His opinions on the subject were, at the time of their publication, quite original; and as they are such as are usually held, to this day, by most intelligent practitioners, the most important will be transcribed here. In a letter to Dr Percival, on the prevention of infectious diseases, published in 1801, Dr H. says—“I have long thought that there is no subject on which a physician could employ his time and ability more advantageously for the benefit of his fellow-creatures, than in the investigation of febrile contagion, in order to ascertain the laws by which it is communicated, and by what means it may be prevented. It is well known to be the cause of very extensive destruction in the army, the navy, and in large towns.” “In 1777 I began to ascertain, by clinical observations, (_i. e._ observations made at the bedside of a patient,) according to what law the small-pox infection, and, in 1780 and 1781, according to what law the febrile infection, is propagated.”—“I found that the pernicious effects of small-pox miasms (that is, airs or vapours) were limited to a very narrow sphere. In the open air, and in moderate cases, I discovered that the infectious distance does not exceed half a yard.”—“Hence it is probable that, even when the distemper is malignant, the infectious influence extends to but a few yards from the poison.”—“I soon also discovered, that the contagion of fevers was confined to a much narrower sphere.” “You will recollect, my dear friend, that at this time (1781) my attention was much engaged in the investigation of the nature of the small-pox poison. I was struck with the difference of the periods in those two maladies during which the infection remains in a latent state, that is, the interval of time which elapses between the patient’s exposure to the pestilential influence and the commencement of the fever. In the typhus, this period appeared to be much longer than in small-pox.” The period between the exposure to what is considered infection, and the period of the manifestation of disease, certainly does vary in different distempers. In those in which palpable contagious poisons are produced, and where they are palpably applied to the system, the interval is known, and seldom varies; but in those where a palpable poison is not recognised, or where it is said to act exclusively through the air, it is found that the interval is sometimes short, sometimes long, and manifests none of that precision almost always observed in reference to the first class of diseases. Dr Haygarth again says, “When the room of a patient ill of an infectious fever is spacious, airy, and clean, few or none of the most intimate attendants will catch the disease.”