Microscopical investigations led to the belief that the source of this dire pest was a microbe, allied to _micrococcus_. If the germs of this lowly organism find a lodgment on the tender skin of a larva, they propagate with immense rapidity, and cause the death of the young insect. Then, wafted about the hive by the currents produced in ventilation, they pass from one part to another; or, attaching themselves to the bodies of adult bees, they are carried from cell to cell, and each of these thus infected, in its turn, becomes a new centre of deadly plague. Dr. Schönfeld in Germany made a series of interesting experiments, which he considered conclusive on the question of the origin and spread of this disease. From a small piece of foul-brood he propagated, by suitable means, large quantities of the fatal so-called _micrococcus_, and with it he was able to infect a healthy stock. He, moreover, established the fact that the dried germs float readily in the air. Placing some of the foul-brood in a bell-glass, in which he inserted lightly a plug of cotton-wool, he caused a gentle atmospheric current to pass into the glass, and out by the tube. Then, moistening the cotton-wool with water, and putting some of the liquid under a microscope, he detected what he concluded to be numerous spores. This circumstance throws a light on the contamination of the different hives in an apiary, through one that has become infected; as, no doubt, during the process of ventilation, many germs of the disease find their way out of the entrances. It is probable that robber-bees are also very frequently the carriers of contagion. Taking advantage of the dwindling down of a stock suffering from the disease, these plunderers pilfer the honey, and, in so doing, receive on their bodies the fatal seeds of the malady, which they then carry to their own stocks. In this way the existence of the pest in one community may become the cause of its extension throughout a neighbourhood. It is, therefore, of the utmost importance that the signs of the appearance of the evil should be constantly watched, and very stringent measures applied whenever its existence is ascertained.