THE ABSOLUTE ZERO 243 



was indefinite, for different gases are liquefied at different 

 temperatures. 



The Absolute Zero 



A certain characteristic property of permanent gases 

 enables us, however, to obtain an absolute zero. The 



coefficient of expansion of gas is — ; the curve representing 



the expansion of gas at different temperatures, when pro- 

 duced backwards, cuts the abscissa at the absolute zero 

 (— 273 ° C), where the volume of gas is imagined to have 

 shrunk into nothing. The absolute zero is a hypothetical 

 abstraction, for the volume of a gas can never be reduced to 

 zero ; the gas changes its condition and becomes liquefied 

 before reaching — 273 . The adoption of the absolute zero 

 for the physical scale has, among others, the following 

 advantage. The volume of a given quantity of gas is thus 

 found (within limits) to be strictly proportional to the 

 temperature as represented on the absolute scale. Thus, 



y_ = T 

 V T' 



where V and V are the volumes of the gas for the tempera- 

 tures T and T' represented on the absolute scale, pressure 

 being supposed to be constant. The volume of a gas is 

 thus a linear function of the absolute temperature. 



/ will now show that, on ascertaining the proper physio- 

 logical zero, photosynthetic activity is also found to be a linear 

 function of temperature as represented on the physiological 

 scale. We shall also find a similar relation when the varying 

 factor is C0 2 -content, or light, the physiological zero being 

 appropriately determined for each of these factors. 



Advantages of the Adoption of the 

 Physiological Scale 



I proceed to show that the adoption of the physiological 

 scale not only enables us to formulate a general law 

 of photosynthesis, but it affords us means of determining 



