456 Prof. C. Timiriazeff. [Apr. M, 



I know by my own experience that it is rather difficult to admit such 

 formidable figures. But some analogies may perhaps help us in this 

 respect. If we strike a flint with a bit of steel in a direction normal 

 to its surface we obtain but a very moderate elevation of temperature, 

 but if we hit it in the right tangential direction, so as to concentrate 

 the whole expended energy on a small particle, it is easily brought to 

 incandescence. Or adopting an analogy much nearer to our case; 

 with a condensing lens we obtain temperatures amounting to thousands 

 of degrees* by increasing the effect in a plane vertical to the falling 

 rays. But we may arrive at the same result by a condensation of the 

 effect in the direction of the falling ray, by diminishing the thickness 

 of the absorbing layer. It has indeed been calculated that a platinum 

 foil not thicker than 1/500 of a millimetre, might be made to melt on 

 being exposed to sunlight, the effect of re-radiation, of course, being 

 eliminated. Practically the reduction of the thickness of the absorbent 

 metallic parts is the plan adopted for augmenting the sensibility in all 

 modern thermoscopic apparatus the bolometer, the thermopile of 

 Kubens and others. I suspect that it was also the plan adopted by the 

 plant, so that we have one occasion more to repeat with Mr. Horace 

 Brown that the plant seems to know more about physics than we are 

 ready to admit. 



Of course these high temperatures, as such, are out of the question, 

 and if I dwell on the corresponding thermometric equivalent of radiant 

 energy accumulated in so small a space and in so short a time, it is 

 only in order to show the analogy with the dissociation process so well 

 studied by chemists. In reality we probably have here the converse 

 of what Wiedemann calls chemirluminescence, that is a direct trans- 

 formation of radiant energy into chemical work without the transient 

 intermediate stage of a high temperature. In this way the cumulative 

 or integrating effect so characteristic of the photo-chemical process is 

 easily understood. AYhen the carbon dioxide is absent, the radiant 

 energy which can no longer be directly transformed into chemical 

 work is used as heat in the process of chlorovaporisation. 



Returning to our curve, which represents the connection between the 

 intensity of light and the quantities of carbon dioxide dissociated in 

 the leaf (fig. 10), and comparing it now with Le Chatelier's dissociation 

 curves (fig. 11), we may perhaps admit that their likeness, which 

 cannot fail to strike one, might be attributed to the same cause that 

 about a certain point where the curve takes a horizontal direction, the 

 temperature in the chlorophyll film if I may be allowed, for the sake 

 of analogy, once more to use this term that the intensity of the 

 radiation accumulated in this small space, is so high that the dissocia- 

 tion of carbon dioxide will be complete. As a consequence, past this 



* My colleague, Professor Zserassky, has obtained in this way temperatures 

 amounting to thousands of degrees. 



