128 LABORATORY EXERCISES IN BACTERIOLOGY. 



taking the plate out of the culture dish and thus exposing it to atmospheric contamina- 

 tion. To offset these objections the small flat dishes known as Petri dishes have been 

 introduced. They are used for precisely the same purposes as the plates and may 

 be thought of as individually covered plates. In their employment the same manipu- 

 lations are to be practised as in case of the preparation of plate cultures; but there 

 is not the same need of a perfectly level surface in spreading the medium over the 

 bottom of such a dish, nor is it requisite that the medium should be immediately 

 solidified by cold, as is done in case of plate cultures, to prevent the liquid medium 

 from flowing over the edges of the plate. The only objectionable feature in their 

 use a very real objection, but not nearly so serious as when sets of plates are being 

 arranged in a large culture dish is the chance of entrance of contaminating organisms 

 from the air when the lid of the dish is raised for the introduction of the liquefied 

 medium. From imperfect fitting of the lid to the dish, particularly in careless hand- 

 ling, there is some danger of entrance of bacteria from the outside; but, as already 

 suggested, this may be largely obviated by sealing the edge of the cover over the 

 dish by a strip of paper or rubber band or other device. 



Esmarch's Tubes (Rolled Tubes). As a further development of the same 

 principle Esmarch suggested the absence of need of exposure of the medium, either 

 in dish or on plate, if the liquefied material after inoculation be spread over the interior 

 surface of the tube itself and solidified in this position. This idea leaves little to be 

 desired if the amount of the medium and the size of the tube be properly proportioned. 

 For reasons before mentioned gelatine lends itself better than the other media for 

 the purpose involved. Obviously, however, tubes of this form in which gelatine 

 is used as the medium should not be exposed to temperatures above 25 to 28 C., 

 lest the gelatine be liquefied. Rolled tubes of gelatine-agar or of plain agar may, 

 however, be incubated without danger. In preparing a rolled tube a block of ice 

 should be obtained, and its upper surface cut nearly, but not quite, level. A small 

 groove should be cut in the ice, leading from the higher edge over the slightly sloping 

 surface and its irregularities melted away by applying a tube of hot water in the groove. 

 The tube of liquefied gelatine is now inoculated, the infectious matter well diffused 

 by agitation, and it is then laid in the groove on the ice block, care being taken that 

 the gelatine does not come too close to the stopper. Here it is steadily rolled around 

 in the groove until the whole interior surface from the bottom to within about half 

 an inch of the stopper is evenly coated with a film of the gelatine and the latter has 

 become solid. The same end may be accomplished by twisting the tube held in a 

 nearly horizontal position under a tap of cold water, but the first method is more 

 satisfactory. 



In such a tube all the advantages of isolation of the colonies 'for transfer to fresh 

 media, and for enumeration or examination, exist as in case of plates or Petri dishes, 

 and the procedure is to be strongly commended. 



Flasks. What has been said of tubes applies to flasks, with whatever difference 

 the size of the latter may occasion. The flask cultures are only used when large quan- 

 tities of some isolated species are desired. 



ILLUSTRATIVE PROCEDURES. 



Examination of Air. The mere fact of the presence of microbic life in the atmos- 

 phere is almost axiomatic, and has been illustrated by the experiences gained in exercise 

 5 or 23, or by the decomposition of media or of organic substances used in our daily 



