672 ANIMAL PHYSIOLOGY. 
Proteids are amorphous ; insoluble in alcohol and ether ; some of 
them soluble in water ; soluble with change of constitution in strong 
acids and alkalies, and lsevo-rotatory. 
Tests for Proteids,—1. With Millon’s reagent (mercury dissolved in 
its own weight of nitric acid, and the solution diluted with twice its 
volume of water) a precipitate, rendered red by boiling. 2. Heated 
with strong nitric acid, they become yellow. On adding ammonia or 
caustic soda, or potash, the yellow is replaced by an orange (aantho- 
proteic reaction). 3. On adding caustic alkali and a drop or two of 
-copper sulphate, a violet color is produced, which can be deepened by 
boiling. 4. To the suspected fluid add enough acetic acid to render it 
decidedly acid, and then a few drops of potassium ferrocyanide. A 
white precipitate indicates that proteids are present. 5. To the. fluid 
rendered decidedly acid, add a strong solution of sodium sulphate and 
boil. Ifa precipitate falls, some proteid was present. 
The first three tests are the most reliable, and apply to all classes of 
proteids. 
PROPERTIES AND CLASSIFICATION OF THE PROTEIDS. 
I. Native Albumins. 
These occur naturally in the tissues and fluids of the body. They 
are soluble in water, are not thrown down by the alkaline carbonates, 
by sodium chloride, or by very dilute acids. Their coagulation-point 
lies below 70° C. They may be dried with change of color to a pale 
yellow, but remain soluble. 
1. Egg-Albumin.—This may be obtained for purposes of experiment 
by cutting up raw white of egg with scissors, diluting with water, strain- 
ing through cotton, and afterward through filter-paper. The resulting 
fluid is almost colorless at first, but on standing darkens gradually. It 
may be precipitated by strong alcohol, which does not seem to alter 
its chemical constitution, or by strong acids, when a great chemical 
change takes place. Various mineral salts, as silver nitrate, mercuric 
chloride, etc., form with albumin insoluble compounds. ‘Whether albu- 
min ever exists entirely free from combination with salts in the animal 
body is a question; probably not. 
By the addition of strong acetic acid or caustic alkali, a clear, jelly- 
like mass results, being, in the first case, acid-albumin, and in the second 
alkali-albumin. It is leevo-rotatory to the extent of 35°5° (—35°5°). 
2, Serum-Albumin,—This compound greatly resembles the foregoing, 
but may be distinguished by the following characteristics: (a) Serum- 
albumin is not, like egg-albumin, coagulated by ether. (0) Serum-albu- 
min is Jess readily coagulated by strong hydrochloric acid, and any pre- 
cipitate formed is easily dissolved by excess of acid, in which respects it 
is the reverse of egg-albumin. (c) Coagulated serum-albumin is readily 
soluble in strong nitric acid, the reverse holding for egg-albumin. (d) 
The specific rotation of egg-albumin is —35°5°; of serum-albumin, —56°. 
(e) Serum-albumin occurs in blood, lymph, chyle, milk, and pathological 
