458 THE TRUE ALBUMINOUS BODIES. 



proteids brownish-yellow; also sulphuric acid and cane-sugar, which stain them 

 purple- violet. 



Precipitation: (i) By boiling. (2) By strong alcohol. (3) By "salting." 

 Most proteids are precipitated by the addition of neutral salts to their solutions 

 to the point of complete saturation, especially if the reaction be acid. If the 

 addition of salt be made gradually, some of the albumin can thus be separated in 

 crystalline form. (4) Nitric acid precipitates albumin, as does also metaphos- 

 phoric acid. (5) Further precipitants are the salts of the heavy metals (iron 

 chlorid, lead acetate, copper sulphate, platinum chlorid, mercuric chlorid in solu- 

 tion with hydrochloric acid). (6) Precipitation is caused by acetic acid and 

 potassium ferrocyanid, also by tannic acid, picric acid or trichloracetic acid. (7) 

 Mercuric-iodid, potassium-iodid on addition of hydrochloric acid, phosphotungstic 

 and phosphomolybdic acids also precipitate albumin. 



Animal proteids. 



Albuminous bodies can be divided into several characteristic groups : The first 

 group comprises albuminous substances in the strict sense, designated genuine 

 albuminous substances or proteins, which are soluble in water or in dilute saline 

 solutions and are levorotatory. This first group comprises the albumins and the 

 globulins. 



The albumins are soluble in water and precipitable by complete saturation 

 with ammonium sulphate, but not by means of sodium chlorid or magnesium 

 sulphate. 



Serum-albumin has been prepared in crystalline form by Giirber. By 

 diffusion almost all of its salts, and thereby its coagulability by heat, can be re- 

 moved. It is precipitated by strong alcohol. It is readily soluble in concen- 

 trated hydrochloric acid, acid-albumin, which is soluble in water, being precipitated 

 on addition of water. 



Egg-albumin, C 80 H 122 N2 SO 24 + H 2 O, has been prepared in crystalline form 

 by Hofmeister. It occurs in the white of birds' eggs and exhibits a specific rotation 

 of polarized light of 37.8. After injection into the veins or beneath the skin, 

 or even after introduction into the intestine in large amount, it appears partly 

 unchanged in the urine. It is precipitated by agitation with ether. Its composi- 

 tion is C^.zgHy.aeNisSj.og. 



Lactalbumin. 



Muscle-albumins, that is, the proteid bodies in the aqueous extract of muscle. 



The globulins are insoluble in water, the majority soluble in dilute salt- 

 solutions. They contain less sulphur and yield a more marked xanthoproteic 

 reaction than the albumins. In solution they are coagulated by a temperature of 

 75 C. and they are precipitated by abundant addition of water. Dilute acids con- 

 vert them into acid- albumins. They are precipitated by saturation of the solu- 

 tion with magnesium sulphate and also by semisaturation with ammonium 

 sulphate, by very dilute acids, as well as by carbon dioxid. The globulins include : 



Serum- globulin, the presence of which in the urine is described on p. 496. 



Fibrinogen, from which fibrin results. The substances from which this is 

 produced are described on p. 69. Stroma-fibrin is considered on p. 72. 



Myosinogen. 



Vitellin, which occurs in the yolk of birds' eggs and likewise in the crystalline 

 lens, perhaps also in the chyle and in the amniotic fluid, is not precipitable 

 by saturation of a neutral salt-solution with sodium chlorid. Crystalline vitellins 

 occur as yolk-plates in the eggs of fish, frogs, tortoises. In the eggs of birds and 

 in tissues the vitellins are amorphous. 



Alkali-albuminates. Potassium and sodium, also calcium hydroxid and 

 barium hydroxid, form combinations with proteids, and the more rapidly the more 

 concentrated the alkaline solution and the higher the temperature. These com- 

 binations are designated alkali-albuminates. They exhibit especially marked cir- 

 cumpplarization, are not coagulated on boiling and are precipitated from solutions 

 by acids, which combine with the alkali. If, for example, egg-albumin be mixed 

 with a solution of potassium hydroxid, potassium albuminate is formed as a 

 gradually developing jelly, which is soluble in boiled water. 



Acid-albuminates. If proteids are dissolved in strong acids, for example 

 hydrochloric acid, they acquire the properties of so-called acid-albumin, which ex- 

 hibits great similarity to alkali-albuminate (also the specific rotation) . This body 

 is insoluble in water and neutral salt-solutions, readily soluble in dilute hydrochloric 

 acid. They are thrown out of solution by the addition of much salt (sodium 

 chlorid or sodium sulphate). Also neutralization by alkali causes precipitation, 

 though boiling does not. On cooling, the boiled (concentrated) fluid becomes 



