STARTING INVESTIGATIONS. 193 



foot has been referred to on page 70. A considerable number 

 of the illustrations in all the text-books are made from micro- 

 scopic sections. The technical skill required to make instruc- 

 tive preparations of animal tissues is not yet possessed by 

 many teachers. 



The fluids of the body pass through membranes by an 

 obscure process called osmosis. It may be demonstrated by 

 dissolving off the shell of an egg in dilute acetic acid or 

 vinegar, and then immersing it for a day or two in water 

 or dextrose solution. Measurements taken before and after 

 treatment show a marked increase in the dimensions of the 

 egg, and if dextrose has been used its presence in the egg may 

 be tested with Fehling's solution. A simpler method is to 

 remove a half-inch circle of the shell from the wide end; also 

 the membrane over the cavity, without wounding the inner 

 membrane. Rest the narrow end of the egg on a napkin ring 

 in a glass of water. By and by the inner membrane will 

 protrude through the opening, and stretch until it finally 

 bursts. 



Reflecting sunlight upon the wall from a half -inch square of 

 mirror pressed by its lower edge against the pulsing artery 

 in the wrist, will show that the artery fills suddenly and 

 contracts comparatively slowly. 



Starting Investigations. — It cannot be emphasized too 

 strongly that the teacher should seek in the children's interests 

 for the lines along which he should pursue nature studies — 

 for the topics of Nature Study lessons. Even when the study 

 starts under the impetus of native interest it may often 

 tax the teacher to maintain the interest until a substantial 

 educational end is gained. 



Besides watching the children's actions and conversations 

 to discover points of interest the teacher may awaken it by 



