The .MiciioscoPE. 171 



heating, either upon the freshly-cnt surface of a boiled potato, 

 which is then covered by a bell-glass, or into a test-tube partly tilled 

 with fluid gelatine, which is tirst shaken thoroughly so as to dis- 

 tribute the introduced germs as much as possible, and is then poured 

 upon a glass plate Avhere it hardens, and is also covered by a bell- 

 glass. In either case the introduced organisms, rapidly multiplying 

 by self-division, form small colonies, each original germ being the 

 starting point of one. Up to this point, the admixture of foreign 

 and undesired germs floating in the atmosphere, is unavoidable. It 

 is, however, an interesting and vei-y valuable fact, that the colonies 

 respectively formed by different genera, and even species of bacteria 

 and their allies, present marked differences of appearance even to 

 the naked eye, so that there is little liability to error from this 

 soiu'ce. After the colonies have grown sufficiently to enable them to 

 be identified, a test-tube partially tilled with a solidified preparation 

 of sterilized gelatine, agar-agar, or similar substance is quickly 

 inoculated by introducing with a needle-point a minute quantity of 

 material from what has been ascertained to be the desired colony on 

 the potato or glass-plate. The test-tube is then closed by a wad of ster- 

 ilized cotton or glass-wool and is placed in the incubator at the tem- 

 perature best suited to the contained organisms. The growth of 

 the latter is rapid and also distinctly peculiar in the different species? 

 so that an experienced investigator, by holding to the light a tube 

 containing a pure culture of such organisms, can determine the 

 species merely by the appearance of the colony, which sometimes 

 spreads over the top of the gelatine in the tube; sometimes grows 

 only in the path made by the needle, and in other cases takes the 

 form of a spiral, a nail, a bunch of grapes, etc. Throughout the 

 entire process the utmost care is taken to prevent the introduction 

 of germs other than the one to be studied. Every portion of the 

 apparatus and the culture-media used are sterilized with the greatest 

 precau^tion and even the hands of the investigator are bathed in ger- 

 micide solutions at all the important steps of the procedure. When 

 a perfectly pure culture of some germ has been thus obtained, the 

 further study of its characteristics, both in the colony and under 

 the microscope, becomes comparatively easy, and valuable experi- 

 ments of inoculation upon living animals, etc., are made possible. 

 The immensely valuable results already obtained by Pasteur, Koch and 

 many others, are a guarantee of what may be reasonably hoped for 

 in the near future by the study of a subject, the immense importance 

 of which can hardly be over estimated. 



