ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 343 



bottom is perfectly flat and of equal thickness, and permits of the 

 media being more evenly spread. They stand sterilization perfectly 

 well, and the bottom is clearly visible up to the edge. I have by 

 numerous experiments ascertained that tin does not appreciably affect 

 the growth of bacteria ; aluminium is less suitable, but duralumin could 

 be used instead of tin, if sufficiently thin rims could be made with this 

 alloy.* 



3. A more expensive form of metal rim has also been made for me, 

 which consists of two concentric rings of well-tinned metal (block tin or 



Fig. 23. Fig. 24. 



1^^ - "^' 1 ^^=~n 



1. One half of a Petri dish with a metal lid to replace the other 



half. The unevenness of the bottom of the Petri dish is 

 exaggerated. 



2. A plating dish made by forcing a glass plate into a flexible 



metal rim. 



duralumin would be better) : the inner is screwed into the outer, wliich 

 is provided with a flange, against which the glass bottom plate is fixed 

 when the inner rim is screwed home (fig. 25). The advantage of this 

 form, which is more expensive, is that the glass plate can be replaced 

 easily when broken. 



•Jr. A much older design (which I used as far back as 1887, in order 

 to obtain even layers of cultivating media, and plates more suitable for 

 photographic purposes than those made in Petri dishes) is even simpler, 

 but somewhat less convenient than the previous forms. It consists of 



Fig. 25. Fig. 26. 



3 4 



3. A " plating dish " made by fixing a glass plate by screwing 



two metal rings together. 



4. A "plating dish" made by fixing a rim of metal to a glass 



plate by means of coagulated albumen or collodion. 



a square or round tin frame ; one surface is perfectly flat, and is fixed 

 to a bottom glass plate by means of coagulated blood serum, collodion, 

 or oxidized linseed oil (fig. 26). 



To make a plating dish with such a frame, a perfectly clean glass 

 plate of suitable size and shape is placed over a metal plate heated to 

 the temperature of boiling water. The surface of the frame which is to 

 be fixed to the glass is painted evenly with fresh blood serum or white 



* The Action of some Metals upon Certain Water and other Bacteria. Journ. 

 Boy. San. Inst., xxxv. No. 6 (1914). 



