4 oo THE POPULAR SCIENCE MONTHLY. 



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 for- 

 ward 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 under- 

 stood by any one who has noticed a large screw in motion, or the 

 shadow of a slowly-turning corkscrew. 



Leeuwenhoek, a Dutchman, published, in 1G84, the earliest observa- 

 tions 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 uncleauliness 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 Mliller 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 ("Infusions- 

 thierchen") was published in 1838. The different forms were grouped 

 in one family, the^Vibrionidce, and, as the title shows, were still sup- 

 posed 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 

 loug 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 



