THE CELL 45 



then bright red, and sinks to the bottom of the glass, 

 leaving the liquid colourless. Therefore grape sugar 

 produces a red precipitate in our blue liquid ; or, in other 

 words, this blue liquid, otherwise called Fehling's 

 solution, by changing colour reveals the presence of 

 grape sugar. This reaction is so delicate that it will 

 betray in a liquid the presence of the most minute 

 quantity of this sugar. Thus we have in Fehling's 

 solution a valuable reagent for detecting the presence of 

 very small quantities of grape sugar. In iodine we have 

 a similar reagent for detecting the presence of starch. 

 I take a large beaker of water, add to it a few drops of 

 starch solution and stir. I have thus in the liquid 

 minute traces of starch. I add to it a few drops of 

 iodine solution, yellow in colour, and the liquid at once 

 turns blue. In the same way if I drop iodine solution 

 on a lump of dough or a piece of bread, I get a dark blue, 

 almost black spot, because starch is contained in both 

 substances ; but if I drop some iodine solution on a 

 piece of gluten, I do not get any black spot, because the 

 starch has been previously washed out with water. 

 So iodine stains the colourless starch blue, and therefore 

 serves as a reagent for detecting starch. We have now 

 to find means for a similar detection of cellulose. Iodine 

 by itself does not stain it blue, but iodine and zinc 

 chloride will. We have only to drop this solution on a 

 sheet of white paper, which, as we know, is cellulose, 

 to produce on it a blue spot. Such are our reagents, 

 our means for detecting the most widely diffused 

 carbohydrates, grape sugar, starch and cellulose. 



Now let us pass to another group, that of albuminoids. 

 These are found either in solution, as in the juice of the 

 cabbage, or in a solid form, e.g. the gluten we have just 

 obtained from our wheat grain. As soon, however, as 

 we heat cabbage juice, we see it turn into flakes : the 

 albumen has ' set ' or coagulated in the same way as 

 an egg ' sets ' when it is boiled. Chemistry presents a 



