412 CONSIDERATION OF CARBON COMPOUNDS. 



2. Millorfs reagent colors them purple-red on heating. This reagent 

 is a solution of mercuric nitrate, containing some excess of nitric acid ; 

 it is best made by dissolving 1 part of mercury in 2 parts of nitric 

 acid of a specific gravity of 1.42, and diluted with 2 volumes of water. 



3. Biuret-reaction. A few drops of dilute cupric sulphate solution 

 and then an excess of potassium hydroxide added, give a violet color. 



4. Heating of equal volumes of proteid solution and a saturated 

 aqueous solution of ammonium sulphate causes the precipitation of 

 all proteids except that of peptones. 



5. A few drops of solution of 1 part of cane sugar in 4 parts of 

 water and then strong sulphuric acid added, produce a purple color. 



6. They are often precipitated by highly diluted acids, but redis- 

 solved by boiling with strong hydrochloric acid, forming a violet-red 

 solution. The precipitated proteids are also generally dissolved by 

 caustic alkalies. 



7. They are also precipitated by tannin, carbolic and picric acids, 

 by potassium ferrocyanide and acetic acid, by lead acetate, mercuric 

 chloride, and by most salts of heavy metals. (The use of egg-albu- 

 min in cases of poisoning by metallic compounds depends on this 

 property.) 



Classification. Our present unsatisfactory state of knowledge 

 regarding proteids, the close resemblance which they show in prop- 

 erties, and the difficulties which are met with in separating them in 

 a pure state, make it difficult to arrange these bodies properly. 

 However, eight classes are now generally distinguished. They are : 

 I. Native or true albumins ; II. Globulins ; III. Derived albumins 

 or albuminates ; IV. Fibrins ; V. Coagulated proteids ; VI. Albu- 

 moses ; VII. Peptones ; VIII. Amyloid substance or Lardacein. 



Class I. Native or true albumins. These proteids are soluble 

 in pure water ; the solutions become turbid at 60 C. (140 F.), and 

 are coagulated, especially in presence of a dilute acid, at or below 75C. 

 (167 F.). Strongly alkaline solutions are not precipitated by heat- 

 ing, and the presence of too much free acid may also prevent coagula- 

 tion. Coagulated albumin is dissolved by strong solutions of alkali 

 hydroxides. Native albumins occur in the whites of birds 7 eggs, in 

 milk, in the plasma of the blood, chyle, lymph, etc., as also in plants. 



a. Serum-albumin is found dissolved in blood-serum (in human 

 blood to the extent of about 4 to 5 per cent.), in lymph, chyle, trans- 

 udations, and, in very small quantities, in milk. Pathologically it 



