86 



SCIENTIFIC NKWS. 



[Jan. 27, iS 



it by boiling in water and subsequent soaking in alcohol. 

 Then place it in a dilute solution of iodine (tincture of 

 iodine and water), of a light brown colour. If the leaf 

 has just been 'i exposed to light for some hours it will 

 turn purple in consequence of the presence of abundant 

 starch ; if not, it will remain unchanged. The experi- 

 ment is best made in the evening of a sunshiny day. 

 One leaf may be taken from a plant which has been ex- 

 posed to full light, another from a plant which has been 

 completeli' shaded by a flower-pot inverted over it. 



The precise way in which sunlight effects the decom- 

 position of carbonic acid in the presence of chlorophyll 

 has been carefully investigated, and during the last year 

 or two, facts have been brought to light which materially 

 affect current views. 



It was once supposed that chlorophyll was capable of 

 direct conversion into starch, and that as it became used 

 up in this way, more was formed. There is, however, no 

 direct proof that chlorophyll is capable of such conver- 

 sion, and there is no reason to believe that the chloro- 

 phyll in a green leaf diminishes in proportion to the for- 

 mation of starch. 



Some years ago Pringsheim was led, mainly by the 

 study of the spectrum of chlorophyll, and of the action 

 of variously-coloured light upon plants, to suggest a new 

 explanation. He taught that a solution of chlorophyll 

 allows such rays to pass as promote the decomposition 

 of carbonic acid, but stops such as promote the reverse 

 action, viz., oxidation of the carbon. His experiments 

 tended to prove that the decomposing rays were the yellow 

 and green rays, which pass almost unaltered through 

 chlorophyll, and that the oxidising rays are the blue, 

 violet, and ultra-violet rays, which are quenched by chloro- 

 phyll. The chlorophyll, according to Pringsheim, acts 

 not as a direct reducing substance, but as a screen, keep- 

 ing off the rays which undo the work of reduction. There 

 was from the first one startling difficulty in the path of this 

 screen-theory. It supposed, contrary to all analogy, 

 that the absorbed rays were actually injurious to the 

 tissues, while the rays which pass almost unabsorbed do 

 the work of assimilation. 



It has been found possible by improved methods to 

 determine more exactly the effect of different rays of the 

 spectrum upon green plants. The older observers, using 

 a slit to separate light of a particular degree of refrangi- 

 bility, were in this dilemma, that if they used a very 

 narrow slit, they got a light of no effective intensity, 

 while if they widened the slit they got an impure light, 

 in which different kinds were mixed. Timiriazeff has at 

 length got over this difficulty, by using a pure light of 

 known refrangibility, and very delicate means of estimat- 

 ing its effect. He exposes a green leaf to such light, and 

 soon gets a tiny bubble of gas. This he collects in a 

 dainty syringe, formed out of a thermometer-tube, and 

 uses the tube not only to collect but to measure the gas. 

 The bubble is then expelled from the syringe, and made 

 to pass through pyrogallic acid, after which it is returned 

 to the syringe and remeasured. Ihe difference between 

 the first and the second measurement gives the amount 

 of oxygen present in the bubble, the rest of the gas being 

 nitrogen. A very short exposure being required, the 

 same leaf may be exposed many times under the same 

 conditions to light of different colours and qualities. 



Timiriazeft's experiments bring out the very interest- 

 ing result that the decomposition of carbonic acid is 

 chiefly effected, not by yellow or yellow-green light, as 

 was till lately supposed, but by red light, and that the 



maximum effect corresponds with the conspicuous absorp- 

 tion-band seen between B and C in the chlorophyll- 

 spectrum. 



Timiriazeff has endeavoured to penetrate yet further 

 into the mysterious action of chlorophyll. The photo- 

 chemical researches of Vogel, Becquerel, and Abney 

 have revealed the existence of substances, among which 

 chlorophyll is to be reckoned, which may be called 

 sensitizers. They absorb rays of light and transfer the 

 vibrations of their own molecules to another substance 

 less easily afiected by light, whose decomposition is thus 

 effected. For instance, an ordinary photographic plate, 

 when exposed to the solar spectrum, does not record the 

 the red rays at all ; iodide of silver is not sensitive to red 

 rays. But if the collodion is tinctured with chlorophyll, 

 the red rays affect the plate, and the chlorophyll absorp- 

 tion-band becomes visible. The sensitizer must be capa- 

 ble of decomposition, by the rays which it absorbs, for it 

 is only while undergoing decomposition that it can set 

 up decomposition in another substance. Now chlorophyll 

 is known to undergo such decomposition and the chloro- 

 phyll-spectrum can be printed on paper stained with 

 chlorophyll. The rays which decompose chlorophyll are 

 the same rays which decompose carbonic acid most 

 powerfully. 



The red rays of the spectrum are therefore capable of 

 doing chemical work, and in particular of decomposing 

 carbonic acid. Their special fitness for this work depends' 

 partly upon their energy, and partly upon the amplitude 

 of their vibrations. Abney and Langley have shown 

 that the maximum of solar energy is attained at a point 

 in the spectrum between B and C, which precisely corre- 

 sponds with the absorption-band of chlorophyll. What 

 chlorophyll does is evidently to transform into chemical 

 work the rays which possess the greatest energy. 



It is the red rays which possess the greatest amplitude 

 of vibration, and Timiriazeff compares them to the high 

 waves which cause a skiff to founder. It is just in that 

 part of the spectrum where the waves run highest that 

 the molecule of carbonic acid undergoes dissociation. 



*-^t^'^5«if-» — 



WANT OF A SANITARY SYSTEM IN 

 RUSSIA. 



THE Vice-Director of the Russian Medical Depart- 

 ment, Dr. Bubnoff, has recently issued a report on 

 Sanitation in Russia, in which he remarks that no 

 working sanitary system can be said to exist in the 

 empire. This is, indeed, only too true, as all the 

 reports which pass into the archives of the Medical 

 Department must impress on the minds of those who, 

 like Dr. Bubnoff, are responsible for the sanitar}' con- 

 dition of Russia. The death-rate in some districts ranges 

 from 60 to 80 per 1,000, of which an enormous propor- 

 tion is due to mortality amongst children — in some 

 places, indeed, as much as 79 per cent, of the total 

 mortality. In spite of a high birth-rate, the population 

 is increasing only at the rate of i per cent. Every year 

 some 2,800,000 persons die in Russia. According to 

 Dr. Bubnoft's statistics, somethmg like 1,000,000 of these 

 deaths may fairly be traced to preventable causes. There 

 is a great paucity of medical men, of whom there are 

 only about 5,000 altogether for 100,000,000 inhabitants, 

 and these of course tend to congregate in the towns, so 

 that in the rural districts doctors are to be met with only 

 at long intervals. 



