MILK. 91 



homogeneous ; and the burden of proof rests with those who 

 profess to be able to demonstrate the existence of .an invest- 

 ing membrane. Robin, one of the highest authorities on 

 these subjects, argues against the existence of a membrane, 

 and opposes the observations of those who assume to have 

 demonstrated it by explanations of the phenomena produced 

 by reagents, which do not involve, as a necessity, the pres- 

 ence of such a structure. The arguments in favor of its ex- 

 istence are not very satisfactory ; and the experiments upon 

 which they are based relate chiefly to the action of ether upon 

 the globules before and after the action of other reagents. 



If a quantity of milk be shaken up with an equal volume 

 of ether, the mixture remains opaque ; but if a little potash 

 be added, the fatty matters are dissolved, and the mixture 

 then becomes more or less clear. These facts are all that 

 can be observed without following out the changes with the 

 microscope. Robin has shown that the fatty particles are 

 acted upon when the milk is thoroughly agitated with ether 

 alone ; and that the opacity is then due to the fact that the 

 ether, with the fat in solution, is itself in the form of an 

 emulsion. If the opaque mixture of milk and- ether be ex- 

 amined with the microscope, globules are seen, larger than, 

 the ordinary milk-globules, much paler, and possessing much 

 less refractive power. These he supposes to be composed 

 of fat and ether. If potash be added, either before or after 

 the addition of ether, the constitution of the whole mass of 

 liquid is changed, and it becomes somewhat transparent, 

 though by no means perfectly clear. 1 It is assumed that, in 

 the first instance, the ether does not attack the globules, be- 

 cause it has no effect upon the membrane which is supposed 

 to exist, and that the potash acts upon the membrane, allow- 

 ing the ether then to take up the fat ; but if the observations 

 of Robin be correct, it is evident that this view cannot be 

 sustained. 



If dilute acetic acid be added to a specimen of milk under 



1 ROBIN, Lemons sur les humeurs, Paris, 1867, p. 399, et seq. 



