a. Nucleo-albumins.

with nucleins. Casein.

Compounds of albumins

b. Glycoproteids. Compounds of albumins with carbohydrates. Mucin, mucoid, and hyalogen.

c. Hemoglobins. Compounds of albumins with coloring-matters containing iron. Hemoglobin, oxyhemoglobin, methemoglobin, carbon-monoxid hemoglobin, etc.

d. Nucleins. Compounds of albumins with phosphoric acid or nucleic acid.

The Albuminoids.-a. The keratins (found in hair, epidermis, nails, etc.).

b. The elastins (found in elastic tissue, the intima of the arteries).

c. Collagen (found in the organic part of bones; is the principal constituent of gelatin).

Besides those compounds which are classified above, are the albumoses and peptones formed by the digestion of protein substances. These forın a class entirely separate from the above. They will be classified in the section on Digestion.

Egg-albumin.-To prepare the solution for use in the following experiments take 20 c.c. of eggalbumin in a flask of 400 c.c. capacity, and add 150 c. c. of water. Shake the mixture vigorously and filter through a folded filter. The residue on the filter consists of the membranous network in which the albumin is enclosed in the egg.

I. Test the reaction with litmus-paper.

2. Heat 5 c.c. of the solution to boiling in a testtube. Note that the solution becomes opalescent, but coagulation does not take place. Add a drop

of acetic acid. Note the immediate separation of the albumin.

3. Heat 5 c.c. of the solution with 2.5 c.c. of sodium hydroxid solution. Alkali albuminate is formed, which is soluble. The albumin does not coagulate. Add dilute acetic acid drop by drop, and note the commencing coagulation of the albumin when the alkali is neutralized and the solution becomes faintly acid.

4. To 5 c.c. of the solution add mercuric chlorid solution. A heavy precipitate forms, insoluble in excess of the reagent. Add a concentrated solution of sodium chlorid. The precipitate dissolves.

5. To 5 c.c. of the solution add nitric acid till a precipitate forms. Then add an equal volume of alcohol. Note the persistence of the precipitate.

6. The following reactions are to a greater or less extent characteristic of all proteids. Dilute the egg-albumin solution used in the foregoing tests with 2 times its volume of water.

a. The Biuret Test.—Mix equal parts of albumin solution and potassium hydroxid. Heat gently and add a few drops of a 2 per cent. solution of copper sulphate. Allow the copper solution to flow down the side of the test-tube. Note the formation of a pink or violet color.

b. Xanthoproteic Reaction.—Add an equal part of strong nitric acid to the albumin solution. Boil. A yellow solution is formed. Place about I c. c. of the solution in a porcelain dish, and add strong potassium hydroxid till the mixture is strongly

alkaline.

orange.

Note the change from deep yellow to

c. Millon's Test.-To Co of -11 add

The Coagulation-temperature of Albumins.—This test is performed in a series of test-tubes, so that all the tubes may be observed under identical conditions.

Take 5 test-tubes, and label them carefully, making a note of the mixture which is put in each.

In I. put 5 c.c. of undiluted egg-albumin.

In II., III., IV., and V., use the diluted solution previously employed for the precipitation-tests; 5 c. c. are placed in each.

To II. add 1 c. c. of 10 per cent. sodium chlorid. To III. add 4 drops (o. 2 c. c.) of the same solution. To IV. add 2 drops of glacial acetic acid.

To V. add 1 c. c. of a 1 per cent. solution of glacial acetic acid.

Place the tubes together with a thermometer in a beaker containing water, and raise the temperature of the water gradually, noting the temperature at which change occurs in any of the tubes. Raise

the temperature of the water in the beaker to boiling. Note any change that occurs after the temperature has reached 100° C.

After the temperature has been maintained at 100° C., for five minutes add to tubes II. and III. a couple of drops of dilute acetic acid. Note the change which occurs.

To tubes IV. and V. add 1 c.c. of sodium-chlorid solution. Does a change take place? What are the conclusions which can be drawn from these experiments?

Under ordinary circumstances, when urine is tested for albumin, the amount of sodium chlorid present in the urine is sufficient to cause coagulation when the acid is added.

Acid Albumin.-Take 5 c. c. of the dilute albumin solution and add a few drops of hydrochloric acid. Boil the solution. Precipitation does not take place. Acid albumin is formed, which is soluble. Cool and neutralize the acid with potassium hydroxid. Note the change in appearance on the slight excess of acid. Use litmus-paper to find the neutral point.

Precipitation of Protein Substances by Saturation with Easily Soluble Salts.-This method is one of great importance as a means of separating proteins. In some cases it is the only means of separating substances such as serum-globulin and serumalbumin, which are closely allied to one another, and of which the greater part of the reactions are identical.

Heat a water-bath to 35° C. Place in a test-tube

IO c.c. of a dilute albumin solution, and add powdered ammonium sulfate till the crystals are no longer dissolved. Note the appearance after some time of a precipitate. This contains all the globulin and albumin found in ordinary eggalbumin. Filter off the precipitate and test the filtrate with the biuret test. Is a protein substance present in the filtrate ?

Treat the precipitate on the filter with cold water. Test the filtrate with acetic acid and heat. Note the ready solubility of the precipitate in water.

Prepare a similar tube, saturating the solution with crystalline magnesium sulfate. Heat at 35° C. for the same length of time. Note the formation of a very slight precipitate of egg-globulin contained in the egg-albumin. Magnesium sulfate precipitates globulins, but does not precipitate albumins. Filter the solution, and apply the acetic acid and heat test. Note the copious precipitate of albumin which has not been precipitated by the magnesium sulfate.

THE BLOOD.

The blood as existing in the body may be classified into arterial and venous. These two kinds are distinguished by their content of oxygen and of carbon dioxid. They are also distinguished by their color, which is due to the blood-pigment being in different stages of oxidation. In arterial blood the pigment consists for the most part of oxyhemoglobin, characterized when in sufficient quantity by its bright-red color and its spectrum.