FERMENTATION PRODUCTS. 61 
other complex nitrogen compounds, 2. ¢., putrefaction, is also sometimes called fer- 
mentation, and at present there is really no very sharp line to be drawn. Consult 
Green and Duclaux for the English and French views (Bibliog., XX). The student 
should observe : 
(x) Gases. Amount, rate of development, kinds-(carbon dioxide, oxygen, hydro- 
gen, nitrogen, marsh gas). 
(2) Acids. Volatile and non-volatile (lactic, acetic, butyric, etc.). 
(3) Alcohols (ethyl, methyl, butyl, glycerin, mannit, etc.). 
(4) Ethers and esters. 
(5) Aldehyds, sugars, gums. 
(6) Albumoses, peptones, amido-bodies. 
The isolation and determination of the amount of these various products 
belongs to the province of the chemist, but the work should be done in the bacteri- 
ological laboratory and under the eye of the biologist if all sorts of errors, due to 
the unsuspected multiplication of intruding organisms, are not to creep in and 
render the work worthless. Only some crude determinations, as of proportion of 
the various gases evolved, may be made by the bacteriologist who is not a chemist. . 
The volume of gas evolved from day to day may be measured in fermentation-tubes 
(fig. 49). Frost has devised a convenient gasometer for roughly estimating it (see his 
Laboratory Guide, plate I). ‘These may be made in any laboratory out of cardboard. 
If the gas is carbon dioxide it may be absorbed by shaking with 10 per cent 
NaOH. ‘To do this, fill the bowl (fig. 49) even fullof the strong caustic-soda water, 
place the thumb or forefinger over the mouth so as not to include any air, invert the 
tube so that the gas shall flow into the bowl and come into contact with the alkali, 
and shake vigorously until all of the carbon dioxide is absorbed. ‘Tilt the fluid 
back into the open end, and remove the finger so as to equalize the pressure. If 
any gas remains after equalizing the air-pressure, place the finger over the mouth of 
the tube, tilt the gas into the bowl and apply a lighted match close to the mouth as 
the finger is removed. If it is hydrogen or marsh gas it will explode in the open 
end of the tube when the finger is removed and a flame applied. If it is nitrogen it 
will not support combustion (see Bibliog., XX, especially ’90 Smith and ’93 Smith), 
How distinguish marsh gas from hydrogen ? 
Organisms easily inhibited by their own acid products may be kept alive a 
much longer time by adding a little calcium carbonate to the bouillon or agar. 
In simple tests for acids, discard bright blue litmus paper, which is very 
sensitive to carbonic acid (try carbonated water on it), and use instead a good 
grade of reddish-violet (neutral) litmus paper. Such paper may be made in the 
laboratory (the best way) or may be purchased of H. Struers, Copenhagen. 
ALKALIES (AMMONIA, AMINS, CARBONATES OF THE ALKALI METALS). 
Determine rapidity of formation. Note that they are often masked by the 
simultaneous formation of acids. Try the litmus test and Nessler’s test. Do not 
put Nessler’s solution into the culture fluid, but expose it to steam from the culture. 
Observe the behavior of the organism when grown in peptone rosolic-acid water 
