DIRT ESTIMATION. 



391 



Crystalline particles usually indicate road dust. In this case 

 a little of the deposit is placed on a slide and warmed with dilute 

 hydrochloric acid, which is evaporated nearly off ; a drop of 

 water is added and also a drop of a solution of potassium ferro- 

 cyanide. A blue colour, due to iron, is obtained from road 

 dust. An estimation of the dirt may be made by allowing the 

 milk to stand, and measuring the amount in a graduated tube 

 (Fig. 43). 



Fig. 43. Dirt Estimation in Milk. 



