112 NATURE TEACHING 



probably derived. (A selection of these stones should 

 be kept in the school.) 



In frosty weather select one or two pieces of sand- 

 stone, and of brick, of very porous character. Wash off 

 all dust and loose pieces, place them in a vessel of water 

 so that they are about half covered, and leave them to 

 soak for two or three hours. Put them out of doors 

 where they can become thoroughly frozen, and allow 

 them to remain exposed to two or three nights' frosts. 

 Then bring them into a warm room, and when they have 

 thawed completely, examine carefully to see whether any 

 small fragments have been split off. The stones or bricks 

 should be placed on clean plates or saucers when put 

 out in the cold, in order that any small pieces broken off 

 can be more easily seen. 



Mechanical Analysis of Soil. 



Separation of soil particles by means of water : This 

 operation may be conducted so as to give quantitative 

 results of interest and value, if a small amount of appar- 

 atus is procurable. For this work it is necessary to have 

 three sieves with holes of known sizes ; brass sieves with 

 circular perforations are preferable to those of wire. 

 A suitable set consists of three sieves with holes of 

 2, i, and \ millimetre respectively. (i mm. = J T indi-) 

 A small scale or balance for weighing the separated 

 gravel, sand, etc., is also required. 



With these proceed as follows : From a well mixed 

 sample of soil weigh out 50 grammes (if oz.), stir this 

 well with water in a glass or cup, and pour the water 

 through the sieve with 2 mm. holes ; the sieve resting 



