36 SCIENTIST 



and an apparatus for measuring light intensity, we can 

 come up with some pretty good figures on how much hght 

 of exactly what wavelength is necessary to produce a given 

 amount of change in direction of the potato sprouts. From 

 there we can go on to analyze the production of green 

 material in the tips of the sprouts in relation to the amount 

 of Hght of a given wavelength. As we proceed, we will 

 find that the process of plant growth depends on the way 

 the green material absorbs the Ught and uses it to combine 

 the carbon dioxide (CO2) in the air with water to produce 

 carbohydrate. The speed of the process depends in part 

 on the amount of CO2 and water available and in part on 

 the temperature, the availabihty of nitrogen, phosphorus, 

 potassium, and several other elements. The effect of each 

 one of these factors can be analyzed by separating it out 

 in much the same way we separated the possible effect 

 of the fresh air from the effect of the hght. 



Often it is not possible to remove a factor as completely 

 as we removed the Hght or the fresh air. In such cases we 

 content ourselves with holding this factor constant. For 

 example, in the first part of our experiment we managed 

 to hold temperature essentially constant by doing the ex- 

 periment in a cellar. In technical terms we were "con- 

 troHing" for the effect of temperature. Much of the time 

 and effort of scientists goes into designing methods for 

 controlling the effects of variables Hke temperature, pres- 

 sure, and acidity, so as to observe the isolated effects of 

 the particular variable in which they are interested. 



Another procedure for isolating certain important ele- 

 ments from complex situations is almost purely intellectual 

 in nature. It is known as abstraction and depends upon 

 what seems like the simple ability to see what the members 

 of a set of things have in common with one another. Until 

 recently much scientific work in biology was devoted to 



