54 LESSONS IN CHEMISTRY. 



same, and is able at once to detect its presence by a property 

 which it possesses of producing a beautiful purple colour when 

 brought into contact with a solution of the metallic-looking 

 substance iodine,* which I have described as an ingredient of 

 the sea and of the plants which live in its waters. {See note^ 

 p. 48.) Like cellular tissue, starch contains no nitrogen, but 

 consists solely of carbon, and of hydrogen and oxygen united 

 in the proportions in which they exist in water. Starch, as 

 the mode in which it is procured shows, is insoluble in cold 

 water, but dissolves, as you are aware, in boiling water, pro- 

 ducing a jelly-like liquid. By the influence of several che- 

 mical agents, and also by means of a peculiar substance 

 generated in plants, starch can be made to undergo important 

 changes, which, as they are of great interest in connexion 

 with the growth of plants, it will be necessary briefly to 

 consider. 



Q6. When a portion of farina or any kind of starch is 

 placed in a flask with water and boiled, a thick jelly is pro- 

 duced, which when dried has the appearance of glue, is, like 

 the starch itself, insoluble in cold water, and is in the same 

 way coloured blue by a solution of iodine. If we make an 

 infusion of barley, and add to it the starch jelly, and keep 

 them some time together, no change is produced; the starch 

 remains undissolved ; but if the grains of barley employed in 

 making the steep have been allowed to vegetate in the field, 

 or have been made to vegetate by art, by " malting" as it is 

 termed, there is a most surprising difference in the effect 

 produced. The starch is seen to grow gradually more liquid, 

 and in the course of a few minutes its consistence entirely 

 disappears, and it becomes as thin and transparent as water. 

 If we evaporate to dryness the transparent solution, we do 

 not obtain a jelly-like mass such as would result from evapo- 

 rating a smiple solution of starch, but a yellow powder which 

 differs from it in being readily soluble in cold water. The 

 solution of this powder is not rendered blue, but of a wine- 

 red colour, by the addition of iodine, showing that the starch 

 originally contained in the liquid has undergone some singular 

 change, and in fact, that the elements which compose it are 

 no longer united in the same form. 



6^. This yellow powder possesses the properties of a gum, 



* A solution of iodine in spirits of wine is sold by thte apothecarj' as 

 tincture of iodine. 



