lesomeness of the food supply.

[Illustration: FIG. 71.--A PETRI DISH, SHOWING COLONIES OF
BACTERIA PRODUCED BY ALLOWING A HOUSE FLY TO CRAWL OVER SURFACE.

(From Minnesota Experiment Station Bulletin No. 93.)]

CHAPTER XXI

LABORATORY PRACTICE

Object of Laboratory Practice, Laboratory Note-book, and Suggestions
for Laboratory Practice.--The aim of the laboratory practice is to give
the students an idea of the composition, uses, and values of food
materials, and the part which chemistry takes in sanitation and
household affairs; also to enable them by simple tests to detect some of
the more common adulterants in foods.

Before performing an experiment, the student is advised to review those
topics presented in the text which have a bearing upon the experiment,
so that a clear conception may be gained of the relationship between the
laboratory work and that of the class room. The student should endeavor
to cultivate the power of observation and to grasp the principle
involved in the work, rather than do it in a merely mechanical and
perfunctory way. Neatness is one of the essentials for success in
laboratory practice, and too much emphasis cannot be laid upon this
requisite to good work. The student should learn to use his time in the
laboratory profitably and economically. He should obtain a clear idea of
what he is to do, and then do it to the best of his ability. If the
experiment is not a success, repeat it. While the work is in progress it
should be given undivided attention. Care should be exercised to prevent
anything getting into the sinks that will clog the plumbing; soil,
matches, broken glass, and paper should be deposited in the waste jars.

[Illustration: FIG. 72.--APPARATUS USED IN LABORATORY WORK.

See page 301 for names.]

A careful record of the experiments should be kept by each student in a
suitable note-book. It is suggested that those students desiring more
time in writing out the experiments than the laboratory period affords,
take notes as they make the various tests, and then amplify and
rearrange them in the evening study time. The final writing up of the
notes should, however, be done before the next laboratory period.
Careful attention should be given to the spelling, language, and
punctuation, and the note-book should represent the student's individual
work. He who attempts to cheat by copying the results of others, only
cheats himself. In recording the results of an experim