308 Mr. J. Dewar on the Chemical Efficiency of Sunlight, 



that amount of energy wbich is necessary to decompose again 

 the carbonic acid and water into their elements^ to separate the 

 oxvgeDj to give it back to the atmosphere, and to collect the 

 carbon and hydrogen of the water and carbonic acid in the 

 body of the plant itself. It is not yet possible to show that 

 there exists an accurate equivalent proportion between the power 

 or energy of the solar rays which are absorbed by the green 

 leaves of plants, and the energy w hich is stored up in the form 

 of chemical force in the interior of the plants. We are not yet 

 able to make so accurate a measurement of both these stores of 

 er.ergv as to be able to show that there is an equivalent pro- 

 portion. AVe can only show that the amount of energy which 

 the rays of the sun bring to the rank is completely sufficient to 

 produce such an effect as this chemical effect going on in the 

 plant. I will give you some figures in reference to this. It is 

 found in a piece of cultivated land producing corn or trees ; one 

 may reckon per year and per square foot of land 0'036 lb. of 

 carbon to be produced by vegetation. This is the amount of 

 carbon which during one year, on the surface of a square foot 

 in our latitude, can be produced under the influence of solar 

 rays. This quantity, when used as fuel and burnt to produce 

 carbonic acid, gives so much heat that 291 lbs. of water could 

 be heated 1^ C. Now we know the whole quantity of solar 

 light which comes down to one square foot of terrestrial surface 

 during one second, or one minute, or one year. The whole 

 amount which comes dow^n during a year to one square foot is 

 sufficient to raise the temperature of 430,000 lbs. of w^ater 1° C. 

 The amount of heat which can be produced by fuel growing 

 upon one square foot during one year is, as you see from these 

 figures, a very suiall fraction of the whole amount of solar heat 

 which can be produced by the solar rays. It is only the 1477th 

 part of the whole energy of solar light. It is impossible to de- 

 termine the quantity of solar heat so accurately that we could 

 detect the loss of so small a fraction as is absorbed by plants 

 and converted into other forms of energy. Therefore, at pre- 

 sent, we can only show that the amount of solar heat is suffici- 

 ent to produce the effects of vegetable life, but we cannot yet 

 prove that this is a complete equivalent ratio. '^ This estimate 

 is, strictly speaking, the mean agricultural efficiency of a given 

 area of land, cultivated as forest ; and considering that active 

 growth only takes place during five months in the year, we may 

 safely adopt -^ J-g- of the total energy of sunlight as a fair value 

 of the conserved power, on a given area of the earth's surface 

 in this latitude during the course of the summer. As chloro- 

 phyl in one or other of its forms is the substance through 

 which light becomes absorbed and chemical decomposition en- 



