As a proof of the efficacy of this practice, even before the virus has fully exerted itself on the system, I shall lay before my reader the following history: 

By a reference to the treatise on the Variolae Vaccinae it will be seen that, in the month of April, 1798, four children were inoculated with the matter of cow-pox, and that in two of these cases the virus on the arm was destroyed soon after it had produced a perceptible sickening.  Mary James, aged seven years, one of the children alluded to, was inoculated in the month of December following with fresh variolous matter, and at the same time was exposed to the effluvia of a patient affected with the smallpox.  The appearance and progress of the infected arm was, in every respect similar to that which we generally observe when variolous matter has been inserted into the skin of a person who has not previously undergone either the cow-pox or the smallpox.  On the eighth day, conceiving there was infection in it, she was removed from her residence among those who had not had the smallpox.  I was now anxiously waiting the result, conceiving, from the state of the girl’s arm, she would fall sick about this time.  On visiting her on the evening of the following day (the ninth) all I could learn from the woman who attended her was that she felt somewhat hotter than usual during the night, but was not restless; and that in the morning there was the faint appearance of a rash about her wrists.  This went off in a few hours, and was not at all perceptible to me on my visit in the evening.  Not a single eruption appeared, the skin having been repeatedly and carefully examined.  The inoculated arm continued to make the usual progress to the end, through all the stages of inflammation, maturation, and scabbing.

On the eighth day matter was taken from the arm of this girl (Mary James) and inserted into the arms of her mother and brother (neither of whom had had either the smallpox or the cow-pox), the former about fifty years of age, the latter six.

On the eighth day after the insertion the boy felt indisposed, and continued unwell two days, when a measles-like rash appeared on his hands and wrists, and was thinly scattered over his arms.  The day following his body was marbled over with an appearance somewhat similar, but he did not complain, nor did he appear indisposed.  A few pustules now appeared, the greater part of which went away without maturating.

On the ninth day the mother began to complain.  She was a little chilly and had a headache for two days, but no pustule appeared on the skin, nor had she any appearance of a rash.

The family was attended by an elderly woman as a nurse, who in her infancy had been exposed to the contagion of the smallpox, but had resisted it.  This woman was now infected, but had the disease in the slightest manner, a very few eruptions appearing, two or three of which only maturated.