;56 



NATURE 



[August 13, 1908 



covered. A hall for demonstrations and lectures lias just 

 been erected, and we ventured to suggest that demonstra- 

 tions of school-music would be welcome. 



Special Institutions. — The work that is being done in 

 schools for 'the blind, the deaf, and the mentallv defective 

 calls for respectful acknowledgment. Cases showing what 

 is being accomplished, so far as material products are con- 

 cerned, can be seen near the entrance. The moral benefit 

 to the pupils cannot be expressed. 



It reflects credit on the committee and the secretaries that 

 the whole of this wonderful collection was in place at the 

 opening of the exhibition. The objects are displayed in 

 an admirable manner, and furnish innumerable suggestions 

 of value to the practical teacher. 



French Education Exhibit. — Although not large enough 

 to furnish grounds for comparison of French with English 

 organisation of education, the French section contains many 

 interesting features. It is housed in the corridors between 

 Shepherd's Bush and Wood Lane, unfortunately rather 

 distant from the English section. The Ecoles Profession- 

 elles, I'Enseignement Technique, and the Ecoles Primaires 

 Superieures contribute ; Lille, Toulon, St. Etienne, Nimes, 

 Dupuy, and Rouen are represented. 



Much of the manual work is excellent, and teachers of 

 chemistry may glean some useful hints from the apparatus 

 and diagrams, which are clearly displayed. There has been 

 an attempt to introduce some really artistic adornment into 

 certain of our own elementary schools ; but we still have 

 much to learn in this respect. Our authorities would do 

 well to pay attention to the charrning pictures sent bv the 

 Soci^t^ Artistique de I'Art k I'EcoIe. Incidentally, w-e 

 observed that Arabic was included in the curricila of some 

 pupils whose note-books we inspected. 



L'ndoubtedly the space allotted to the French education 

 section is too small, and hence the display falls short o( 

 our expectations. We admit that those expectations were 

 high. In justice to the work performed in bringing 

 together the exhibit, we should add that the interesting 

 quality of what we could see considerably strengthened 

 our desire for a fuller display of recent achievements by 

 our neighbours in the field of education. 



G. F. Daniell. 



THE ELECTROCHEMISTRY OF LIGHT. 

 T N the April and May numbers of the Journal of Physical 

 Chemistry, Mr. Wilder D. Bancroft contributes two 

 long articles under this heading, long chiefly because of 

 the very extensive quotations from the writings of Grott- 

 huss, Herschel, H. W. Vogel, E. Vogel, Timiriazeff, 

 Acworth, v. Hiibl, Bothamley, and others whose work 

 bears upon the subject. The object of the communication 

 is " to bring the various catalytic actions of light under 

 one head so far as possible," and to show that this may 

 be done by accepting two laws enunciated by Grotthuss 

 sorne ninety years ago : — (i) that only those rays of light 

 which are absorbed can produce chemical action ; (2) that 

 the action of a ray of light is analogous to that of a 

 voltaic cell. The action, therefore, is regarded as electro- 

 lytic, and sensitisers, whether "optical" or "chemical," 

 are viewed as depolarisers. The fundamental conception 

 of Grptthuss, that the action of light is essentially electro- 

 lytic in character, is held to be sound and to accord with 

 modern notions, though the language in which he expressed 

 it may be somewhat obscure. 



The author proceeds to show that the decomposition of 

 various salts containing silver, iron, copper, mercury, 

 chromium, uranium, manganese, vanadium, and moly- 

 bdenum, as the result of light action yields the same pro- 

 ducts as those resulting from electrolytic action, but that 

 some substances are light-sensitive only in the presence of 

 a suitable depolariser (or absorber of one of the products 

 of the decomposition). Herschel 's account of his experi- 

 ments on the action of light upon iron salts and ferro- 

 and ferricyanides is quoted in full from the Philosoohical 

 Transactions. When paper is impregnated with a mixture 

 of potassium ferricyanide and ferric chloride and exposed 

 to light, the ferric chloride is reduced and TurnbuU's blue 

 is formed, further exposure giving a brown substance of 

 unknown formula. The author records that since his 



writing Mr. Schluederberg has succeeded in producing 

 this brown substance by electrolytic means. Herschel's 

 observation that by the continued exposure of a Prussian- 

 blue print to light the colour was bleached, but that the 

 colour returned when the print was left in the dark, and 

 that this reversal took place even when the iron salt was 

 exposed alone and the ferricyanide added afterwards, is 

 explained by the supposition that the light, after it has 

 reduced the iron of the ferric ammonium citrate to the 

 ferrous state, by its prolonged action produces a reducing 

 agent powerful enough to reduce the ferricyanide, the 

 white ferrous ferrocyanide that results being re-oxidised in 

 the dark. 



The analogy between the oxidation of organic bodies by 

 the action of light and by electrolysis is not so easy to 

 trace for want of facts. Whether the oxygen (of the air) 

 or the dye is the depolariser must be decided experimentally 

 in each case, and " there is one conclusive way " of 

 answering this question. " If the active light is light 

 which is absorbed by the substance to be oxidised and not 

 by the o.xygen, then the substance to be oxidised has been 

 made active by the light and the oxygen is the depolariser* 

 If the active light is absorbed by oxygen and not by the 

 substance to be oxidised, then this latter is the depolariser, 

 and the oxygen is made active by light. If the active 

 light is absorbed by both, it is possible that each is made 

 active and that each is also the depolariser. In this last 

 case, however, the results should be checked by experi- 

 ments with another oxidising agent and another reducing 

 agent. While light can only act in case it is absorbed, it 

 does not follow that all light which is absorbed acts to 

 any appreciable extent." In the bromination of organic 

 compounds, Schramm and Zakrzewski have shown that 

 the most effective rays correspond to the weaker bromine 

 absorption bands in the yellow-green and orange instead 

 of the stronger bands in the greenish-blue and blue. The 

 researches of Herschel on the action of light on the colour- 

 ing matter of flowers are explicable by the Grotthuss 

 theory, and Timiriazeff in his Croonian lecture (1903) 

 showed the strict applicability of the law so far as regards 

 the correspondence between the absorption of light and its 

 chemical action in the case of chlorophyll. 



In some cases the depolariser changes the sensitiveness 

 of the system with regard to certain rays. These sub- 

 stances are generally distinguished as " optical sensitisers," 

 but, the author says, " a more rational distinction would 

 be between depolarisers with marked absorption bands and 

 depolarisers without marked absorption bands." The dis- 

 covery of the action of " optical sensitisers " by H. W. 

 Vogei in 1873, that is, the possibility of sensitising photo- 

 graphic plates for the less refrangible rays by means of 

 dyes, and Eder's work that led him to the conclusion that 

 the absorption of silver bromide dyed with eosin and the 

 maximum of the photographic sensitising action of eosin on 

 silver bromide exactly coincide in the spectrum, are detailed 

 by copious extracts from the writings of these investigators. 

 Edcr distinguished between the absorption of the dyed 

 silver salt and a dyed gelatin film or aqueous solution of 

 the dye, the former giving an absorption of greater wave- 

 lengths, in accordance with Kundt's law. Acworth, who 

 worked under apparently ideal conditions, comparing the 

 maxima of absorption and sensitiveness by estimating them 

 in the same emulsion, found that the sensitiveness maxi- 

 mum was displaced towards the red as compared to the 

 absorption maximum, and Wiedemann accounts for this 

 by suggesting that the light at the place of maximum 

 absorption may cause increased vibration within the mole- 

 cule, resulting in radiations or heat waves, but without the 

 amplitude of vibration in the molecule attaining a sufficient 

 magnitude io result in any decomposition or chemical 

 change of the molecule. Mr. Bancroft accepts Acworth 's 

 experimental results, but considers that his absorption 

 curves show the sum of the absorptions of the dyed gelatin 

 film and the dyed silver bromide, inste.ad of the absorption 

 of the latter alone, and that therefore his results do not 

 disprove Eder's conclusions that the maxima of absorption 

 and photographic effect coincide. 



Concerning the mode of action of such sensitisers, he 

 states that the theory of Grotthuss enables us to make a 

 definite statement with regard to them, " and one that 

 differs to a certain e.xtent from any of the previous ones. 



NO. 2024, VOL. 78] 



