52 THE DAIRY OF THE FARM. 



milk may be read off upon the scale by the thickness of 

 the layer which it exhibits. A series of such tubes in a 

 frame are needed for comparing in this way the milk of 

 different cows. 



Another instrument, for the same purpose, depends for 

 its indications on the fact that opacity of milk depends 

 upon the corpuscles of fatty matter which are suspended 

 in it, and that consequently the more cream it contains the 

 greater will be the obstacle opposed to the passage of light. 

 It consists of two tubes, one of which may be pushed into 

 the other like the joints of a telescope, and the end of each 

 tube is closed with glass, so that when milk is poured into 

 the outer tube by a small opening on the side, by pushing 

 in the inner tube, a layer of milk of any thickness may be 

 obtained. The apparatus is placed on a stand, and the 

 value of the milk is estimated by the thickness of the 

 layer of it through which the light of a small wax 

 taper at a fixed distance can be observed, the value of 

 the milk being in the inverse ratio of the transparency ; 

 for the larger the amount, of fat present, the greater, of 

 course, will be the opacity. The thickness of the layer 

 of milk is measured by a scale on the instrument, and a 

 table sold with it shows the percentage of cream to which 

 it corresponds. 



These and other devices are expedients for determining by 

 mere observation the relative quality of different samples of 

 milk. For the exact determination of its composition a 

 tedious process of analysis is required, on being subjected 

 to which it is shown to consist of casein, butter, sugar of 

 milk and water, besides soluble and insoluble mineral 

 matters. The quantities of these in a sample of milk 

 vary very considerably. 



