164 COLLOIDS IN BIOLOGY AND MEDICINE 



nin, yet the casein obtained by the addition of acids and that obtained 

 by rennin are not identical. Furthermore, casein may be separated 

 from the crystalloid portions of milk by ultrafiltration (H. BECHHOLD 

 and by centrifugation (H. FRIEDENTHAL*). We are indebted to E. 

 LAQUEUR and 0. SACKUR* as well as T. B. ROBERTSON* (who gives a 

 bibliography) for the exhaustive chemical studies of casein upon which 

 we base our remarks. In water, casein is completely insoluble and 

 decidedly acid. A piece of casein stains damp blue litmus paper red. 

 According to L. MICHAELIS the isoelectric point occurs when the H 

 ion concentration is 2.10~ 5 . Casein forms salts soluble in water with 

 alkalis and alkaline earths. One grain of casein binds 8.81 c.c. 1/10 

 normal alkali (using phenolphthalein as an indicator). From this 

 the combining weight of casein is 1135, and a common multiple of 

 this is its molecular weight. E. LAQUEUR and O. SACKUR deduced 

 from the conductivity of sodium-casein solution with increasing 

 dilution, that casein was a tetra- or hexabasic acid and that, therefore, 

 its molecular weight lay between 4540 and 6810. T. B. ROBERTSON, 

 as a result of his investigations, comes to the contrary conclusion, 

 that only a single carboxyl group is available for union with a base. 

 W. VAN DAM* has investigated the diminution in H ion concentration 

 of lactic acid solution upon adding casein and concludes from it, that 

 a basic group unites with four replaceable H atoms in a casein mole- 

 cule. 



In solution, casein salts are hydrolytically dissociated and, in 

 fact, it follows from the following experiment that casein (acid) 

 forms a hydrosol. A neutral solution of casein-sodium solution is 

 slightly opalescent and becomes clear upon the addition of an alkali. 

 The solution of casein-lime salts are still more opalescent since the 

 alkaline earth salts are weaker bases. Casein-sodium does not 

 diffuse through parchment; the membrane must have a decided 

 difference in potential since the sodium ion has a strong tendency to 

 diffuse. 



E. LAQUEUR and 0. SACKUR showed that the internal friction of 

 casein salt solutions increased proportionately to the electrolytic 

 dissociation, and that every diminution of electrolytic dissociation 

 was accompanied by a diminution of internal friction. The casein 

 ions are thus responsible for high internal friction. 



Hemoglobin. 



Hemoglobin, the coloring matter of blood, has only recently been 

 studied by P. BOTTAZZI. It is preeminently suited for colloid-chemi- 

 cal investigation on account of its color, ease of crystallization and 



