The small-pox, by a treatment diametrically opposite to that which reason, and a perfect knowledge of the nature of the disease, would have dictated, hath, through a series of many ages, preserved all the virulence with which it first burst into Europe. But experience hath taught us, that, when produced by inoculation, it is much less fatal. Why? Is it because those that are inoculated are previously prepared? No: that is a very insufficient reason; for you have heard, that, in the Levant, preparation is disregarded; and also, that some of the most successful operators in this nation think it of little importance. We must therefore search for another cause; to the discovery of which let us consider, in what respect the communication by inoculation differs from that in the natural way. In the latter case the variolous _miasmata_ are conveyed into the body either with the air into the lungs, or with the saliva into the stomach: in the former, it is received into the system by means of the lymphatic vessels which are distributed over the surface of the body. There is yet another difference, perhaps a very essential one, namely, that in the natural infection, it is communicated by volatile particles, which probably may be in their nature more virulent than those which are fixed. For my own part, I am of opinion that the small-pox is a disease of the lymphatic system only, and my opinion seems to be confirmed by the impossibility of communicating the infection by inoculating with the blood. Be this as it may, it is indisputably true, that the crude lymph is sufficient to give the infection, and that there is no necessity to draw blood at the time of incision. Add to this, the frequent tumour of the lymphatic glands in the axilla soon after the operation. From these premises, it seems rational to conclude, that the general success of inoculation is chiefly to be attributed to our mixing the fixed variolous ferment with the lymph on the surface of the body, by which means the viscera, most essential to our existence, are less affected, and the poison rendered less virulent by dilution. But the wonderful success of the present practice of inoculation remains yet to be accounted for. I have told you, that the most malignant diseases may be rendered less malignant by proper treatment. The malignancy of the small-pox hath been considerably abated by inoculation, and that malignancy hath been still farther diminished by the gradual introduction of the anti-phlogistic and antiseptic plan. So that in all places where inoculation hath been long practised, and the patients thus treated, the small-pox will naturally become a mild disorder, and the ignorant operators themselves will be surprised at their unexpected success.