SAND AND CLA V 6i 



containing a little caustic soda. The mixture is stirred 

 well, allowed to stand for several minutes, and the 

 supernatant liquid poured off. The sediment which 

 remains behind is twice more treated in the same 

 manner, the several washings being retained for the 

 subsequent separation of the clay (39 a). The liquid 

 should eventually pour off perfectly clear. The residual 

 matter is washed into a weighed dish, dried, ignited, 

 and weighed. It consists of the fine gravel and sand, 

 the- percentage of which in the soil can thus be 

 calculated. 



(d) Microscopical Examination. — The residual sand 

 should be examined with a powerful lens, or under the 

 microscope. The quartz particles, SiOg, show a crystal- 

 line structure and are generally transparent, though 

 possibly coloured with a little ferric oxide. 



Flakes of mica are common and may usually be 

 detected, whilst rounded fragments of coloured opaque 

 material may consist of more or less altered felspar, 

 oxide of iron, or other minerals. 



39. Clay, 



{a) The various washings from determination of the 

 sand (38 a) are united, and the liquid just made acid 

 by the addition of hydrochloric acid. The sediment 

 which settles out on standing consists of "clay." The 

 clear liquid is poured off, the sediment washed into 

 a weighed evaporating dish, dried on the water-bath, 

 ignited to a dull red heat, and weighed. This gives 

 the percentage of clay. 



{b) Hygroscopic Moisture and Water of Hydration. — 

 The hygroscopic moisture in a sample of clay is deter- 

 mined in the usual manner by drying about 2 grams 

 in the steam oven to constant weight. 



The chemically combined water is estimated by 



