TEXT-BOOK OF BACTERIOLOGY. 89 



made, in the most different quarters, to facilitate this part of the 

 process, is a clear proof that a thoroughly satisfactory manner of 

 proceeding has not yet been found. By experience, we have found 

 it best to pass it through filter paper — through an ordinary folded 

 filter. All other means — cotton-wool, glass-wool, etc. — are of little 

 use. The filtration takes place in the steam generator or with the 

 hot-water funnel. Both these aids may be dispensed with if only 

 the precaution is taken of " previously wetting the filter with boil- 

 ing water." If this is not done, the inside of the paper becomes 

 covered with a thin coating of solidified agar and the filtration is 

 stopped in its verj' beginning. 



The process of filtration is one that always requires a great 

 amount of patience and perseverance. It is better, in many cases, 

 to avoid filtration altogether. 



The food agar is used in a manner somewhat different from gel- 

 atin ; it is chiefly its surface that we employ as a basis of develop- 

 ment for bacteria, and it is not necessary to be so particular about 

 its complete and faultless transparencj'. Pour the agar out of the 

 flask in which it has been boiled and cleared from the sediment 

 into smaller vessels, whence it can be poured out for use as occa- 

 sion may require. Or, again, pour the whole mixture, after several 

 hours' boiling, into tall, narrow cylinders — measuring glasses— 

 which are then placed for some time in the steam generator. Here 

 the opaque particles settle down more or less completely, and por- 

 tions can now be removed from the upper transparent strata with 

 pipettes for immediate use. 



The agar obtained in this manner, or the one previously de- 

 scribed, is next poured, in quantities of about 10 c.cm., into sterilized 

 test-tubes, and made thoroughly germ-free by three heatings of 

 half an hour each in the steam generator. 



The agar is allowed to solidify in an oblique position, in order 

 to obtain a large surface for use. A small quantity of water 

 always separates and remains at the bottom of the glass, and does 

 not disappear by evaporation for some time. 



Even the best, most carefully-filtered agar, which in its liquid 

 state was perfectly transparent, generally becomes somewhat cloudy 

 and opaque as soon as it hardens, but this appearance, if it occurs, 

 is of no moment as indicating a fault in the process of preparation. 

 As in the case of the bouillon gelatin, so also in the case of 

 bouillon agar, numerous additions are made for special purposes. 

 Such are grape sugar, formate of soda, resorcin, litmus, etc. For 

 observing the process of reduction, for instance, add to a litre of 

 prepared agar about 40 c.cm. of a saturated solution of litmus. 



