12 PRACTICAL AGRICULTURAL CHEMISTRY [27-28 



27. Borax Bead. — Fuse a little borax on the loop of a 

 platinum wire until it becomes like a bead of colourless 

 transparent glass. Now take a very little manganese dioxide, 

 and heat in the outer blowpipe flame ; a port-wine tint will 

 be imparted to the bead. Next heat in the inner flame ; 

 the bead will become colourless. It will thus be seen 

 that the two flames may give rise to different coloured 

 beads. 



28. Use of Test-papers. — Certain vegetable colours 

 are affected by the addition of certain chemicals. The 

 test-papers in common use are made by soaking paper in 

 litmus or turmeric solutions. They are usually purchased 

 in little books. 



Litmus occurs normally as a blue substance. Its 

 solution in water or alcohol is at once turned red by acid 

 bodies. This reddened litmus is then sensitive to alkalies, 

 which give the original blue colour again. 



Turmeric is normally of a yellow colour. Its solution 

 is turned brown by alkalies, the yellow colour being 

 restored by acids. 



Experiment. — Try the effect of solutions of hydro- 

 chloric acid, sodium hydrate, and sodium chloride on 

 blue and red litmus and turmeric papers. It will be 

 found the first gives an acid, the second an alkaline, and 

 the third a neutral reaction. 



Thus by means of test-papers we have a method of 

 ascertaining when a reagent is added in excess, provided 

 that reagent give a different reaction to the liquid to which 

 it is added. 



