318 NEW ZEALAND NATURE-STUDY BOOK 
Again the solubility of limestone in carbonated water can be 
made clear, by passing a stream of carbon-dioxide through water 
holding carbonate of lime in suspension. Ina limestone district 
the furrows traversing the surface of rocks should be noticed and 
explained. 
The action of water in carrying away loosened particles and 
sediment may be exemplified by referring to the work done on the 
roadside or in the playground during a shower of rain. 
The action of frost can be shown by immersing in a freezing 
mixture (made by mixing together salt and snow or salt and ice) 
a test tube filled with water. In a short time the temperature of 
the water will be lowered, and the liquid itself converted into 
ice—when the expansion of the water during the process of freezing 
will crack the test tube. In this connection reference should also. 
be made to the bursting of water pipes in frosty weather. 
In fact numerous ways of illustrating this phase of the 
subject will occur to the thoughtful teacher. Every advantage 
should be taken of local conditions to make the subject real, and 
to encourage pupils to question Nature and watch for her answers. 
XVII.—THE FORMATION OF SOIL. 
The soil is formed from the rocks, and the sub-soil may 
be considered intermediate between the two. In many 
places —road cuttings, river-banks, sea-cliffs, land-slips,—the 
transition from the one to the other is plainly to be seen, 
the soil giving place to sub-soil, which gradually becomes 
more compact and harder until it merges into the 
underlying rock. 
The disintegrating action of weathering agents is not 
confined to bare and exposed rock-surfaces, but is still 
continued below the weathered mass. Indeed, owing to 
the greater ease with which weathering agents can obtain 
access to the material composing the crumbling crust, 
decomposition and disintegration are there carried on with 
increased rapidity. Other agents now begin to take part in 
the process. Seeds germinate, and, sending down their roots, 
work still further on the fabric of the soil. This becoming 
