Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 19 



pie, in the body of a rabbit they may be carried in and out of organs, into cells 

 and perhaps out again. 



The travels and destinations of such labeled isotopes are detected most com- 

 monly by the now familiar Geiger-MuUer counter. This apparatus detects and 

 amplifies each radioactive disintegration of an atom. The number and rate of 

 disintegrations are a measure of the amount of labeled material present. In 

 general the use of tracers is directed toward investigations of the constant 

 buildup and breakdown, and the come and go of chemically active molecules 

 in the living organism. In this way it has been learned that thyroxin, the 

 iodine-containing amino acid that is so important in the functioning of the 

 thyroid gland, is manufactured by muscle and in the intestine as well as in 

 the thyroid gland. Recent studies on the metabolism of rabbits by means of 

 radioactive isotopes have shown that radioactive phosphorus administered to 

 adult animals enters their bones and the enamel and dentine of their teeth. 

 This shows that such hard substances, deposited in early youth, do not stay 

 unchanged for a lifetime, but are continually exchanging material with the cir- 

 culating blood. 



States of Matter 



Molecules are continually affected by the attraction of their neighbor mole- 

 cules. Their relative sizes and the distances between them determine the 

 strength of their mutual attraction and the state of the substance in which they 

 are contained whether gas, liquid, or soUd (Fig. 2.9). Changes of matter from 

 one state to another involve a change in energy, usually the giving off or 

 absorption of heat. 



In gas, the molecules are scattered away from each other; their movements 

 are rapid and disorderly and they take zigzag turns into their surroundings. 

 The volume of a gas is dependent upon temperature and pressure. The gas 

 spreads through all available space but is compressible because it does not 



RELATIVE DENSITIES 



MIXTURES 



Gas 





Liquid 



Solid 



B Solution 



Suspension 



o;.oi.?jo 



Emulsion 



Fig. 2.9. A. Diagrams showing the relative densities of molecules in a gas, 

 liquid, and solid. B. Diagrams of mixtures: solution thoroughly dissolved and 

 homogeneous; suspension with particles of one substance undissolved; emulsion 

 with very large undissolved droplets. 



