ANIMALS AND THEIR PBODUCTS. 131 



white ; but, if dissolved in water, it does not render 

 the water white, but leaves it transparent; whereas 

 milk is white, opaque, not transparent, which shows 

 that the substances combined with water to form it 

 are not perfectly dissolved, but that a portion of them 

 at least are only suspended in water. The same ap- 

 pears also from the fact that some of them separate*by 

 standing. 



236. The curd of milk exists in the form of little 

 sacks, or bags, each enclosing a globule of oil. These 

 little sacks of oil are so nearly of the same weight 

 with the water, that, by the slightest agitation, they 

 are kept diffused nearly equally throughout. They 

 are, however, a trifle lighter than the water. This 

 gives them a tendency to the surface ; and it accounts 

 for the fact that the milk in the upper portion of the 

 cow's udder, that which is drawn last, is the richest. 

 It is so with milk standing in a pail only a short time 

 — the top is richer than the average of the whole. 

 This slight tendency of the curd-sacks, which en- 

 close the oil, of which butter is made, to rise to^the 

 surface, is the principle on which the cream is sep- 

 arated. 



237. Owing to the upward tendency being so very- 

 Blight, milk should be set in broad, shallow pans. A 

 given quantity, set in such pans, will produce more 

 cream than if set in deep vessels. We must suppose 

 that the oil is lighter than the curd-sack in which it is 

 contained. Those sacks which contain most, rise 

 first ; those which contain less, rise more slowly ; and 



