202 THE BOOK OF NATURE STUDY 



strongly heated parts with its carbonic acid to pass into the 

 state of quicklime. Quicklime in its turn has a great attraction 

 for water and carbonic acid ; it greedily takes up water to 

 become slaked lime, and slaked lime will quickly absorb car- 

 bonic acid to go back to carbonate of lime. Pour a little dilute 

 hydrochloric acid upon a piece of chalk in a dish, there is a 

 violent effervescence due to the carbonic acid expelled by the 

 stronger hydrochloric acid ; repeat the same experiment with the 

 different samples of soil, some of them will be sure to effervesce a 

 little, but with others the bubbles of carbonic acid will be barely 

 perceptible because when there is less than about i per cent, 

 of carbonate of lime in the soil the carbonic acid dissolves in the 

 liquid as fast as it is set free. Take a little of the fine clay or 

 even of the clay soil, rub it up into a paste with water, and then 

 make up two large jars of turbid clayey water, using distilled 

 or rain water ; add to one of them a little lime or a few 

 cubic centimetres of clear lime-water and put the jars aside 

 to stand. As before, they will take some time to clear, but the 

 lime in the one jar will bring about a much more complete and 

 earlier clearing, as though it had transformed the clay into 

 coarser particles more of the nature of fine sand. It is now 

 easy to understand why the brick worked up with lime gave 

 various indications of having been rendered more coarsely 

 grained, such as its reduced shrinkage on drying ; the lime com- 

 pounds have the power of making the finest clay particles bunch 

 up together or " flocculate," until they behave like a smaller 

 number of larger ones. All the soluble salts of lime act thus, 

 sulphate of lime or gypsum, for example, and carbonate of lime, 

 because it becomes so readily bi-carbonated and dissolved by the 

 carbonic acid in the soil water. 



Flocculation of the clay particles is not, however, the only 

 action brought about by the carbonate of lime we have recognised 

 in the soil ; it also behaves as an anti-acid or neutraliser of the 

 injurious acids which are often produced by the decay of organic 

 matter in the soil. Moisten a little of the peaty soil and leave 

 it for some time in contact with a piece of blue litmus paper ; 

 as a rule the litmus will be reddened by the acids contained in 

 the decaying vegetable matter of the peat. Now grind up a 



