TEXT-BOOK OF BACTERIOLOGY. 85 



mains opaque. Sometimes the reason is that it was boiled too long- 

 or too violently after the neutralization, so that substances have 

 been dissolved which remain in the liquid as obstacles to transpar- 

 ency, and cannot be held back by the filter. The same fault is also 

 seen when the opposite mistake has been made, when it has been 

 heated for too short a time— i.e., not long enough to separate all 

 the coagnlable albuminous matter. 



It is not very easy to overcome these difficulties. The best way 

 is to let the gelatin run through the filter, cloudy as it is, and then 

 endeavor to make it clear. To do this, add some uncoagulated 

 albumin [the best being the white of a hen's egg] to the solution 

 and then boil again. Soon the white of egg curdles, it conglobates 

 and carries away the suspended particles with it. Now filter once 

 more, and a clear and beautiful gelatin will be obtained. The suc- 

 cess of this measure is, in fact, so sure and perfect that, as a rule, 

 we do not wait to see whether the gelatin clears unaided, but in all 

 cases, about one and a half hours after the neutralization, we add 

 the white of an egg and then boil again for another half -hour. 



One other case may be cursorily mentioned in which the gelatin 

 may be spoiled. If the solution be poured into new, unused glasses, 

 sometimes it will be found that it afterward becomes cloudy again. 

 This is because the glasses, as they come direct from the glass- 

 works, often contain some traces of alkali; their surface is not 

 chemically neutral, as may easily be proved, and this small quantity 

 of alkali suffices in certain cases to cause loss of transparency in 

 the gelatin. It may happen, for instance, that of 100 test-tubes 

 with gelatin, all prepared in the same manner, 20 or 30 afterward 

 become cloudy, while the rest remain clear. It is therefore desira- 

 ble to clean the glasses, for the first time, with acidulated water. 



When all these difficulties above enumerated have been over- 

 come, pour the clear, transparent gelatin into sterilized test-tubes, 

 not allowing the gelatin to touch the mouth of the tube where the 

 stopper comes, else the cotton-wool sticks to the glass and gives 

 trouble afterward. 



Of course the gelatin can be used in bottles, in Erlenmeyer's 

 flasks, or in dishes, as well as in test-tubes. In all cases, however, 

 the gelatin must be thoroughly sterilized. This, of course, is done 

 in the steam generator. Yet it has been found that gelatin will 

 not bear the prolonged heating which would be necessary to make 

 it germ-free by one single operation ; it is apt to lose its capacity 

 of solidification, in which case it becomes useless for our purposes. 

 We therefore put it, for three successive days, into the steam gen- 

 erator, each time for twenty-five minutes. 



