ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 99 
this fact as additional evidence for his “slim-membran” theory, the 
modern notion of adsorption can account for such staining without 
assuming a mucoid surface at all. 
A milk from which the cream has been separated may be used as 
the medium for emulsifying such oils as coconut oil, and cotton- 
seed oil, to yield artificial milks, emulsions of great stability. Such 
a result decidedly tends to disprove the notion of a mucoid substance 
being necessary for the existence of the fat in milk ‘as small 
globules. 
Cream is formed by the rising of the fat globules through the 
denser milk serum to the surface. The density of the liquefied 
fat is about 0°92, whilst that of the milk serum is over unity. The 
diameters. of the fat globules vary between 0:01 mm. and 0:0016 mm. 
Thick cream contains about 56 per cent. fat and 39 per cent. water, 
whilst thin cream contains about 29 per cent. fat and 64 per cent. 
water. The fat content varies between very wide limits, and the 
percentage of fat is in inverse ratio to the density of the cream. 
The purchase of cream is based on its thickness, which is usually 
measured by its viscosity. Thickness is often induced by the aid of 
such substances as gum tragacanth, gelatine, starch, “ viscogen” 
(lime and sucrose-syrup), or by the process of homogenising, 7.e., 
forcing the cream through minute orifices (at a suitable temperature 
below 60°C.) under pressure. (See section on homogenised milk.) 
Since it is impossible to whip cream which has undergone homo- 
genisation, gelatine, agar-agar, or gum tragacanth is added to the 
extent of about 0-1 per cent.,so as to induce permanent frothing 
on beating up the cream. It is interesting to note that homogenised 
cream cannot be churned into butter. 
Butter. 
When milk or cream is churned, the globules of fat coalesce and 
_ granules are formed, which are then worked together to give a mass 
of apparently homogeneous texture. The average fat content of 
butter is 83-5 per cent. fat and the water content about 13 per cent. 
When examined under the microscope with crossed Nicol prisms, a 
uniformly dark field is viewed, whereas margarine under similar 
conditions presents a dark field containing bright portions and 
indistinct crystalline structures. The fat in milk, on churning into 
butter, has no crystalline structure, whilst margarine fats have, 
owing to their repeated melting and cooling. 
As to the exact changes occurring when milk fat is churned into 
butter, there is still divergence of opinions. The upholders of the 
“slim-membran” theory, argue that the mucoid substance envelop- 
ing the fat globules is rubbed off, and the globules therenpon coalesce. 
Fleischmann inclines to regard the process of churning as being the 
solidification of superfused fat globules, but this theory is discredited 
_ by the fact that the fat globules in milk are rapidly solidified by 
- mere cooling. 
It is very likely, however, that there is an adsorption layer of 
_ some kind around the fat and that during churning this layer is 
_ continually thinned out by the impacts of the various globules, 
_ eventually permitting coalescence to small nuclei, which grow by 
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