390 



SCIENCE 



[N. S. Vol. XXXVI. No. 926 



clustrial purposes land which should be 

 devoted to raising foodstuffs. An approxi- 

 mate calculation shows that on the earth 

 there is plenty of land for both purposes, 

 especially when the various cultivations 

 are properly intensified and rationally 

 adapted to the conditions of the soil and 

 the climate. This development is the real 

 problem of the future. 



II 



Technical organic industry may yet ex- 

 pect great help from photochemistry un- 

 derstood in the sense above expressed and 

 the competition between this anc^-the chem- 

 istry of coal tar will be a great incentive 

 for new progress. It is also true that 

 human genius will always tend to proceed 

 along lines selected by itself, and there is 

 no question but that the great development 

 in the coal tar industry has been due in 

 part to this splendid spirit of indepen- 

 dence. It may be asked whether there are 

 not other methods of production which may 

 rival the photochemical processes of the 

 plants. The answer will be given by the 

 future development of photochemistry as 

 applied to the industries and on this I 

 have a few ideas to express. The photo- 

 chemical processes have not had so far any 

 extensive practical application outside of 

 the field of photography. From its very 

 beginning photography has aroused a great 

 deal of interest; it was taken up technic- 

 ally and, as usually happens in similar 

 cases, it had a rapid and brilliant success. 

 But notwithstanding the many applica- 

 tions photography represents only a small 

 part of photochemistry. So far, photo- 

 chemistry has only been developed to a 

 very slight extent, perhaps because chem- 

 ists have been attracted by problems which 

 seemed more urgent. So it happens that 

 while thermochemistry and electrochem- 

 istry have already i-eached a high degree 



of development, photochemistry is still in 

 its infancy. Now, however, we notice a 

 certain awakening due to a series of studies 

 concerning general problems and special 

 processes, especially in the organic field, in 

 which my friend Dr. Paul Silber and my- 

 self have taken an active part. Two recent 

 puiblications, one by Plotnikow and the 

 other by Benrath, bear witness to this. 

 But much remains to be done both in the- 

 oretical and general photochemistry as well 

 as in the special branches. 



The photochemical reactions follow the 

 fundamental laws of affinity, but have a 

 special character. They are especially 

 notable for the small temperature coeffi- 

 cient and are, however, comparable — a fact 

 which is not without technical importance 

 — to the reactions which take place at very 

 high temperatures. According to a bril- 

 liant idea of Plotnikow, luminous radia- 

 tions produce a different ionization from 

 that due to electrolytic dissociation; the 

 separation of an ion requires a quantity 

 of light which is determined by the theory 

 of Planck and Einstein. The question is 

 therefore related to the most recent and 

 profound speculations of mathematical 

 physics. 



For our purposes the fundamental prob- 

 lem from the technical point of view is 

 how to fix the solar energy through suit- 

 able photochemical reactions. To do this 

 it would be sufficient to be able to imitate 

 the assimilating processes of plants. As is 

 well known, plants transform the carbon 

 dioxide of the atmosphere into starch, set- 

 ting free oxygen. They reverse the ordi- 

 nary process of combustion. It has always 

 seemed probable that formaldehyde was 

 the fii'st product of the assimilation; and 

 Curtius has at last demonstrated its pres- 

 ence in the leaves of the beech trees. The 

 artificial reproduction of a similar process 

 by means of ultraviolet rays has already 



