I am indebted to you for several favors unacknoleged. I have waited till I could inform you that some variolous after vaccine inoculations had proved that I had preserved the matter of the cow pox in it’s genuine form. Dr. Coxe of Philadelphia has ascertained this, having recieved his vaccine matter from hence. to this is added your information that the matter I sent you produced the genuine disease: and consequently those in Virginia who recieved the matter from me are now in security. knowing how little capable the people in general are of judging between genuine & spurious matter from their appearance, or that of the sore, I endeavored in the course of my inoculations at home to find some other criterion for their guide. with this view I was very attentive to discover whether there be not a point of time counted from the vaccination, when the matter is genuine in all cases. I thought the 8. times 24 hours furnished such a point; I governed myself by it, and it has been followed here succesfully by Dr. Gant. but your experience, so much greater, can inform us whether this rule is a sure one: or whether any other point of time would be still more certain. to the eye of experience this is not necessary: but for popular use it would be all-important: for otherwise the disease degenerates as soon as it gets into their hands, and may produce a fatal security. I think some popular criterion necessary to crown this valuable discovery. Accept assurances of my great esteem & respect.