Experiments refpeSiing the Vaccine Difeafe, 325 

 difeafe : and the demonflration in courfe could be given of 

 propofition I, on the ground of the 3d and 3d propojitioiiy 

 that unfufceptibility of the conftitution to the agency of the 

 variolous poifon is produced by the variolous difeafe, if this 

 were not already proved by abundant experience. At a fu- 

 ture time, however, I (hall relate the obfervations and expe- 

 riments to confirm this a priori conclufion 5 ift, becaufe 

 thefe proofs will increafe the validity of the 3d propofition ; 

 and adly, becaufe I do not mean to offer this demonftration 

 as Infallible, like mathematical. 



From the preceding reafonlng it may be imagined, that I 

 confider the cow-pox and fmall-pox as only varieties of the 

 fame fpecies of difeafe ; and that therefore the name variola 

 vaccina is appropriate, although I endeavoured to fliow that. 

 it was unjuft, and tended to miflead, by giving erroneous 

 notions, {Inquiry^ p. io8)|| But it niuft here enter into our 

 contemplation, that the fame ftate of an animal or other fub- 

 ftance, in a certain refpeft, may be produced by very differ- 

 ent things ; and the phenomena attending their agency may 

 be very different from one another. It is fo in the inftances 

 under confideration j and further, in order to ellablifli refem- 

 bling things to be varieties of the fame f.ecies, we ou<iht to 

 be able to trace them to one common orig-in, or to fliow 

 that they all agree in what fliould be reckoned effential pro- 

 perties. Now hitherto it has not appeared that the cow- 

 pox has arifen from the fmall-pox, or the fmall-pox from 

 the cow-pox. If it be faid, that in fonie of the eruptive in- 

 flancesof the cow-pox, the puftules could not be diftinguifh- 

 ed from the fmall-pox, it (hould be confidered that it has 

 not been yet (liown, that in any cafe the fmall-pox has 

 changed into the cow-pox ; — that the cow is fufccptible by 

 moculation of the human matter of the cow-pox, but not of 

 the fmall-pox ; and that the pulhiies refembli;)g the fmall- 

 pox, which occur in the cow-pox, afford matter which, I be- 

 lieve, produces in foine cafes (if not, [)erhaps, in fo great a 

 proportion as originally) the cow-pox m its ufual miM way, 

 Y 3 t'ii. 



