98 



SCIENTIFIC AMUSEMENTS. 



form by filtering the watery liquid and evaporating the clear liquor 

 to dryness in a basin heated by the steam bath. What is left undis- 

 solved by both the ether and water is mainly the caseine of the milk. 

 The analysis of milk, in order to determine its quality and 

 purity, is performed in very much the same way as the above 

 experiment, except that considerable precautions are requisite in 

 the manipulation in order to obtain exact results. A measured 

 quantity of the milk to be examined is put in the evaporating 

 basin first of all, this having been carefully weighed on a delicate 

 weighing-machine or "balance" made specially for purposes of the 

 kind. This is simply an extremely delicate pair of scales enclosed 

 in a glass case (fig. 49), with doors and sliding glass panels to 



Fig. 49. Chemical Balance. 



enable the weights, &c., to be put on the pans, and then all 

 currents of air kept off by closing the doors ; a mechanical 

 arrangement prevents the balance being able to swing until a catch 

 is released by means of a rod worked by a mill-headed button or 

 disc, or by a lever pressed by the finger. When the water has all 

 been driven off and the residue is quite dry, the basin is weighed 

 again ; subtracting from this weight that of the empty basin, the 

 difference represents the quantity of " total solids " contained in 

 the milk, viz., fatty matters, milk sugar, casein, &c., &c., all 

 lumped together. When the residue is treated with ether, the 

 ethereal solution of fatty matters is transferred to another weighed 

 dish, being filtered through paper if not quite clear; after the 

 ether has evaporated, the fatty matter left on the basin is heated 



