THE SCOPE OF BIOLOGY 23 



Examine a little of the starch water under a microscope. Sketch some 

 of the starch grains. Make a thin section of a bit of potato with a razor 

 and examine under a microscope, noting the starch grains. Add a little 

 iodine solution and again examine with a microscope. 



Boil the starch water over a flame. As it comes near to the boiling 

 point the mass will become thick and pasty (starch paste), due to the 

 bursting of the starch grains by heat. Place a little under the microscope 

 and look for grains. Add to the paste a little iodine and it will turn a 

 brilliant blue. 



Test for Sugar. Put a little glucose or dextrose in a test tube con- 

 taining a considerable quantity of water. Add to this a few drops of 

 weak H2SO 4 and a few drops of NaOH; boil. The presence of sugar is 

 determined by the appearance of a brownish red precipitate, which goes 

 through a series of color changes, but finally remains as a brownish red 

 sediment at the bottom of the tube. 



FATS 



One of the simplest tests for fat is to place the material in which the 

 fat is supposed to be upon a sheet of common paper. The paper will be 

 rendered transparent by absorbing the oil of the fat-containing tissue. 



Fat Emulsion. Fat has the property of being readily divided into 

 minute particles which, when mixed with water, float in the liquid, form- 

 ing what is known as an emulsion. Place a few drops of olive oil in a 

 test tube half full of water. The oil will rise to the top of the water and 

 appear as a clear yellowish layer. Put a finger over the mouth of the 

 test tube and shake vigorously. The whole contents of the tube will 

 turn a milky white, and upon being allowed to stand the milkiness will 

 remain for a long time. Eventually, however, the fat again separates 

 from the water. This milky appearance is produced by the fact that the 

 fat has been divided into minute particles that float through the water 

 and refract the light in such a way as to give a white color. This is called 

 an emulsion. 



Examine, under a microscope with a high power, a drop of milk, noting 

 that it is an emulsion. 



BOOKS FOR REFERENCE 

 Recent Works in General Physiology and Psychology 



CONN, Advanced Physiology, Silver, Burdett & Co., New York. 

 DRIESCH, The Science and Philosophy of the Organism, A. & C. 

 Black, London. 



