CELL-CONTENTS AND CELL-WALLS. 57 



elements). Note also that the starch soon begins to blacken, prov- 

 ing that it contains carbon, and at the same time dirty white fumes 

 are evolved, having a pungent odour somewhat resembling that of 

 burnt sugar. 



Apply a light to the mouth of the test-tube the fumes are 

 inflammable. Introduce a piece of moist blue litmus paper into it 

 the litmus becomes red, showing that the fumes are acid. Introduce 

 a glass rod, on the end of which is a drop of lime-water, into the 

 test-tube. The lime-water becomes milky, showing that carbon 

 dioxide is one of the products of decomposition of starch. This 

 confirms the presence of carbon in starch (since carbon dioxide is a 

 compound of carbon and oxygen). 



When all the volatile matter has been driven off, a black residue 

 of charcoal remains. 



68. Experiments with Starch Solution. Shake up 

 some powdered starch with cold water ; it is not dissolved. 

 Filter, test the filtrate with iodine no blue colour is 

 given. Stir up some dry starch with a little cold water, 

 then add boiling water, and boil until an imperfect 

 opalescent " solution " is obtained ; this, on cooling, will, 

 if strong enough, " set " or gelatinise to form a paste or 

 mucilage. 



(a) Add iodine solution to the starch solution ; the blue 

 colour produced will disappear on heating (the " iodide of 

 starch " is destroyed by heat) and reappears on cooling 

 (run a tap of cold water over the test-tube to cool it). 



(6) To another tube of starch solution add Fehling's 

 solution, and heat. There is no reduction. 



(c) Hydrolyse a portion of the starch solution by boil- 

 ing with a little dilute sulphuric acid for a few minutes ; 

 neutralise with soda or potash, and test with Fehling's 

 solution. Reduction occurs, owing to the conversion of 

 starch into glucose. 



(d) Note that starch is precipitated from solution by 

 alcohol, also by basic lead acetate solution. 



69. Starch Grains in Potato Tuber (Fig. 21). Cut 

 across a Potato tuber, apply some dilute iodine solution to 

 the surface, and note the deep blue or almost black colour 

 due to the abundant starch. In testing for starch, it is 



