November 9, 1905] 



NA TURE 



29 



Border occasionally seen between Light and Dark 

 Regions on Photographic Plates. 



The reason mentioned by Sir Oliver Lodge (p. 5) for 

 the border seen between light and dark regions on photo- 

 ihs is not the only one. In the denser regions of a 

 negative the developer' gets more exhausted or restrained 

 than in the thinner regions, and this affects the adjacent 

 parts. At the junction of a dense and a thin area the 

 edge of the thin part is made thinner by the restraining 

 compounds (bromide, oxidised pyrogallol, &c.) derived from 

 the denser part, while, on the contrary, the edge ol the 

 denser part is made denser by the less exhausted developer 

 flowing from the thin area. This effect is apt to be the 

 more marked when the developer is already well restrained, 

 ,1- by staleness or the addition of much bromide. 



( ambridge, November 4. F. J. Allen. 



The explanation of a well known phenomenon in photo- 

 graphy, given by Sir Oliver Lodge in his letter to you 

 lasl week (p. 5), does not take into consideration the 

 following facts : — 



(1) The " perceptible difference in thickness " between 

 the acled-on and unacted-on portions of a negative is only 

 perceptible to our unaided senses when certain developers 

 are employed containing substances which act powerfully 

 on the gelatin. Most modern negatives certainly have no 

 perceptible difference in thickness, certainly not enough 

 difference to give rise to so marked an effect as that 

 referred to. 



(2) The difference in thickness is most marked in the 

 " carbon " transparencies from which many enlarged 

 negatives are made. Here it can be both seen and felt ; 

 in the other case it cannot. We might therefore expect 

 this cylindrical lens effect to be most marked when using 

 such a transparency, but the careful comparison of a 

 number of enlarged negatives made in these two methods 

 reveals not the slightest difference between them. 



In my own mind I have always accounted for the 

 phenomenon in the following way : — The sensitive film 

 ordinarily can only be approached by the developer from 

 its outward face, hence the action over an area where the 

 light action has been the same is uniform. But if that 

 area is bordered by one where there has been little or no 

 light action, the developer absorbed by such parts is not 

 spent in doing any or much work in those parts, and, so 

 far as any lateral diffusion is concerned, is practically fresh 

 developer. Hence the borders of an exposed portion, where 

 it comes against an unexposed portion, are attacked by 

 fresh developer diffusing both from the front and from the 

 unexposed part, and we should therefore expect to find a 

 border line of greater density there, as in fact we do. 

 For a similar reason we should expect to find a less dense 

 line on the border of the more transparent portion, as 

 is the case, though it is not often so noticeable as the 

 former. 



That this is the true explanation is, I think, made 

 manifest by the fact that the line in question can be quite 

 easily distinguished on plates exposed in Spurge's actino- 

 meter, when there is certainly no opportunity of a 

 "cylindrical lens effect," and especially when develop- 

 ment has been pushed far. R. Child Bavley. 



20 Tudor Street, London, E.C., November 6. 



The Use of Gasoline in Chemical and Fhysical 

 Laboratories. 

 Experimental work has so thoroughly established its 

 claims to a reasonable share in the curriculum of every 

 secondary school that very few schools are now with- 

 out proper laboratories. No inconsiderable number of 

 these schools are, however, beyond the limits of the 

 ordinary gas supply, and the question of providing a sub- 

 stitute for coal-gas has presented no little difficulty. The 

 matter became urgent some time ago at the Llanberis 

 Intermediate School, mainly for heating purposes, but also 

 for lighting. Investigation seemed to point to two possible 

 substitutes — acetylene and gasoline. Both have been used, 

 but not to any very large extent, in this country. An 

 a< count was given in the School World for January 1Q02, 

 ol the use of acetylene in Felsted School. 



NO. 1880, VOL. 73] 



For a small installation, where light is the first con- 

 sideration, it would probably be admitted that acetylem 

 is highh satisfactory, but even for lighting . the use of 

 mantles lias rendered gasoline a very severe rival. I It 

 problem is different when heat is the chief (actor. In most 

 cases of schools tin- gas is required to meet both demands, 

 and gasoline seems to possess the advantage. 



1 he questions for consideration are cost and efficiency. 

 In reference to cost, estimates were obtained to supply 

 the chemical and physical laboratories and to light the 

 whole building, and showed that the initial cost of plant 

 and fitter's work would be about fifty per cent, higher for 

 acetylene than for gasoline, and the estimated cost of 

 maintenance for the former was also much higher. 



Efficiency may be considered under the following 

 leads : — (o) The relative simplicity of the generating plant; 

 l/i| the ease of manipulation; (c) the nearness to which 

 the gas approaches in use to coal-gas; id) the risk of 

 explosion. 



(a I The plant used in the Llanberis School was supplied 

 by the Walworth Manufacturing Co., of Boston, U.S. \ 

 and consists essentially of three parts : — (1) A large shallow 

 cylindrical copper tank, holding 250 gallons, buried some 

 30 feet or more from the building, which is filled with 

 gasoline through a pipe and closed air-tight by a s< n-w 

 cap. Two other pipes, an inlet and outlet, are fitted into 

 the top of the tank and pass under ground to the cellar 

 ol the building. (2) In the cellar a pump, worked b) a 

 weight on pulleys, forces air through the inlet pipe on to 

 the surface of the gasoline in the tank. Evaporation is 

 rapid (gasoline boiling from about 35 C. to 70' C), and 

 the mixture of vapour and air is driven through the outlet 

 pipe into (3) an automatic mixer, by which a definite and 

 known amount of air can be added, so that the proper 

 proportion for burning may be constantly maintained. The 

 whole plant is extremely simple, and was easily put up by 

 a local gas-fitter under my direction. 



(In It requires very little attention. The weight has to 

 be wound up about once a week ; the mixer adjusted, by 

 moving a small wheel along a rod, about once every two 

 or three months ; and the tank filled about every twelve or 

 eighteen months. The frequency of the recurrence of these 

 operations clearly depends on the size of the plant relative 

 to the demands upon it. 



(1 I file burners differ slightly from the ordinary coal-gas 

 bunsens, but give an excellent flame for ordinary laboratory 

 purposes. The most noticeable difference is that the flame 

 is more easily blown out. This gives a little trouble with 

 an ordinary foot blowpipe, but a slight modification, which 

 I hope to carry out, suggested by my friend Mr. B. B. 

 Turner, of Storrs Agricultural College, Connecticut (who 

 has used gasoline for some years, and who brought it to 

 my notice), will probably get over the difficulty. The plant 

 supplies enough gas to light the whole building as well as 

 for laboratory purposes. 



(i/| The risk of explosion is very slight, as any escape is 

 at once detected by the strong smell, and the limits of 

 explosion are narrower than those of coal-gas and very 

 much narrower than those of acetylene. The absence of 

 any heating arrangements to aid the evaporation, such as 

 arc- proposed by some makers, considerably reduces the risk 

 of explosion. J. R. Foster. 



THE AEGER IN THE RIVERS TRENT AND 

 OVSE. 



HAYING had an opportunity of witnessing- the 

 bore, or aeger as it is locally called, in the 

 River Trent at Gainsborough during the recent high 

 equinoctial tides, which did so much damage all along 

 the east coast, I send you the following description, 

 which may interest some of your readers, more 

 especially as I am not aware of any trustworthy 

 account of this bore that has yet been published. 



The Trent is a tributary of the Humber, and joins 

 that river about 16 miles above Hull and 40 

 miles from the North Sea. The width of the Trent 

 at the junction is from 2500 feet to 3000 feet at high 

 water, diminishing to 700 feet ri miles from the 



