242 CHAP. XXVI. THE PHYSIOLOGICAL SCALE 



variation of each factor at a time, as its intensity is gradually 

 increased from a minimum to a maximum. We plot the 

 results in the form of a curve, in which the ordinate represents 

 the photosynthetic activity and the abscissa the intensity 

 of the factor which is undergoing variation. The charac- 

 teristics of the curve enable us to find the relation between 

 the varying factor and the effect produced. 



Scale of Measurement 



In all quantitative measurements it is necessary to adopt 

 a suitable scale. For this we have to fix on two points of 

 reference, a lower and an upper, the interval being divided 

 into a convenient number of units. In the measurement of 

 temperature, for example, the two definite points usually 

 adopted are the temperatures at which water changes its 

 condition to solid or to steam — the melting-point of ice or 

 the boiling-point of water. According to the centigrade 

 scale the interval is divided into ioo parts, the melting-point 

 of ice being taken as zero. The fixing of the zero and of the 

 higher point is, however, arbitrary. For there is no reason 

 why a substance other than water should not have been 

 chosen for the determination of the two points of reference ; 

 the melting-point of solid paraffin would have given a higher, 

 and that of solid mercury a lower, zero-point. Fahrenheit 

 attempted to discover the lowest temperature attainable in 

 his time from a mixture of ice and salt. We have thus two 

 temperature zeros, according to the centigrade or the 

 Fahrenheit scale, the interval between the zero and the 

 boiling-point being divided respectively into'ioo and 212 

 parts. Again, the zero-point and the scale of measurement 

 are by no means final ; they are adopted as a matter of 

 convenience to suit the particular investigation. The 

 zero-temperature, according to Fahrenheit, was, as previously 

 stated, determined from the lowest temperature that could 

 be obtained in his time. Still lower temperatures were 

 produced later in the liquefaction of gases ; but the zero 



