Upon Jenner’s assertion that Cowpox was unaltered by transmission from arm to arm, Pearson remarked, “The fact remained to be proved.” That Cowpox produced a harmless ailment was not, he thought, to be hastily assumed. Dr. William Heberden had recently inoculated 800 poor persons at Hungerford without a mishap, and 1700 had passed through Dr. Woodville’s hands in the current year (1798) with only two deaths; yet how erroneous would be to argue that variolous inoculation was harmless from such special experience! Such instances of success can only be attributed to a certain favourable epidemic state of the human constitution itself, existing at particular times, for the proportion of deaths from inoculation is usually much greater, owing, probably, to certain unfavourable epidemic states. (P. 67.) If Cowpox remained unchanged in transmission from arm to arm, it would be no harmless ailment; for the evidence was distinct that it was frequently a severe one. For example, Edinburgh told Pearson that when suffering from Cowpox he had to give up work and go into an hospital; and Grimshaw that the disease was uncommonly painful, with swellings in his armpits, sore to the touch; and the servant at Rhodes’s farm in the Hampstead Road, who had seen much Cowpox in Wiltshire and Gloucestershire, said the milkers were sometimes so ill that they had to keep their beds for several days, though none ever died of the Cowpox fever. If, however, by transmission from arm to arm, Cowpox became milder, it was not improbable that at the same time it would lose more or less of its protective efficacy. Pearson might have seen and added, that resistance to inoculated Smallpox, when the constitution was in no humour for Smallpox, was no proof that the same constitution would resist Smallpox when epidemic, or in condition for the evolution of the disease. Pearson likewise took objection to Jenner’s evidence (such as it was) that it was possible to take Cowpox after Cowpox, but not Smallpox after Cowpox; saying-- Most of professional men are extremely reluctant in yielding assent to this statement. Some, indeed, reject it in the most unqualified terms. That Cowpox follows Cowpox appears certain, but that Cowpox should avert Smallpox, and not avert itself appears incredible. (P. 44.) Here we see Pearson on the verge of discovery of the illusion, but with all his training and Yorkshire shrewdness he lost the scent, and allowed himself to be deceived; and not only deceived, but to become a prime mover in the deception of the world. Jenner felt the difficulty and replied-- CHELTENHAM, _27th September, 1798_.