94 THE PROTEIN SUBSTANCES. 



tionable whether there exists any proteid body which is soluble in water 

 without the aid of mineral substances. Nevertheless it has not been 

 thus far successfully proven that a native proteid body can be prepared 

 perfectly free from mineral substances without changing its constitution 

 or its properties. 1 



As previously stated, the albuminous bodies are amphoteric elec- 

 trolytes, and are polyacidic bases as well as polybasic acids. The base- 

 and acid-combining powers of various proteids are different, and the 

 maximum acid-combining power may perhaps also be used in the dif- 

 ferentiation of the various proteids (COHNHEIM, ERB, and others). 



The acid-combining power of the proteins has been studied by means of 

 physical methods by SJOQUIST, BUGARSKY, and LIEBERMANN and with the aid 

 of chemical methods by SPIRO and PEMSEL, ERB, COHNHEIM and KRIEGER, and 

 v. RHORER. The methods pursued by COHNHEIM and KRIEGER consisted in pre- 

 cipitating the proteid from acid solution (HC1) with an alkaloid reagent (calcium 

 phosphotungstate) . The reaction takes place as follows : proteid hydrochloride 

 + calcium phosphotungstate = proteid phosphotungstate + calcium chloride. The 

 acid remaining in the filtrate was determined, and when this quantity was sub- 

 tracted from the known original amount in the proteid solution, the difference 

 represented the acid combined with the proteid. If sodium picrate or potas- 

 sium-mercuric iodide is used instead of the phosphotungstate we have, accord- 

 ing to v. RHORER, 2 a method which is the best of all heretofore suggested. 



The proteids can be salted out from their neutral solutions by neutral 

 salts (NaCl, Na 2 S0 4 , MgS0 4 , [NH 4 ] 2 SO 4 , and many others) in sufficient 

 concentrations. By this salting out the properties remain unchanged 

 and the process is reversible, as on diminishing the concentration of the 

 salt the precipitate redissolves. The various proteids act in an essentially 

 different manner toward the same salt, and also for one and the same 

 proteid the behavior toward different neutral salts is different, as some 

 cause a precipitate, while others on the contrary do not precipitate. 



The behavior of various proteids with one and the same salt, such 

 as MgSO 4 or (NH 4 )2S0 4 , is often made use of in the isolation of the 

 proteid, and special methods of separation are based upon fractional 

 precipitation. HASLAM S has recently shown that these methods may 

 lead to great errors, and give good results only under special conditions. 



The conditions are different from those of salting out, when the pro- 

 teid solution is precipitated by salts of the heavy metals. Here the 

 precipitates (often called metallite albuminates) are not true combina- 



1 See E. Harnack, Ber. d. d. chem. Gesellsch., 22, 23, 25, and 31; Werigo, Pfliiger's 

 Archiv, 48; Biilow, ibid., 58; Schulz, Die Grosse des Eiweissmolekiils, Jena, 1903. 



2 Pfliiger's Arch., 90. In regard to the literature on this subject see Cohnheim,. 

 Chemie der Eiweisskorper, 2. Aufl., pp. 107-109. 



a See Cohnheim, Chemie der Eiweisskorper, 2. Aufl., 1904, pp. 144-148; Pinkus, 

 Journ. of Physiol., 27; Pauli, Hofmeister's Beitrage, 3, p. 225; Halsam, Journ. of 

 Physiol., 32. 



