tory, sinuous motion. Another variety, less frequently found, is larger, and has the form of a corkscrew, with from one and one-half to three or even four complete turns. These spiral forms are perfectly rigid, and rotate about their long axis with great rapidity, moving rapidly forward and backward with the regularity, although not the deliberation, of a pendulum. Their rotation about the long axis gives rise to the appearance of a wavy, serpent-like motion, which has deceived many observers, and it is very probable that the smaller vibriones mentioned above owe their apparent sinuous motion to the same cause. The ease with which one may be deceived on such a point will be readily understood by any one who has noticed a large screw in motion, or the shadow of a slowly-turning corkscrew.
Leeuwenhoek, a Dutchman, published, in 1684, the earliest observations of bacteria of which we have any record. He found them chiefly in the matter picked from between his own teeth and those of his acquaintances, and, animated apparently by the same spirit for which his countrywomen are so noted, he defends himself against a possible charge of uncleanliness by mentioning that he habitually brushed his teeth after every meal; but he also records that he found the largest quantity of bacteria between the uncared-for, broken teeth of an old man. He supposed these bacteria to be animals, and, indeed, gave the name eels to some large ones which he found in vinegar, whose motions were so active that he was "obliged to kill one before the limner could portray it." In the eighteenth century Müller made a classification of the forms then known, but it was not until after the great improvements made in the construction of microscopes, about 1820, that Ehrenberg gave the complete description and classification which have served almost until the present day. His book ("Infusionsthierchen") was published in 1838. The different forms were grouped in one family, the Vibrionidæ, and, as the title shows, were still supposed to be animals.
To-day they are known to be plants, and the different varieties are supposed by many observers to represent only different periods of development. Robin asserted several years ago that the ordinary rod bacteria could develop into the long thread bacteria, and even into the long filaments of leptothrix found so constantly in the mouth; but this was not generally believed, and the latest complete classification, that of Ferd. Cohn, published in 1872, is based upon the absence of such a developmental relationship. The study of these plants is rendered very difficult by their extreme smallness, and all attempts to cultivate them under the microscope, in "wet chambers," have failed to disclose the secrets of their growth, on account of the abnormal conditions in which they are necessarily placed. A considerable depth of liquid seems to be essential, as do also the presence of air, and protection against shocks or jars, and movement of the liquid; consequently, they can be studied only by comparing the forms found at intervals during
