106 TEXT-BOOK OF BACTEKIOLOGY. 



facilities for judging the colonies in connection with their liquefying 

 power, as the gelatin. As a whole, therefore, the agar plates are 

 less useful for studying peculiarities of growth in the diilerent 

 species of bacteria upon a solid food medium. 



It would be almost impossible to overrate the importance of 

 the glass-plate system for our investigations. ' The glass plate is 

 the invaluable, altogether indispensable means which leads us 

 safely through the intricate world of bacteria, discovers the finest 

 differences of species, and is able to give an answer to the most 

 difficult questions at all times. 



The more we advance in practice and experience the more we 

 learn to prize this method, which is distinguished at once \)y the 

 certainty of its operation, the rapidity with which it accomplishes 

 its purpose, the ease of its manipulation, and the almost unlimited 

 extent of its applicability. 



These advantages are in fact so apparent that one can scarcely 

 comprehend how some investigators can ignore them. He who 

 brings substances known or supposed to contain a mixture of bac- 

 teria into a solid culture medium and then leaves them to their fate, 

 shows clearly that he does not, or perhaps will not, understand the 

 chief advantage of a plate — i.e., the separation of germs and the 

 consequent opening of a possibility to each one germ of attaining its 

 fair development without being crowded or overgrown by others. 



Take this case : we are seeking in some organ — say the lungs — 

 a particular species of bacteria, and we put portions of lung tissue 

 into a test-tube containing gelatin or agar in order to let the 

 germs develop. Perhaps there may be only ten of them altogether, 

 and of the ten only two belonging to the species sought, the others 

 being indifferent to us. The plate is sure, in all cases, to present 

 these two, and to present them in their characteristic forms. If 

 they remain with the others in the test-tube without being poured 

 out on a glass plate or Petri dish, the two may not be able to main- 

 tain themselves against the other eight and will be lost for our 

 observation. We then obtain an altogether false result from our 

 examinaton; we have indeed employed the solid food medium, but 

 we have obtained no advantage from it. 



VII. PURE CULTURES. 



The next step must be to definitely separate from each other 

 the diflferent species which the plate process has enabled us to dis- 

 tinguish. 



The plates remain good for a limited time only, and the liquefy- 



