188 BACTERIOLOGY. 



small number of colonies to be of value for further study. 

 It is, therefore, advisable to destroy this plate as soon as 

 it has been found to be of no value. It should be placed in 

 the solution of mercuric chloride, for at least over night. 



The object of the plate method, it will be remembered, 

 is to secure a perfect separation of the various bacteria 

 that may be present. The farther apart these organisms 

 are, at the time the gelatin solidifies, the better it will be 

 for subsequent study. On a good plate the colonies will be 

 5-10- mm. apart. They can be then transplanted without 

 any risk of touching a neighboring colony. 



Plate No. 2 will sometimes show a successful dilution. 

 This will often be the case, if but one loopful of gelatin is 

 transferred. At other times, the colonies on this plate 

 may be so numerous as to make it of but little value. 



Plate No. 3 should show perfectly distinct and well 

 separated colonies. When studying and transplanting 

 these growths, care should be taken not to pick out an 

 accidental colony which owes its origin to some organism 

 that dropped down from the air. Such a foreign colony is 

 usually large, and grows rapidly; moreover, it is likely to 

 be the only one of its kind on the plate. A single large 

 colony, especially if unlike the others in appearance, 

 should always be avoided. This is true when working with 

 pure cultures. When, however, some blood or a portion of 

 an organ is used for making plates the single colony in that 

 case may be of great importance. 



After having made a careful study of the various col- 

 onies with the unaided eye, the plates should then be given 

 a careful microscopic examination. The plate should be 

 placed upon the stage of the microscope and the colonies 

 closely studied under a low-power, the No. 3 objective. 

 Further characteristics can thus be brought out which have 

 escaped the eye. 



A study of the micro-organisms which compose the 

 colonies should now be made. This is done by making 



