December 20, 1900J 



NA TURE 



179 



LETTERS TO THE EDITOR. 



\Tht Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.^ 



Chemical Products and Appliances at the Paris 

 International Exhibition. 



I SEE that attention has been called in these columns to the 

 excellence of the German catalogue of instruments of precision 

 distributed at the recent Exhibition. It will be of interest if, as 

 a member of the jury representing Great Britain for Class 87, I 

 may be permitted to add that the German catalogue explanatory 

 of the collective chemical exhibits of that country is also a 

 remarkable production, worthy of a permanent place on our 

 library shelves. Copies of this work, printed and got up in a 

 highly artistic way with German text, were distributed among 

 the members of the jury by my friend, Geh. Rath. Dr. Otto N. 

 Witt, the titular member representing Germany on our jury. 

 A French edition was afterwards to be had on application to 

 the custodian of the German exhibits. The work consists of 

 over 200 pages, each emljellished with a coloured floral design 

 as a heading, and contains a general introduction, giving an 

 account of the development of German chemical industry, the 

 value of the production in this branch of manufacture for the 

 year 1897 being estimated at 47,395,132/. (947,902,645 marks). 

 The introductory part, which is from the pen of Dr. Witt, is 

 followed by a special part containing the history, an account of 

 the nature, of the products manufactured, the equipment of the 

 factory and scale of production, and a list of the exhibits of each 

 of the ninety firms represented in the collective exhibit. Those 

 who visited the Exhibition and made an examination of the 

 chemical exhibits of the various countries will have formed their 

 own conclusions as to their respective positions in the scale of 

 chemical industry. At any rate, it is not the object of this letter 

 to institute invidious comparisons — I merely wish to point out 

 that it is not pnly in instruments of precision that the German 

 catalogue reveals the industrial eminence of that country. 



R. Meldola. 



Electricities of Stripping and of Cleavage. 



In the ordinary process of giving a glazed surface to photo- 

 graphic paper prints by leaving them to dry face downwards 

 upon clean glass, enough electricity is, I find, developed at the 

 moment of separation between the dry glazed print and its glass 

 support to produce a pretty bright illumination in the dark. 

 "Solio " and other gelaiino-chloride printing papers being very 

 liable to adhere obstinately to a glass plate in this process, I 

 have only constantly employed it with albumenised printing 

 paper, and have then often noticed strong electrical attraction 

 between the glass plate and the freshly separated paper. Not 

 jail glass plates, but apparently only very hard unhygroscopic 

 ones, with a low percentage of soda in their composition, serve 

 the purpose well ; and even on these the print must not be freed 

 from superfluous water by any pressure, but by swinging the 

 plate until the water is sufficiently expelled to leave the glass 

 and paper adhering firmly to each other. The paper can be 

 then further freed from water by wiping it on the back, very 

 lightly, with a soft cloth, and any intrusive air-bubbles seen 

 through the glass can be driven out by stroking the back, of the 

 print very lightly with the finger. Left then to dry quite hori- 

 zontally with the paper upwards, the latter will in hot, dry 

 weather or in a very dry, warm room separate itself at last more 

 or less completely from the glass ; but in ordinarily damp atmo- 

 sphere, and cold weather, remains, though sensibly quite dry, 

 adhering to it. The slightest warmth of sunshine or of a fire 

 or gas flame applied to the plate is then enough to make the 

 paper crisp, and leave the glass. This it does with audible 

 clicks as the adhesion breaks up here and there, showing that 

 a state of pretty strong tension prevails in the thoroughly dry 

 paper and the coat of albumen until these can break loose from 

 their support. 



The tension is apparently strongest in the albuminous coating 

 of the paper, since the paper curls in towards that face when it 

 is liberated ; and if the process of separation is observed on the 

 face of the print, through the glass, the still adhering white parts 

 of the paper have a greenish, and the loosened parts the ordinary 



NO. 1625, VOL. 63] 



yellowish tinge of such papers, in very perceptible contrast with 

 each other. When well dried spontaneously in a warm, dry 

 place, if, also, the rough back of the dry paper is rubbed smooth 

 before gently warming it to strip it off, there is strong enough 

 electrical attraction between the glass and the released paper to 

 keep the latter flat against the glass while the separation spreads, 

 with clicks and snaps of freeing from the edges, until soon, the 

 tension in the film prevailing, the glazed and loosened print 

 bulges upwards in theSmiddle and its ends curl inwards. In the 

 drum-like shape which it then sometimes assumes I have 

 seen "it rolling about on the under side of the warm glass plate, 

 supported there for some minutes by the mutual electrical 

 attraction between the dry paper and its glass support. 



After trying in vain, for some months past, to see any luminous 

 signs of this strong electrification, by taking the warmed plate 

 when separation was commencing, into an adjoining darker room 

 to watch the operation's progress, to-night, at last, the experi- 

 ment has perfectly succeeded. The conditions in which it did so 

 were not exceptionally favourable ones in any particular, respects 

 that I could iiotice, but a quickly toned, fixed and washed albu- 

 men paper print had dried slowly and thoroughly in a dry, warm 

 place without loosening itself from the glass surface. The paper 

 was not smoothed on the back before holding it pretty close to 

 the dull hot coals of a low fire, which had scarcely time to more 

 than slightly warm it, when clicking sounds announced that 

 splitting from the glass surface had commenced.^ A glance 

 through the glass face showed that loosening from the glass at 

 one end of the print had just begun, and the plate was immedi- 

 ately taken, for the paper to finish freeing itself, into perfect 

 darkness in another room. Though but very slightly warmed, 

 a little waving of the plate, with its transparent glass side up- 

 wards, up and down (which assists the parting by rapid drying 

 and changes of temperature in the paper), presently advanced 

 the cleavage a little step further, and this was marked by an 

 audible snap, and at the same time by a light-flash at the 

 released end of the print, bright enough to have been seen easily 

 by sufficiently watchful eyes, from any part of the moderately 

 large room. The same bright glow followed the line of yielding 

 of the print, while it was then quickly seized by its loose end 

 and stripped from the under side of the glass plate by hand, as 

 a yellow or orange-coloured stripe of gauzy light, about half an 

 inch wide, as bright as the first flicker of whiter gauzy light. 



Considering that the tough coating of dry albumen seems to 

 be stretched on its glass support with considerable strain and 

 force of tension, which is slackened and released immediately 

 behind its advancing line of severance from the rigid glass, per- 

 haps the electric excitation may be due to friction of small 

 rubbing surfaces of the loosened coat of albumen against the 

 glass ; and in that case the example may be one of electrifica- 

 tion by a mechanical, rather than by a molecular form of 

 cleavage like that observed in crystals, of one surface from 

 another. Li Becquerel's well-known experiment of the evolu- 

 tion of electricity and light when a thin lamina of mica is split 

 into two thinner leaves, no definable forces of released tension, 

 leading to electrification by friction of dissimilar portions of the 

 crystal, can be resorted to as probable effective working sources 

 in the mica, or in other crystals, of the electricities developed by 

 their cleavage. But while no evident condition seems, in fact, 

 to predetermine which of the two halves of a split leaf of mica 

 should be found to receive a positive and which a negative 

 charge from the electrifying forces which the cleavage ex ercises, 

 the common experience that glass rubbed with silk is positively, 

 and the silk negatively, electrified in a determinate and certain 

 way could easily afford a test, and might furnish some assurance 

 of the sufficiency of the above view's account of the origin in 

 niechanical friction of the glass's and paper's opposite electrifi- 

 cations, if the two bodies, stripped asunder, are found to be 

 endued with electrical charges of invariable kinds agreeing with 

 those which glass and dried white of egg acquire when they are 

 rubbed together. But I have not yet had an opportunity of 

 putting the question to this test by the simple use of an electro- 

 scope, although the experiment could be easily performed with 

 that appropriate equipment as the persistency of the charges on 

 the glass and on the paper is plentifully long enough to allow of 

 their complete investigation. 



1 These sounds are chiefly due to tough adhesions common at ragged 

 points along the paper edges, and overcome from time to time by the spread- 

 ing separation and increasing tension. Across the open print surface the 

 separation spreads silently and smoothly, except across occasional spots of 

 softened albumen with unduly tough adhesion, which may also there some- 

 times occasion snapping sounds in parting. 



-"iO ^ 



