192 THE NATURE STUDY COURSE. 



hydrochloric acid into a bottle containing two ounces of water 

 and adding as much pepsin as will lie on the point of a pen- 

 knife. Put in this mixture, and observe the digestion of, a 

 little bit of minced lean meat, or a bit of boiled white of egg 

 sliced, or better than either the flaky white of egg coagulated 

 by stirring it as it falls drop by drop into slightly acidulated 

 boiling water. Set the bottle in water at blood heat. 



The cooling of the body by perspiration may be explained 

 or illustrated by laying wet muslin or wet batting on a ther- 

 mometer bulb and observing the descent of the mercury. This 

 descent will take place even if the muslin or batting had been 

 wet in warm water. 



Carbon dioxide in expired air may be demonstrated by 

 passing the breath through a tube or straw into limewater. 

 (Lime water may be made by putting a cupful of freshly 

 slaked-lime into a quart of water ; stirring it well three or 

 four times in an hour and then settling it. Siphon off and 

 bottle the clear limewater below the scum on the surface.) 

 It is estimated that a person exhales about a half-pound of 

 carbon per day. Weigh out that quantity of charcoal to show 

 the pupils that its conversion into gas by burning would be 

 the equivalent of the C0 2 exhaled by each one in twenty-four 

 hours. 



Iodine in a water solution gives a blue color to any 

 substance containing starch. Fehling's solution boiled with 

 any substance containing grape-sugar gives a reddish-orange 

 re-action. These re-agents may be used to show that complete 

 digestion by saliva converts starch into grape-sugar. 



The coagulating effect of alcohol on albumen is shown by 

 putting some white of egg into brandy or whiskey. 



Apparatus and Models. — The demonstration of the circula- 

 tion of blood in the capillaries of a tadpole's tail or a frog's 



