76 FOURTH REPORT — 1834. 



be permeable, in both directions, by gaseous and vaporous fluids. 

 Through the same membrane, it is probable that contagious 

 emanations are chiefly admitted into the sanguiferous vessels. 

 Certain poisons (prussic acid, for instance,) have been traced by 

 their odour and chemical qualities into the blood*. But as we 

 have no tests of contagious poisons, it must remain conjectural 

 that they also are admitted into the blood-vessels, and circu- 

 late with that fluid. Even were that point established, it would 

 remain to be determined whether they act by producing che- 

 mical changes, or by at once affecting the nervous expansions, 

 and through them the great nervous centres. 



XVII. The theory which has been framed to account for the 

 spread of contagious emanations, is founded on the same prin- 

 ciple as that assumed to explain the diffusion of aqueous and 

 other vapours, viz. that a chemical affinity exists between vapours 

 and atmospheric air, producing a kind of solution analogous to 

 that of saline bodies in water. But this theory, though inge- 

 niously supportedf, is superseded by the more probable views of 

 Dr. Dalton, that in all mixtures of elastic fluids, whether gases 

 or vapours, with each other, chemical affinity has no share in 

 the effect, but that they maintain their state of equilibrium by 

 their respective elasticities alone J. In our atmosphere, for ex- 

 ample, the oxygen and nitrogen gases, which are its constant 

 ingredients, and the carbonic acid and aqueous vapour, which 

 vary a little in their proportions, are diffused through each other 

 by their respective elasticities, according to certain mechanical 

 laws. This is not the fit place for a detail of the evidences, on 

 which Dr. Dalton originally founded his opinion, nor of the ad- 

 ditional arguments deducible from the experiments of Mr. Gra- 

 ham §. It is sufficient to remark, that the probabilities are 

 greatly in favour of the new theory, which, by analogy, may be 

 extended to the contagious vapours. These effluvia, it is pro- 

 bable, are also diffused through the atmosphere, not by a pro- 

 cess of solution, but by the elasticities inherent in them as va- 

 pours; which elasticities are amenable only to variations of 

 temperature and pressure, and are totally independent of changes 

 in the proportions of the ingredients of the atmosphere. 



XVIII. The activity of contagious emanations has been as- 

 certained to be confined within very moderate distances from 



• Christison On Poisons, 8vo, 1829, p. 561. " Poisons," the same writer 

 observes, " act on the mucous membrane of the pulmonary air-cells, with a ra- 

 pidity not surpassed by their direct introduction into a vein." p. 22. 



f Chiefly by Dr. Haygarth. See his Inquiry, and also his Sketch. 



X Manchester Memoirs, vol. v. series i. 



§ Transactions of the lloyal Society of Edinburgh, 1832. 



