EDWARD JENNER 151 



They who are not in the habit of conducting experiments may not 

 be aware of the coincidence of circumstances necessary for their be- 

 ing managed so as to prove perfectly decisive; nor how often men 

 engaged in professional pursuits are liable to interruptions which dis- 

 appoint them almost at the instant of their being accomplished. 



[However, I feel no room for hesitation respecting the common origin 

 of the disease, being well convinced that it never appears among the 

 cows (except it can be traced to a cow introduced among the general 

 herd which has been previously infected, or to an infected servant), 

 unless they have been milked by someone who, at the same time, has the 

 care of a horse affected with diseased heels. 



The spring of 1797, which I intended particularly to have devoted 

 to the completion of this investigation, proved, from its dryness, re- 

 markably adverse to my wishes; for it frequently happen^, while the 

 farmers' horses are exposed to the cold rains which fall at that season 

 that their heels become diseased, and no Cow-pox then appeared in the 

 neighbourhood.] 



The active quality of the virus from the horses' heels is greatly 

 increased after it has acted on the nipples of the cow, as it rarely hap- 

 pens that the horse afifects his dresser with sores, and as rarely that 

 a milk-maid escapes the infection when she milks infected cows. It 

 is most active at the commencement of the disease, even before it 

 has acquired a pus-like appearance ; indeed I am not confident whether 

 this property in the matter does not entirely cease as soon as it is 

 secreted in the form of pus. I am induced to think it does cease, and 

 that it is the thin darkish-looking fluid only, oozing from the newly- 

 formed cracks in the heels, similar to what sometimes appears from 

 erysipelatous blisters, which gives the disease. Nor am I certain 

 that the nipples of the cows are at all times in a state to receive the 

 infection. The appearance of the disease in the spring and the early 

 part of the summer, when they are disposed to be affected with spon- 

 taneous eruptions so much more frequently than at other seasons, 

 induces me to think that the virus from the horse must be received 

 upon them when they are in this state, in order to produce effects; 

 experiments, however, must determine these points. But it is clear 

 that when the Cow-pox virus is once generated, that the cows cannot 



