64-66] EXPERIMENTS ON SOILS 2J 



them to stand for an hour or so. The clay will scarcely 

 have settled at all in the one to which no lime-water has 

 been added, whilst the other will have become almost clear 

 owing to the clay having fallen to the bottom of the liquid 

 as though it were fine sand. 



64. Limestone in Soil. — In paragraph 39 it was shown 

 that when hydrochloric acid is added to marble (which is a 

 form of limestone) effervescence takes place, and carbonic 

 acid gas is evolved. Take a small quantity of soil in a 

 test-tube and moisten it with water. Fill the test-tube 

 about half full of dilute hydrochloric acid. If the soil 

 contains considerable quantities of limestone the effer- 

 vescence will be seen at once. Should only traces be 

 present the effervescence may be detected by holding the 

 mouth of the test-tube close to the ear, when the sound 

 caused by the gas coming off will be distinctly heard. 



65. Test for Lime. — Lime is oxide of calcium ; for a 

 full set of tests for that metal see paragraphs 1 78-1 81. 

 It usually exists in soils combined with carbonic acid. 

 To test for lime in a soil : boil the mixture of boil and 

 acid described in paragraph 64 for a minute, then add 

 ammonia to it until it is alkaline, and filter. To the clear 

 liquid so obtained add a little ammonium oxalate and allow 

 the liquid to stand for a few minutes. A white precipitate or 

 cloudiness shows that the soil contained lime. 



66. Test for Organic Matter (Humus).— Place a 

 little soil on a piece of platinum foil and hold it with a pair 

 of crucible tongs just over the top of a Bunsen flame. The 

 soil will first of all darken in colour until nearly black, then 

 it will become lighter again until it is of much the same colour 

 as it was before heating. The reason of these changes is that 

 the organic matter (which always contains carbon) becomes 

 charred, and the carbon so formed gives a dark colour to 

 the substance. After a while the carbon itself burns, and 

 leaves the soil as it was before the experiment, except that 

 the organic matter has been burned away. 



