From this account it is not difficult to conceive that animals, though exceedingly small, may yet emit such poisonous effluvia as will destroy much larger ones in their neighbourhood. It will by no means be incredible that, had one or two such offensive animals been thrown into a jar containing gold-fishes,[86] the whole of these beautiful inhabitants would have perished at once. Let us suppose such a thing to have actually happened; that a malicious person had put them in over night, and in the morning the proprietor of the fishes finds them all dead, and the water offensive to the last degree. He sends for a neighbouring philosopher, who, happening to be ignorant of the existence of such animals, endeavours to account for the phenomenon upon some of the received principles of philosophy. How much theory would here be wasted, and what endless disputes might ensue without even a _possibility_ of arriving at the truth! Just so it is with epidemic diseases. The cause is invisible, and, until it becomes discoverable by our senses, it can never be known; for, as has already been observed, a cause never can be known merely by its effects, unless we have seen it, or somebody who has seen it gives us information. And this will certainly be found to hold good in every instance, even from the Supreme Cause himself to the diminutive insect just mentioned. [86] The gold-fish is a small species of carp, brought originally from China. They are adorned with the most beautiful and resplendent colours, and are frequently kept in jars for pleasure. They subsist entirely on the water, without any other food. This is by Dr. Fordyce said to be the case with all fishes, provided the water be impregnated with oxygen. Lastly, I shall consider another _possible_ source of epidemics, which has been hinted at by others. Allowing that infectious matter proceeds from the body of a diseased person, as much must issue from a single patient as is sufficient to bring the disease upon thousands, and with regard to the small-pox and some other distempers we certainly know that it is so. This infection is dissipated in the atmosphere, and intimately combined with it, so that it becomes imperceptible and harmless; but we have no reason to suppose that it is annihilated, or cannot be re-produced in its pristine state. Water, though perfectly dissolved, and to appearance deprived of existence in the air, may yet be precipitated from it, and pour down upon us in deluges. What happens in one case may happen in another. The infectious matter, dissolved in the air, may by some natural cause be precipitated from it, overshadowing whole regions, and, if it be not powerful enough to produce the epidemic of itself, may certainly _predispose_ to it in such a degree, that the slightest additional cause will bring it on.