SECT. XXXIII. 2. DISEASES OF SENSATION. 457 



' have not experienced the abforptions of coramoa 

 matter. 



It is probable, that more certain proofs might 

 have been found to fhew, that common matter is 

 infectious the fir ft nine it is abfnrbed, tending to 

 produce fimilar abfceffes, but not the fecond time of 

 its abforption, if this fubject had been attended to. 



8. 'I hefe contagious difeafes are very numerous, 

 as the plague, fmall-pox, chicken-pox, meafles, fcar- 

 let-fever, pemphigus, catarrh, chincough, venereal 

 difeafe, itch, trichoma, tinea. The infectious ma- 

 terial does not feem to be diflblved by the air, but 

 only mixed with it perhaps in fine powder, which 

 foon fubiides ; fince many of thefe contagions can 

 only be received by actual contact, and others of 

 them only at fmall diftances from the infected per- 

 fon, as is evident from many perfons having been 

 near patients of the fmall-pox without acquiring the 

 difeafe. 



The reafon why many of thefe- difeafes are re- 

 ceived but once, and others repeatedly, is not well 

 underflood ; it appears to me, that the conftitution 

 becomes fo accuftomed to the iiimuii of thefe in- 

 fectious materials, by having once experienced them, 

 that though irritative motions, as hectic fevers, may 

 again be produced by them, yet no fenfation, and 

 in confequence no general inflammation fucceeds ; 

 as difagreeable fmells or taftes by habits ceafe to be 

 perceived ; they continue indeed to excite irritative 

 ideas on the organs of fenfe, but thefe are not fuc- 

 ceeded by fenfation. 



There are rn.my irritative motions, which were 

 at firft fucceecled by fenfation, but which by fre- 

 quent repetition ceafe to excite fenfarion, as explain- 

 ed in Sect. XX. on Vertigo. And, that this circum- 

 ftance exifls in refpect to infectious matter appears 

 from a known fact ; that nurfes, who have had the 

 fmall-pox, are liable to experience fmall ulcers on 



their 



