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.

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 experiment, the student should state briefly and clearly the following:

Fig. 73.—Balance and Weights.

List of Apparatus used in Experiments

Directions for Weighing.—Place the dish or material to be weighed in the left-hand pan of the balance. With the forceps lay a weight from the weight box on the right-hand pan. Do not touch the weights with the hands. If the weight selected is too heavy, replace it with a lighter weight. Add weights until the pans are counterpoised; this will be indicated by the needle swinging nearly as many divisions on one side of the scale as on the other. The brass weights are the gram weights. The other weights are fractions of a gm. The 500, 200, 100 mg. (milligram) weights are recorded as 0.5, 0.2, and 0.1 gm. The 50, 20, and 10 mg. weights as 0.05, 0.02, and 0.01 gm. If the 10, and 2 gm., and the 200, the 100, and the 50 mg. weights are used, the resulting weight is 12.35 gms. No moist substances should ever come in contact with the scale pans. The weights and forceps should always be replaced in the weight box. Too much care and neatness cannot be exercised in weighing.

Fig. 74.

Fig. 75.—Pouring
Reagent from Bottle.

Directions for Measuring.—Reagents are measured in graduated cylinders (see Fig. 74). When the directions call for the addition of 5 or 10 cc. of a reagent, unless so directed it is not absolutely necessary to measure the reagent in a measuring cylinder. A large test tube holds about 30 cc. of water. Measure out 5 cc. of water and transfer it to a large test tube. Note its volume. Add approximately 5 cc. of water directly to the test tube. Measure it. Repeat this operation until you can judge with a fair degree of accuracy the part of a test tube filled by 5 cc. In the experiments where a burette is used for measuring reagents, the burette is first filled with the reagent by means of a funnel. The tip of the burette is allowed to fill before the readings are made, which are from the lowest point or meniscus. When reagents are removed from bottles, the stopper should be held between the first and second fingers of the right hand (see Fig. 75). Hold the test tube or receptacle that is to receive the reagent in the left hand. Pour the liquid slowly until the desired amount is secured. Before inserting the stopper, touch it to the neck of the bottle to catch the few drops on the edge, thus preventing their streaking down the sides of the bottle on to the shelf. Replace the bottle in its proper place. Every precaution should be taken to prevent contamination of reagents.

Fig. 76.—Microscope and Accessories.

1, eye-piece or ocular; 2, objective; 3, stage; 4, cover glass; 5, slide; 6, mirror.

Use of the Microscope.—Special directions in the use of the microscope will be given by the instructor. The object or material to be examined is placed on a microscopical slide. Care should be exercised to secure a representative sample, and to properly distribute the substance on the slide. If a pulverized material is to be examined, use but little and spread it in as thin a layer as possible. If a liquid, one or two drops placed on the slide will suffice. The material on the slide is covered with a cover glass, before it is placed on the stage of the microscope. In focusing, do not allow the object glass of the microscope to come in contact with the cover glass. Focus upward, not downward. Special care should be exercised in focusing and in handling the eye-piece and objective. A camel's-hair brush, clean dry chamois skin, or clean silk only should be used in polishing the lenses. Always put the microscope back in its case after using.