120 BACTERIOLOGY. 



filled with cold water, or that cold water can constantly 

 be passed through it by means of a rubber connection 

 with a spigot. The inlet for the water should be just 

 above the bottom of the box, and the outlet just beneath 

 the top and slightly turned upward and then downward, 

 so as to insure the complete filling of the space with water. 

 The box should be sufficiently strong to resist the pres- 

 sure of the water. A convenient size is from 20 to 25 

 cm. in diameter, and of about 1.5 to 2 cm. high. It 

 is simple in construction, and can be made by any cop- 

 per spinner. An idea of its construction is given in 

 Fig. 28. 



Fig. 28. 



Metal cooling-stage. 



When gelatin or agar-agar is to be cooled it is only 

 necessary to place the dishes containing it on top of this 

 box and start cold water circulating through it. 



EsMARCH Tubes. — The modification of Koch's 

 method which insures the greatest security from con- 

 tamination by outside organisms and requires the small- 

 est supply of apparatus is that suggested by v.Esmarch. 

 It differs from the other methods thus: the dilutions 

 having been prepared in tubes containing a smaller 

 amount of medium than usual — as a rule, not more 

 than 5 to 6 c.c. — are, instead of being poured out upon 



