30 FRACTICAL AGRICULTURAL CHEMISTRY [73-75 



a test-tube with distilled water, then add a few drops of 

 hydrochloric acid and a few drops of litmus solution. The 

 acid will colour the litmus red. Now add a few drops of 

 lime-water and shake up. If the liquid be still red (i.e. if 

 it be still acid) add a few more drops of lime-water and 

 shake up again. Continue doing so until the colour 

 changes to blue. When this takes place all the acid will 

 have been neutralised. 



* 73. Action of Lime on Salts of Iron.— Soluble salts 

 of iron are often injurious to crops. Lime has the property 

 of rendering these salts insoluble. Dissolve a crystal of 

 ferrous sulphate (copperas) in half a test-tube of cold water. 

 Add a little lime-water. The iron in the solution will be 

 rendered insoluble and a dark-coloured precipitate of fer- 

 rous hydrate will be formed. 



74. Sulphur in Gas-lime.— Lime which has been 

 used in the purification of coal-gas, called gas-lime, is some- 

 times used as a manure and insecticide, but unless it is 

 exposed to the action of the air for a considerable time it 

 acts as a plant poison, as it contains calcic sulphide. To 

 show the presence of a sulphide, place a little gas-lime in a 

 test-tube and add a little dilute hydrochloric acid. Effer- 

 vescence takes place and the calcic sulphide is decomposed, 

 forming sulphuretted hydrogen, which may be recognised 

 by its offensive smell. 



* 75. Sulphur in Gypsum.— Gypsum (sulphate of lime) 

 contains sulphur, but as a plant-food. Treat a little gypsum 

 as in the last experiment. No sulphuretted hydrogen will 

 be given off. Mix a little gypsum in a mortar with twice its 

 weight of powdered charcoal. Place a little of this mixture 

 on a piece of platinum-foil and heat over a Bunsen flame 

 until all the charcoal is burned off. Allow to cool, then place 

 the mass in a test-tube and add a little dilute hydrochloric 

 acid. Sulphuretted hydrogen will now be evolved, showing 

 that sulphur was present in the gypsum. The action of the 



