i88 



NA TURh 



[December 22, 1S92 



glass by a feeble electrical discharge ; as well as two permanent 

 marks; noticed by Ettrick, which betray a disintegration of the 

 surface. 



I have found that when a stronger discharge is employed inore 

 complex phenomena of a similar kind are produced. A six-inch 

 Wimshurst machine is arranged with extra condensers, as if to 

 pierce a piece of glass. If this is about four inches square the 

 spark will generally go round it. For a day, more or less, there 

 is only a bleared watei'y track, -^^ inch wide, when the glass is 

 breathed upon ; but after this time others develop themselves 

 within the first, a fine central black line with two white and two 

 black on either side, the total breadth being the original x^s- inch. 

 These breath-lines do not precisely coincide in position with the 

 permanent scars, but the central one is almost the same as a 

 permanent mark, which the microscope shows to be the surface 

 of glass fractured into small squares of considerable regularity: 

 on either side is a grey-blue line always visible, which Riess 

 ascribes to the separation of the potash. After several months 

 I found two blue lines on either side, which I believe were not 

 visible at first. Of course these blue lines may be seen on most 

 Leyden jars, where they have discharged themselves across the 

 glass. 



In 1842 Moser, of Konigsberg, produced figures on polished 

 surfaces by placing bodies with unequal surfaces near to them; 

 the action was ascribed to the power of light, and his results were 

 compared with those of Daguerre. Moser says, " We cannot 

 therefore doubt that light acts uniformly on all bodies, and that, 

 moreover, all bodies will depict themselves on others, and it 

 only depends on extraneous circumstances whether or not the 

 images become visible." In general, the multitude of images 

 would make confusion ; it can only be freshly polished surfaces 

 that are free to reveal single definite impressions. However 

 great Moser's assumption may be, there are many achievements 

 of modern photography that would be as surprising if they were 

 not so familiar. I have not the means of knowing the precise 

 form of Moser's methods : in the experiments which follow there 

 is usually contact and light pressure, and if they are iiot wholly 

 analogous, they may for that cause help to generalize the idea : 

 in none of these is electricity applied. 



A piece of mica is freshly split, and a coin lightly pressed for 

 thirty seconds on the new surface : a breath-image of the coin 

 is left behind. At the same time it may be noticed that the 

 breath causes abundant iridescence over the surface, whilst it is 

 in a fresh state. . It is not clear how the electricity of cleavage 

 can have an active agency in the result. 



It is familiar to most people that a coin resting for a while on 

 glass will give an outline of the disk, and sometimes faint traces 

 of the inner detail when breathed upon. 



An examination-paper, printed on one side, is put between 

 two plates of glass and left for ten hours, either in the dark or 

 the daylight : a small weight will keep the paper in continuous 

 contact, but this is not necessary if thick glass is used. A perfect 

 breath-impression of the print is made, not only on the glass 

 which lay against the print, but also on that which faced the 

 blank side of the paper. Of course the latter reads directly, 

 and the former inversely ; the print was about one year old, and 

 presumably dry. 



More oftt-n both impressions are white, sometimes one or other 

 or both are black. At other times the same one may be part 

 white and part black, and they even change while being 

 examined. 



During a sharp frost with east winds early in March, 1890, 

 these impressions of all kinds were easy to produce, so as to be 

 quite perfect to the last comma ; but in general they are difficult, 

 more especially those from the blank side. 



At the best period those from the blank side of the paper were 

 white and very strong ; also there were white spots and blotches 

 revealed by the breath. They s-eemed to correspond with slight 

 variations in the structure of the paper, and suggest an idea that 

 the thickness of the ink or paper makes a minute mechanical 

 indentation on the molecules : the state of these is probably ten- 

 der and sensitive under certain atmospheric conditions, as happens 

 with steel in times of frost. 



The following experiments easily succeed at any time : — Stars 

 and crosses of paper are placed for a few hours beneath a plate 

 of glass : clear white breath-figures of the device will appear. 

 A piece of paper is folded several times each way to form small 

 squares, then spread out and placed under glass : the raised lines 

 of the folds produce white breath-traces, and a letter weight 

 that was above leaves a latent mark of its circular rim. 



NO 1208, VOL. 47] 



Some writing is made on paper with ordinary ink and well 

 dried : it will leave a very lasting white breath-image after a few 

 hours' contact. If, with an ivory point, the writing is traced 

 with slight pressure on glass, a black breath-image is made at 

 once. Of course this reads directly, and the white one inversely. 

 It is convenient to look through the glass from the other side 

 for inverse impressions, so as to make them read direct. 



Plates of glass lie for a few hours on a table-cover worked 

 with sunflowers in silk : they acquire strong white figures from 

 the silk. 



In most cases I have warmed the glass, primarily for the sake 

 of cleansing it from moisture ; but I have often gone to a heat 

 beyond what this needs, and think that the sensitiveness has 

 been increased thereby. 



It is not easy to imagine what leads to the distinction between 

 black and white, different substances act variously in this respect. 

 I have placed various threads for a few hours under a piece of 

 glass, which lay on them with light pressure : wool gives black, 

 silk white, cotton black, copper white. A twist of tinsel and 

 wool gives a line dotted white and black ; after a time these 

 traces show signs of developing into multiple lines as in the 

 spark figures. 



Two cases have been reported to me where blinds with em- 

 bossed letters have left a latent image on the window near which 

 they lay ; it was revealed in misty weather, and had not been 

 removed by washing. I have not had a chance to see these for 

 myself, but both my informants were accustomed to scientific 

 observation. 



A glass which has lain above a picture for some years, but is 

 kept from contact by the mount, will often show on its inner 

 side an outline of the picture, always visible without breath. It 

 seems to be a dust figure easily removed : possibly heat and light 

 have loosened fine paint particles, and these have been drawn up 

 to the glass by the electricity made in rubbing the outer side to 

 clean it. The picture must have been well framed and sealed 

 from external influences ; most commonly dust and damp get in 

 and obscure such a delicate effect. 



I am not able to suggest simple causes for these varied effects. 

 I am not inclined to think, except in the case of water-colours, 

 which is hardly part of the enquiry, that there is a definite 

 material deposit or chemical chai)ge ; one cannot suppose that 

 imperceptible traces of grease, ineradicable as they may be, 

 would produce complete and delicate outlines. The cleaning 

 off of impressions may at first seem to indicate a deposit ; but 

 this renewal of the surface might rather be like smoothing out 

 an indented tin-foil surface : such a view might explain the case 

 where a blank over-electrified disk is developed into fine detail. 

 The electrified fiijures seem to point to a bombardment, which 

 produces a molecular change, the intensity of electricity bringing 

 about quickly what may also be done by slow persistent action 

 of mechanical pressure. At present it seems as if most of the 

 phenomena cannot be drawn out from the unknown region of 

 molecular agency. 



While experimenting I was not within reach of references to 

 former researches, but I have since done my best to find them 

 out, and to indicate all I have learnt in the body of my paper. 



Poggendorflf, vol. Ivii. p. 492 ; translated in Ajxhives de 

 rElectricite, 1842, -p. 647. 



Riess' " Electrische Hauchfiguren " in *' Repertorium der 

 Physik " ; translated in Archives de l' Electricity, 1842, 

 P- 59r. 



Reiss' " Die Lehre von der Reibungs Electricitat," vol. ii. pp. 

 221-224. 



Mascart, '^EUctricite Stattqtte," vol. ii. p. 177. 



Taylor's " Scientific Memoirs," vol. iii. 



SCIENTIFIC SERIALS. 

 American Journal of Science, December. — An experimental 

 comparison of formulcC for total radiation between 15° C. and 

 110° C, by W. de Conte Steven^. The formulce given by 

 Dulong and Petit, by Rosetti, Stefan, and Weber, were tested 

 for a comparatively small range of difll^erences by a determina- 

 tion of the heat radiated from an iron disc at a distance of 

 about 30 cm. from a thermopile. The results tended to show 

 that H. F. Weber's formula {Sitztmgsber. , Berlin, 1888) agrees 

 most closely with experiment. Stefan's formula, according to 

 which the heat emitted in unit of time is proportional to the 

 fourth power of the absolute temperature, is also fairly accurate, 



