LABORATORY WORK 343 
closed arm. If not, record the bacterium as forming mo gas. Test the liquid in the 
bulb with litmus-paper to see if acid. If gas is formed in any tube place a mark on 
the tube at the top of the liquid to mark the amount of gas. Then fll the bulb 
completely with a 2 per cent. NaOH solution. Place the thumb over the open- 
ing of the bulb so that there will be no gas bubble between the thumb and the 
surface of the liquid. Invert the tube, allowing the gas to flow into the bulb, and by 
turning back and forth mix the gas with the NaOH solution, keeping the thumb in 
position all the time. After mixing turn the tube once more so that all the gas 
will be in the closed arm. Remove the thumb and it will usually be found that the 
level of the liquid in the closed arm rises, because some of the gas has been dissolved 
in the NaOH. This dissolved gas is COs. By determining the level of the gas 
before and after the treatment the proportion of COz2 to the undissolved gas may be 
determined. This is called the gas ratio. 
Tabulate the characters of the different species of bacteria tested, determining 
whether any two of them are alike in all respects. It is by such characters that 
bacteria are described. To determine species is too difficult for a beginner. 
No. 20. Test for Indol.—Make the following: 
Peptone, I gm. 
Distilled water, 100 C.C. 
Dissolve by boiling, place in test-tubes and sterilize as usual. After sterilization 
inoculate tubes with several different pure cultures of bacteria and allow to grow for 
todays. Addtrc.c. of ao.oz per cent. solution (fresh) of potassium nitrite and a few 
drops of concentrated sulphuric acid. Heat gently. Ifa pink color appears it in- 
dicates the formation of indol, a character used to distinguish certain species of 
bacteria (e.g., B. colt). 
No. 21. Putrefaction.—Place in a series of test-tubes with a little cold water 
the following: (a) A bit of raw meat; (b) some white of egg; (c) some flour; (d) some 
crushed beans; (¢) sugar; (f) starch; (g) a bit of melted butter. Set in a warm 
place for two or three days and determine which will putrefy and which will not. 
From the tube containing the meat and the egg, remove a bit of the liquid as soon 
as putrefaction begins and examine under a microscope (both stained and unstained) 
Examine the liquid in the other tubes in thesame way. Remove a little of the putre- 
fying mass from one tube and dilute by placing it in a water blank. Transfer a 
platinum loopful of this dilution to a melted gelatin tube and another to a melted 
agar tube. Mix by gentle agitation and pour into Petri dishes. Allow to grow for 
two days and examine the colonies. Are both liquefiers and non-liquefiers present? 
No. 22. Ammoniacal Fermentation of Urea.—Fill a test-tube or a flask half- 
full of urine and allow to stand for a day or two ina warm place. Note the odor of 
ammonia. Suspend a bit of red litmus-paper™ in the mouth of the tube and note 
that it turns blue from the ammonia fumes. Remove a bit of the liquid with a 
* Filter-paper moistened with Nessler’s solution (used by chemists) is better, 
which should turn yellow to reddish-brown if ammonia fumes arise. 
