318 Frances K. M. DaiiKircsti : 



As sx matter of interest it may be mentioned that the increase 

 of viscosity due to increased acidity imparts to the cream certain 

 definite projierties, viz. : — 



(1) The 2)i-operty of -svliipping, and 



(2) The proj^erty of buttering. 



A cream of sufficient acidit}- (approaching the critical acidity) 

 will i-cadily AvhijD, and having reached this condition, will, with 

 very little further mechanical agitation, form butter. It is pro- 

 bable, therefore, that the butter-maker, in allowing the cream to 

 ripen, not only aims at improving the flavour of the butter, but 

 also takes advantage of this pioperty of the cream acquired by 

 acidification in order that the fat globules may coalesce with the 

 minimum of loss. In making butter from fresh cream, a consider- 

 able loss of fat is inevitable, since the globules in this case have not 

 the strong tendency to coalesce. 



The probable explanation of the critical acidity is that at this 

 ])oint a definite change occurs in the proteins of the cream serum, 

 and the nature of this change is such that the protein, which forms 

 a kind of envelope around the fat globules, imj^edes their free 

 motion in rolling past one another. 



This theory is supjDorted by the fact that in separated milk there 

 is no such sharp rise in viscosity on acidifying as is the case with 

 cream, although at tlie degree of acidity of the milk Avhich corres- 

 ponds to the critical acidity of the cream serum tliere is a definite 

 change in the proteins present, evidenced by a visible precipita- 

 tion. 



The capillary of the viscosimcter employed for the experiments 

 Avith separated milk had only 1/3 of the cross-section of the capil- 

 lary used for the cream, so that there is no reason to suppose that 

 a relatively larger space Avas occupied by the milk in any part of 

 the tube, than by the cream serum, Avhich might otherAvise be thought 

 a possible explanation of the difference in behaviour of the tAvo 

 liquids. 



In Nature of June 1st, 1911, there appeared a short 

 summary of a paper on " Viscosity of Emidsions," by Baucelin, in 

 Avhich the folloAving statement occurred : — " In accordance Avith 

 the Einstein theory, increase of viscosity is found to be independent 

 of the size of particles in suspension, and depends oidy on the 

 total volumei of particles per \init A'olumc." 



On this assumption, since the total volume of the fat globules 

 in any cross-section of the capillary of the viscosimcter could not 

 be supposed to occupy nearly 2/3 of the total space for any of 



