ALBUMINS OR PROTEINS. 131 



first place what amino acids take part in their formation, and then even- 

 tually stereochemical studies will decide the question. We have to 

 depend at present chiefly upon physical differences. Although many 

 proteins may be fairly well characterized in this way, by their solubility 

 in water, in salt solutions, etc., there are many others in which this is not 

 the case. 



The proteins, as a whole, may be first of all separated into two main 

 groups: (1) The Simple Proteins, and (2) The Compound Proteins, or 

 Proteids. The first group is subdivided into the true albumins and the 

 so-called albuminoids. The true proteins comprise (a) the albumins 

 (serum-albumin, egg-albumin, and lactalbumin, etc.); (6) the globulins 

 (serum-globulin, egg-globulin, lactoglobulin, and cell-globulin); (c) the 

 plant-globulins and plant- vitellins ; (d) fibrinogen; (e) myosin; (/) phos- 

 phorized proteins, or the so-called nucleo-albumins (casein, vitellin, nucleo- 

 albumins of cell-protoplasm); (g) histons; (h) protamines. While these 

 groups are fairly well distinguishable, as we shall soon see, the following 

 group, which likewise belongs to that of the simple proteins, is charac- 

 terized more from a morphological point of view. The albuminoids 

 include (a) collagen; (6) ceratin (from hair, feathers, horn, etc.); (c) 

 elastin; (d) fibroin (from silk); (e) spongin and conchiolin; (/) amyloid; 

 (g) albumoid and perhaps the melanins. 



To the compound proteins, or proteids, belong nucleoproteids, hemo- 

 globin, and glucoproteids. 



In the following pages we shall merely devote space enough to such 

 description of the individual proteins as seems absolutely necessary for 

 later discussion. Those interested are referred to the special works on 

 the subject. 1 



We will first take up the simplest albuminous substances, the simple 

 proteins. The albumins and globulins comprise a well-characterized group. 

 They are generally found together ; for example, in blood-serum, milk, and 

 the whites of eggs. The albumins are soluble in pure water. If blood- 

 serum is dialyzed against distilled water, a precipitate will form on stand- 

 ing. This is globulin, which had been held in solution by neutral salts. 

 Precipitation follows as the latter diffuse. The albumins, on the other 

 hand, remain dissolved. They are also soluble in dilute salt solutions, as 



1 An exhaustive description of the individual proteins is found in Otto Cohnheim's 

 Chemie der Eiweisskorper, and in Gustave Mann's The Chemistry of the Proteids, which 

 is based upon the former. We shall follow, as a rule, Cohnheim's classification. Even 

 to-day the chemistry of the amino acids might be used as a basis for a classification, 

 but in order to prevent misunderstandings we will here adhere to the older method. 



The following works are instructive: Viktor Griessmayer: Die Proteide der Getreide- 

 arten, etc. (1897). Leo Morochowetz: Das Globulin des Blutfarbstoffes, etc. Le 

 Physiologiste Russe, 41-47 (1903), and 48-60 (1904). F. Hofmeister: Ueber Bau und 

 Gruppierung der Eiweisskorper, Ergeb. Physiol. (Asher and Spiro), 1, 759 (1902). 



