinion is at vari- 

 ance with all that is known of the diffusion of volatile contagions. 



XVI. We have no decisive evidence, through what channels 

 contagious emanations escape from the animal body. They may 

 issue from the whole of its surface ; but it is probable that they 

 transpire chiefly through that fine membrane, lining the air- 

 cells of the lungs, which the pheenomena of respiration show to 



8vo, published in 1826j)y Longman and Co.) It is desirable that this vahi- 

 able document sliould be made accessible to medical and general readers, by 

 republication in some less voluminous work. 



