204 PHYSIOLOGY. 



furnished by the material of the house, the weight would 

 all be recovered. 



In the continual wasting away of our bodies there is no 

 real loss of matter. Our weight is reduced, but the wastes 

 are still part of the earth or air, and are used again. For 

 instance, a particle of carbon in the carbon dioxid of the 

 expired breath may be taken in through a blade of grass in 

 an adjoining field. A cow may eat the grass, and we may 

 soon take the very same particle of carbon in the flesh or 

 milk of the cow. Or the carbon may be taken by that 

 kind of grass called wheat, and become part of the seed 

 or grain of wheat, and be made into flour and be eaten as 

 bread, and be part of us once more. Or this particle of 

 carbon might be carried by the winds to Florida or Cali- 

 fornia, and become part of an orange, and come again to 

 make part of our bodies. Thus there is a ceaseless round 

 of matter into and out of our bodies. The plants furnish 

 food for us, and we help to make food for them by the 

 wastes of our substance. No one has a monopoly of any 

 portion of matter ; it is now ours, now some one else's. A 

 particle may pass from one animal to another animal, as 

 when we eat flesh or other animal food. But more often 

 the wastes of our bodies go to make part of the air or the 

 soil, and are then taken by some plant before again becom- 

 ing part of our tissues. But we are as unable to destroy 

 matter as we are to create it. 



The Indestructibility of Force. So with energy. We 

 cannot create it and we cannot destroy it. We derive our 

 energy from the food we eat. And this food we get 

 directly or indirectly from the vegetable kingdom. 



An engine gets energy from the combustion of fuel. In 

 the growth of the plant under the influence of sunlight the 



