jfune 13, 1878] 



NATURE 



87 



successfully in the production of electric currents, and although 

 mixtures of photographically sensitive salts have been shown by 

 Smee to produce currents of a similar kind, yet no attempt has 

 been made to examine the proper form of instrument required 

 for the general investigation of electrical actions induced by 

 light on fluid substances. 



This subject has occupied my attention for some time, and the 

 completed investigation I hope to present to the Society. In 

 the meantime the following description will give an idea of the 

 method of investigation. 



A little consideration shows that the amount of current pro- 

 duced by a definite intensity and quality of light acting during 

 a short' period of time on a given sensitive substance in solution, 

 is primarily a function of the nature, form, and position of the 

 poles in the cell relatively to the direction in which the light 

 enters, and the selective absorption, concentration, and con- 

 ductivity of the fluid. 



Diffusive action taking place in such cells complicates the 

 effects and is especially intricate when insoluble substances 

 are formed. In order to simplify the investigation in the first 

 instance, poles that are not chemically acted upon, and a sensi- 

 tive substance yielding only soluble products on the action of 

 light, were employed. For this purpose platinum and chlorous 

 acid or peroxide of chlorine were selected. 



The best form of cell had one of the poles made of fine 

 platinum wire fixed as closely as possible to the inner surface 

 where the light enters, the other pole being made of thicker 

 wire placed deeper in the fluid. 



As the action is confined to a very fine film where the light 

 enters, the maximum amount of current is obtained when the 

 composition of the fluid is modified deep enough to isolate tem- 

 porarily the front pole in the modified medium. Under these 

 conditions the formation of local currents is avoided, and the 

 maximum electromotive force obtained. 



In cells of this construction the amount of current is inde- 

 pendent of the surface of the fluid acted upon by light, so that a 

 mere slit sufficient to expose the front pole acts as efficiently as 

 a larger surface. This prevents the unnecessary exhaustion of 

 material, and enables the cell to be made of very small dimen- 

 sions. By means of such an apparatus the chemical actions 

 of light and their electrical relations may be traced in many new 

 directions. 



The amount and direction of the current in the case of 

 chlorous acid is readily modified by the addition of certain 

 salts and acids, and thus electrical variation may be produced, 

 resembling the effects observed diuring the action of light on 

 the eye. 



Certain modifications taking place in chlorous acid that has 

 been prepared for some time increase its sensibility, and as a 

 general result it is found that the fluid through these alterations 

 increases in resistance. "We hive thus an anomalous kind of 

 battery where the available electromotive force increases with the 

 resistance. The addition of neutral substances which increase 

 the resistance without producing new decompositions, improves 

 the action of the cell. 



Care has to be taken to use the same apparatus in a series of 

 comparative experiments, as infinitesimal differences in the con- 

 tact of the active pole render it difficult to make two instruments 

 giving exactly the same results. Cells have been constructed with 

 two, three, and four poles, and their individual and combined 

 action examined. Quartz surfaces have also been employed 

 instead of glass, thus enabling the chemical opacity of different 

 substances to be determined. 



The electrical currents derived through the action of light 

 on definite salts are strong in the case of ferro- and ferri-cyanide 

 of potassium, but remarkably so in the case of nitroprusside of 

 sodium. 



Of organic acids the tartrate of uranium is one of the most 

 active. A mixture of selenious acid and sulphurous acid in 

 presence of hydrochloric acid yields strong currents when 

 subjected to light in the form of cell described. The list of 

 substances that may be proved to undergo chemical decompo- 

 sition is very extensive, and full details will be found in the 

 completed paper. 



Geological Society, May 8. — Henry Clifton Sorby, F,R,S,, 

 president, in the chair, — Charles Preller Sheibner, Ph.D., was 

 elected a Fellow of the Society, — The following communica- 

 tions were read : — On the glacial phenomena of the Long Island, 

 or Outer Hebrides (second paper), by James Geikie, F,R.S. In 

 this paper the author gave some additional notes on the glacia- 



tion of Lewis, and a detailed account of the glacial phenomena 

 of Harris and the other islands that form the southern portion 

 of the Outer Hebrides. In concluding, the author pointed out 

 that we may now arrive at a true estimate of the thickness 

 attained by the ice-sheet in the north-west of Scotland. If a 

 line be drawn from the upper limits of the glaciations in Ross- 

 shire (3,000 feet) to a height of 1,600 feet in the Long Island, 

 we have an incline of only i in 210 for the upper surface of the 

 ice -sheet ; and of course we are able to say what thickness the 

 ice reached in the Minch. Between the mainland and the Outer 

 Hebrides it was as much as 3,800 feet. No boulders derived 

 from Skye or the mainland occur in the Till of the Outer 

 Hebrides, and this was explained by the deflection of the lower 

 portion of the ice-sheet against the steep wall of rock that faces 

 the Minch, The under part of the ice that flowed across the 

 Minch would be deflected to right and left against the inner 

 margin of the Long Island ; and the deep rock-basins that exist 

 all along that margin are believed to have been scooped out by 

 the grinding action of the deflected ice. Towards the north of 

 Lewis, where the land shelves off gently into the sea, the under 

 strata of the ice-sheet were enabled to creep up and over the 

 district of Ness, and thus gave rise to the lower shelly boulder- 

 clay of that neighbourhood, which contains boulders derived 

 from the mainland. The presence of the overlying interglacial 

 shell-beds proves a subsequent melting of the ice-sheet, and a 

 depression of the land for at least 200 feet. The overlying 

 shelly boulder-clay shows that the ice-sheet returned and over- 

 flowed Lewis, scooping out the older drift-beds and commingling 

 them with its bottom moraine. The absence of kames w'as 

 commented upon, and shown to be inexplicable on the assump- 

 tion that such deposits are of marine origin ; whilst if they be 

 of torrential origin their absence is only what might be expected 

 from the physical features of the islands. The only traces of 

 post-glacial submergence are met with at merely a few feet above 

 present high-water mark. — Cataclysmic theories of geological 

 climate, by James Croll, F.R.S. Communicated by Prof. 

 Ramsay, F.R.S. The author commenced by calling attention 

 to the great diversity of the hypotheses which have been brought 

 forward for the explanation of those changes in the climate of 

 the same regions of the earth's surface which are revealed by 

 geological investigations, such as alterations of the relative dis- 

 tribution of sea and land, of the ecliptic, and of the position of 

 the earth's axis of rotation, all of which, he maintained, have 

 proved insufficient or untenable. Sir William Thomson has 

 lately maintained that an increase in the amount of heat con- 

 veyed by ocean-currents combined with the effects of clouds, 

 winds, and aqueous vapour, is sufficient to account for the 

 former prevalence of temperate climates in the Arctic regions, 

 and this view, the author stated, he had himself been contending 

 for for more than twelve years. He thinks, however, that alter- 

 ations in the eccentricity of the earth's orbit is the primary motive 

 cause, whilst Sir William Thomson believes this to be the sub- 

 mergence of circumpolar lands, which, however, in miocene 

 times, appear to have been more extensive than at present. He 

 pointed out that a preponderance of equatorial land, as assimied 

 by Sir Charles Lyell to account for the milder climate of Arctic 

 regions in miocene times, would rather tend to loss of heat by 

 rapid radiation into space, whilst water is remarkably powerful 

 as a transporter of heat, so that, in this case, equatorial water 

 rather than equatorial land is needed. In speaking of the 

 glacial climate, the author maintained that local causes are in- 

 sufficient to explain so extensive a phenomenon. He indicated 

 that we are only too prone to seek for great or cataclysmic 

 causes, and although this tendency has disappeared from many 

 fields of geological research, this is not the case in all. His 

 explanation of the causes of a mild climate in high northern 

 latitudes is as follows : — Great eccentricity of the earth's orbit, 

 winter in perihelion, the blowing of the south-east trades across 

 the equator perhaps as far as the tropic of Cancer, and impul- 

 sion of all the great equatorial currents into northern latitudes ; 

 on the other hand, when, with great eccentricity, the winter is 

 in aphelion, the whole condition of things is reversed ; the 

 north-east trades blow over into the southern hemisphere, carry- 

 ing with them the great equatorial currents, and glacial condi- 

 tions prevail in the northern hemisphere. Thus those warm and 

 cold periods which have prevailed during past geological ages 

 are regarded by the author as great secular summers and winters. 

 — On the distribution of ice during the glacial period, by T. F. 

 Jamieson, F.G.S. The author believes that a study of the dis- 

 tribution of ice during the glacial period proves that the greatest 



