248 



RESPIRATION AND EXCRETION 



1. Method. — Insert your hand in a clean, dry fruit jar. Wrap 

 a towel over the opening of the jar so as to allow no air to get in 

 between your hand and the sides of the jar. 



Observations. — What happens in the jar? 

 Conclusion. — What is given off from the hand? 



2. Method. — Weigh yourself . Note the weight. Exercise vi- 

 olently for half an hour; weigh yourself again. Note the weight. 



Observations. — Was there any change in weight? 



Conclusion. — How must the change of weight have been 

 brought about and how did the body lose this ? Remember that 

 when oxidation of food or tissue takes place in the body three 

 products, at least, are formed : heat, organic wastes, and water. 



(Food + oxygen = carbon dioxide + water + organic wastes + heat 

 + muscular energy.) 



3. Method. — Take the temperature of the body before and 

 after exercise by placing a clinical thermometer in the mouth. 

 Any change? Account for this by the following experiment. 



4. Method. — Take two thermometers, place a damp cloth 

 around the bulb of one and leave the other exposed without a 

 damp cloth. After some time, so as to allow the water in the 

 cloth to reach the same temperature as the air in the room, 



read the two ther- 

 mometers. 



Observations. — 

 Do they both read 

 the same? How do 

 you account for the 

 difference? Remem- 

 ber that when water 

 evaporates, it takes 

 heat from the air sur- 

 rounding it. 



Conclusion. — Ap- 

 plying this principle 

 to the skin, explain 

 why evaporation from the skin makes us' feel cooler. 



