SECRETION AND EXCRETION 347 



chemically, since the equilibrium of the milk was thus changed. 

 The experimental arrangements by which VOELTZ, at least qualita- 

 tively, established the existence of serum pellicles, are the most 

 fortunate. No value is to be attributed in my opinion to the quan- 

 titative results, since the composition of the pellicles must change 

 while they pass through the layer of water. VOELTZ layered a 

 column of milk about 10 cm. high under a column of water 50 cm. 

 high. The milk globules mounted through the water and were thus 

 freed of all water-soluble ingredients. The cream thus formed was 

 then taken up, freed from fat and the residue determined. The 

 composition proved very variable and qualitatively contained the 

 ash and organized constituents of the milk as far as that could be 

 deduced from the mere determination of ash, organic substance, 

 Mg, Ca and P. 



By emulsifying butter fat in skimmed milk, VOELTZ produced 

 artificial pellicles and compared them with the natural ones. 



In the light of our knowledge of the spreading out of colloidally 

 dissolved substances on surfaces, it ought not to be very difficult to 

 explain the phenomenon of the pellicles of milk fat (incorrectly called 

 serum pellicles). According to G. QUINCKE, a substance spreads out 

 at the interface between two fluids (or a gas and a fluid) if by this 

 means the surface tension of the surface possessed in common is 

 diminished. Oil spreads out, for instance, at the interface between 

 water and air. We know from W. RAMSDEN* and METCALF* (see p. 

 34) that albumin and peptone separate out at the interface between 

 water and air as a solid. G. QUINCKE * 2 showed that a film of gum 

 solution surrounded each globule in the pharmacopeal emulsions. 

 H. BECHHOLD * L bases his explanation of protective colloids which 

 prevent the flocculation of organic colloids and suspensions on this 

 phenomenon. 



Simple consideration shows that a fat globule in an albumin 

 solution must surround itself with a layer of albumin. Since the 

 surface tension at the boundary surface of albumin or serum and fat 

 (0.4 to 1.6) is smaller than that of water and fat (1.6 to 2.4) the 

 albumin of milk must collect on the surface of the fat globules. 

 An experiment of ASCHERSON* shows the correctness of this a priori 

 assumption. ASCHERSON emulsified olive oil in an alkaline solution 

 of egg albumin and observed that the oil droplets were surrounded 

 by an albuminous membrane. The strength and composition of 

 the pellicles of the milk globules vary, of course, not only with the 

 colloidal, but also with the crystalloid constituents, especially the 

 salts. In passing through the aqueous layer (in VOELTZ' experi- 

 ment) the pellicles are again changed, and from the investigation 



