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. 



