I consider that the want of attention to the circumstance of the concentration of decaying emanations is a great reason that the effects of miasmata in producing fever is still a _questio vexata_. Thus there may be many church-yards and sewers evolving decaying matter, and yet no fever may occur in the locality. Some other more modified effect may be produced, according to the degree of concentration of the decaying matter, such as diarrhœa or even dysentery; or there may be no perceptible effects produced, although the blood may still be thrown into a diseased state which will render it susceptible to any specific contagion that approaches. It must be remembered that decaying exhalations will not always produce similar effects, but that these will vary not only according to the concentration, but also according to the state of decomposition in which the decaying matters are. The rennet for making cheese is in a peculiar state of decay, or rather is capable of a series of states of decay, and the flavour of the cheese manufactured by means of it varies also according to the state of the rennet. Just so with the diseases produced by the peculiar state or concentration of decaying matters or of specific contagions. When the Asiatic cholera visited this country many of the towns were afflicted with dysentery before the cholera appeared in an unquestionable form. In like manner the miasmata evolved from church-yards may produce injurious effects which may not be sufficiently marked to call attention until they assume a more serious form by becoming more concentrated. But notwithstanding the absence of marked effects, it is extremely probable that constant exposure to miasmata may produce a diseased state of the blood. Thus I had occasion to visit and report upon, amongst other matters, the state of slaughter-houses in Bristol. These are generally situated in courts, very inefficiently ventilated, as all courts are. I remarked that the men employed in the slaughter-houses had a remarkably cadaverous hue, and this was participated in a greater or less degree by the inhabitants of the court. So much was this the case, that in a court where the smells from the slaughter-house were so offensive that my companion had immediately to retire from sickness, I immediately singled out one person as not belonging to the court from a number of people who ran out of their houses to inquire the object of my visit. The person who attracted my attention from her healthy appearance compared with the others, had entered this court to pay a visit to a neighbour.