A terrible scourge that frequently visited the old world in the middle and dark ages is that known as the “Black Death.” As to its real character and source, the world is yet in ignorance. Whether it was readily conveyed in the atmosphere or not seems a mooted point. Modern medical science has robbed many contagious diseases of their terrors. Small-pox is easily guarded against. Diptheria has no terrors for clean streets. Yellow Jack has little chance against sound sanitation and hygiene. The germ theory of disease has greatly aided disinfective measures. In contagious diseases, infection proceeds chiefly from personal contact with a diseased person or objects that have been touched by him. In malarial diseases there is no danger from personal contact; the disease resulting clearly from a poisoned atmosphere. But in the case of what are known as epidemics, the source of infection is not clear. The disease may attack thousands in a short time, and yet not appear readily communicable by personal contact. Doubtless in these cases the atmosphere is the medium of infection. Hence, disinfective measures are of little or no use against them. So while such can not properly be classed among atmospheric phenomena, yet it would seem that in the atmosphere we find the chief vehicle of the disease. We may not here undertake any discussion of the several deadly contagious diseases that are known to modern medicine. Suffice it to say nearly all of them may be classed as filth diseases, arising from impure food or water, or filthy streets. Most notable of these is perhaps the terrible Asiatic cholera, that has swept Europe frequently, and which is now known to originate in the overcrowding and filth attendant upon the great twelve-yearly festival in honor of a Hindoo idol. Had the people of the middle ages, who regarded its ravages as a visitation of God upon them for their sins, been aware of its origin, they might have been disposed to wonder why they should be punished for the idolatry of a people thousands of miles away. Possibly such reflections might have originated either a new species of crusade, or have opened the missionary movement several centuries earlier than it really began.