66 



LABORATORY EXERCISES IN BACTERIOLOGY. 



(a) Potato Dishes. These are large flat dishes, usually about twenty centimeters in 

 diameter and about eight centimeters in height, with straight sides, and fitted with covers 

 of the same shape as the dish itself, but of slightly greater diameter, so as to permit the 

 cover to easily fit down over the dish (Fig. 18). They are used to protect the large slices 

 of potatoes often used as culture material or the sets of plates in plate cultures. Be- 

 cause of their size and the quality and texture 

 of glass used in their manufacture they cannot 

 well be subjected to heat in sterilization. 

 Therefore, after having been well washed and 

 cleansed after the method detailed for test- 

 tubes (omitting the boiling), it is customary 

 to place in the bottom of each dish a layer of 

 ordinary filter paper (intended to retain 

 moisture for cultures), after which the dish, 

 and, too, the cover, are filled level full with 

 a disinfectant solution (1:1000 solution of 

 mercuric chloride) , which is allowed to remain 

 therein for at least one hour (in practice often 

 over night). The sublimate solution is then 

 poured out and the cover at once adjusted, 

 after which the potatoes or plates may be 

 introduced. There are numerous opportuni- 

 ties, in the adjustment of the cover and at 

 tj.me of introduction of potatoes and plates, 

 for the contamination of the interior by or- 

 ganisms from the air, and the protection of 

 the media by such dishes is therefore not to 



be compared with that obtained in tubes or flasks ; their use, therefore, is by no means 

 so frequent as formerly. Care should be exercised, in the selection of the dishes and 

 covers, that the contact between the cover and the edge of the dish be as perfect as 

 possible. * 



(6) Petri Dishes. These dishes (Fig. 19), of the same general appearance as the 

 large culture dishes, but much flatter, are usually 

 eight or ten centimeters in diameter and one 

 and a half to two centimeters in height. They 

 were introduced to take the place of the plates 

 employed in the separation of bacteria in impure 

 cultures, and are of particular service, as they 

 permit the examination of the gross colonies of 

 organisms developing in the interior by means 

 of the microscope, the unopened dish being 

 placed upon the stage of the instrument and 

 examined with the low powers of the microscope. 

 The same precautions in the cleansing of these 



dishes are to be observed as detailed for tubes. The covers are then applied to the 

 dishes and the whole sterilized in the dry-air oven by the fractional method (the whole 

 process should be performed, as there is usually no opportunity for further steriliza- 

 tion after the introduction of the culture medium). It will be found of advantage 

 to fold the dish and cover in wrapping paper before sterilization, the paper being allowed 



FIG. 20. METAL Box FOR HOLDING 

 GLASS PLATES DURING STERILIZA- 

 TION IN OVEN. 



FIG. 



21. SET OF CULTURE PLATES 

 AND PLATFORMS. 



