Man's physical universe

xanabras

AIR AND WATER AS RAW MATERIALS 657

effectively stop the reactions. In this case it is probable that the surface

of the catalyst is covered with a layer of the poisonous substance

to a depth of one atom or molecule, which thus renders it unable to

adsorb other substances.

The activity of catalysts may also be increased by the use of small

amounts of foreign substances called promoters.

The intermediate-compound and adsorption theories mentioned

above fail to explain some types of catalysis, and no satisfactory general

definition has yet been found. We have a great deal more to learn

about catalysis before we can hope to enter into successful competition

with nature in the synthesis of many natural products.

Catalysis Plays an Important Part in Life Processes.

Microorganisms, sometimes called nature's chemists,

bring about a

host of chemical reactions by a secretion of catalysts called enzymes.

Similar enzymes are produced in the higher plants and animals. It is

the enzymes produced in plants that cause fruits to ripen. The digestion

of foods by animals is greatly facilitated by the enzymes produced

in the mouth, stomach, and intestines. Vitamins contained in foods

act as physiological catalysts in animals, and it

has been adequately

demonstrated that many important reactions in the human body will

not take place in their absence. The human body also secretes important

catalysts, called hormones, which control the efficiency of the

utilization of food, the rate and extent of growth, sexual development

and activities, and many other functions. Some of man's greatest

conquests of diseases have been accomplished by the analysis, synthesis,

and controlled use of these organic catalysts. These physiological

catalysts will be discussed in more detail in Unit X.

Nitrogen Fixation Is an Excellent Example of Sjnithesis Which Requires

Catalysts.

The maintenance of the nitrogen supply of the soil is probably the

most difficult problem involved in keeping up the fertility of the soil

because nitrogen compounds are expensive and easily lost from the soil.

Nitrogen was formerly obtained from manures, and in some sections

of the world, such as China, manures are still

the chief source of nitrogen.

Nitrogen may be added to the soil by legume crop rotation, but

this process takes the land out of service, although it may even then

pay; for example, 1533 pounds of seed cotton were obtained from an

acre of ground after rotation with cowpeas, whereas only 837 pounds

of cotton were obtained when grown year after year without a rotation

of crops.

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