vni THE FORMATION OF IRREVERSIBLE SALTS 299 



Tyndall's experiment (see p. 258), and that their precipitating power 

 is directly proportional to the degree of their colloidal nature, and 

 also pointed out that ox serum, which always shows a strong Tyndall 

 reaction, readily precipitates ferric hydrate. 



He further showed if a colloid solution of gum-mastic was placed 

 in a cylindrical vessel, and if a saturated solution of copper sulphate, 

 or aluminium, or iron chloride, was introduced at the bottom of the 

 cylindrical vessel, care being taken to avoid all air bubbles, that the 

 mastic solution acted as a semi-permeable membrane. For example, in 

 'the case of copper sulphate the presence of free sulphuric acid could 

 be demonstrated in the upper layers of the solution, while the copper 

 hydrate, enveloped in particles of gum-mastic, had become precipi- 

 tated. This conception of Spring, published in 1900, has been again 

 brought forward as something new by several German investigators, 

 namely, by Biltz, 1 Freundlich, 2 Landsteiner and Jagic, 3 Neisser and 

 Friedmann. 4 The author was unable to procure the papers published 

 in the Miinchener periodical, and quotes from Pauli. 5 



While copper sulphate, if it be employed in the manner just 

 indicated, separates freely into H 2 S0 4 and Cu(OH) 2 , which latter then 

 unites with the mastic solution, quite a different result is obtained if 

 the copper sulphate or some other salt and the mastic solution be 

 mixed together, for in this case the flocculi of gum-mastic contain no 

 hydrate of copper . 



It appears to the author that in this last case coagulation is 

 brought about in the following way. The electropositive copper, 

 Cu, set free from the copper sulphate by electrolysis, combines with 

 the neighbouring electronegative mastic-radicals in such a way as to 

 form a compound Cu + Mastic"; this compound is then broken up 

 hydrolytically, and Cu + 2 (OH)' is formed ; the mastic having 

 handed on its negative electrical charge to the hydroxyl radical, and 

 thus having lost its electrical load, fuses with other mastic particles 

 because the absence of a difference in potential leads to an increase in 

 the surface tension (see p. 265). 



7a. Compounds of Albumins with Alkaline Earths 



The salts of the alkalies, alkaline earths, and heavy metals differ, 

 according to Pauli, 6 essentially in this, that the importance of the 

 an-ion, from the precipitation point of view, gradually becomes less 



1 Biltz, Ber. d. deutsch. chem. Ges. 37. 1095. 



2 H. Freundlich, Zeitschr. f. physik. Chem. 49. 129 (1903). 



3 Landsteiner and Jagic, Miinchener med. Wochenschrift, 1903, No. 27. 



4 Neisser and Friedmann, ibid. 1904, Nos. 15 and 19. 



5 ffofmeister's Beitrdge, 6. 253, 1905. 6 W. Panli, ibid. 6. 233 (1905). 



