LIQUID DIFFUSION. 



99 



on a steam bath (or in a sort of oven heated by steam) to dry it 

 completely, and the basin is then weighed after cooling. Sub- 

 tracting from this weight the weight of the second empty basin, 

 the difference represents the weight of fatty matter contained in 

 the milk. 100 parts of genuine cow's milk that has not been im- 

 poverished by allowing it to stand and skimming off more or less 

 of the cream that has then risen, and that has not been adulterated 

 by adding water, ought to yield at least from 2J to 3 parts of 

 fatty matter and from 10 J to 11 J of " total solids!" Eich milks, 

 like that of a good Alderney cow, give higher figures. According 

 to the quantity of fatty matters found, and of the other constituents 

 of the " total solids " present, the analyst forms an opinion as to 

 the genuineness of the milk, and to what extent it has been skimmed 

 or watered, should such adulteration have been made. 



Spontaneous Intermixture of Liquids : Liquid Diffusion. 



Liquids that are miscible together, but of different density, 

 exhibit a remarkable behaviour when they are arranged in 

 different layers, so that the heavier liquid is undermost, viz., that 

 although kept perfectly at rest, and at a uniform temperature so 

 as to avoid intermixture by setting up convection currents (Chapter 

 XX.), the heavier liquid will gradually rise up and disseminate 

 itself throughout the lighter one, and, conversely, the lighter 

 liquid will sink and intermix with the heavier one. This process 

 is termed Diffusion. 



Expt. 90. To show the Diffusion of Salt and Water. Fill 

 a bottle with strong brine, and loosely cork it ; lower it into a 

 large jar full of distilled water, so that the 

 mouth of the bottle is an inch or two below 

 the surface of the water in the jar (fig. 50). 

 Carefully withdraw the cork, and leave the 

 whole to stand for a day or two ; at the end of 

 this time, more or less of the heavy brine will 

 have risen up through the lighter water, 

 whilst some of the lighter pure water will 

 have descended into the bottle, weakening 

 the brine. Carefully take out a spoonful of 

 water from the top portion of the liquid in the 

 jar, place it in a test-tube, and test it with 

 nitrate of silver (Expt. 37). You will observe 

 a more or less marked cloudiness, or precipitate formed, showing 

 that the salt originally contained in the brine in the bottle has be- 

 come partly diffused throughout the pure water outside the bottle. 



Fig. 50. Diffusion 

 Bottle inside Jar. 



