44 
LABORATORY EXERCISES 
To No. 1, I cc. of salt; to No. 2, 3 cc. of salt; to No. 3, 1 cc. of 
sugar; to No. 4, 4 cc. of sugar; to No. 5, 4 cc. of vinegar; to No. 6, 
2 cc. of powdered boric acid; to No. 7, 1 drop of commercial forma¬ 
lin; to No. 8, 4 drops of commercial formalin. Keep the tubes in 
a warm room for two days. At the end of this period, observe the 
odor and appearance of the contents of each tube. What indicates 
bacterial action? Compare the bacterial action in each to that in 
the “ control ” tube. Which substances added to the medium 
have been most effective in preventing bacterial action? Which 
are required in large amounts? Which are harmless? Which 
are dangerous? Where is each used? What is a preservative? 
II. Disinfectants. To the second set of test tubes add the 
following chemicals: 
To No. 1, one drop of five per cent mercuric chloride solution; 
to No. 2, 6 drops of the same mercuric chloride solution; to No. 3, 
1 drop of five per cent carbolic acid solution; to No. 4, 6 drops 
of the same carbolic acid solution; to No. 5, 1 drop of tincture of 
iodine; to No. 6, 6 drops of tincture of iodine; to No. 7, J cc. of 
slaked hme solution; to No. 8, 1 cc. of slaked hme; to No. 9, 1 
drop of four per cent chloride of lime solution; to No. 10, 5 drops 
of the same chloride of hme solution. Set the tubes aside in a warm 
place for two days. Then observe the bacterial growth in each. 
Compare the tubes with the control ’’ and with one another. 
Which is the clearest? Does the amount of a disinfectant need to 
be considered? Which of these disinfectants are widely used? 
Discuss the use of each. Define “ disinfectant.’’ How does a 
disinfectant differ from a preservative? 
EXERCISE 47 
THE EFFECT OF HEAT UPON BACTERIA 
Make an infusion by steeping some chopped timothy hay in 
water. Allow this to cool and then add to it some faucet water in 
which the hay was previously washed. The solution should then 
be filtered until it is clear. 
