313 



BREATH FIGURES. 



BREATH FIGURES. 



311 



army ; or to the gabionnade, that is, the row of gabiona placed on end 

 and filled with earth, which the sappers construct for the protection of 

 the troop's in the trenches, or on the breach which is made in a ram- 

 part. A breast-work, however, differs from an epaulement, which is 

 also a mass of earth or other material raised to cover troops or artillery 

 when in situations exposed to the fire of the enemy, in being con- 

 structed to fire from. 



The intrenchments with which the Greeks and Romans protected the 

 ground occupied by their armies were breast- works, which in wooded 

 countries frequently consisted merely of felled trees; and in other 

 circumstances were formed of earth protected by palisades, or by the 

 interwoven branches of trees planted on the top of the bank of earth. 

 The same denomination might be applied to the continuous lines which 

 were formerly raised for the protection of armies ; but as these are not 

 now recommended by engineers, and as instead of them a number of 

 separate redoubts are usually formed at intervals from each other to 

 contain artillery, the word breast-work is little used, the protecting 

 masses of earth generally receiving the name which is given to those 

 which crown the ramparts of a permanent fortification. 



BREATH FIGURES. If we write with a blunt wooden point upon 

 a clean surface of glass or other polished material, and then breathe 

 upon it, the characters will become visible : or if we first breathe 

 upon the glass, and then trace some characters with a blunt point in 

 the midst of the breath, the characters can be brought out again by 

 breathing on the surface. Hence the Germans call the figures which 

 are produced by the breath, Jlauchfyuren ; while the French philoso- 

 phers term them fyures roriques, or roric figures, from the Latin for 

 " dew," ro, gen. roris. 



These figures, which may be produced in a variety of ways, first 

 excited attention in the year 1842, when Moser of Konigsberg 

 announced to Humboldt the remarkable proposition that " if any two 

 bodies be brought sufficiently near together, and face to face, one of 

 them impresses ita image on the other." Thus, if a coin be left for 

 some time upon a polished metal plate, the plate will retain an impres- 

 sion of the com, which may be made visible by exposure to the vapour 

 of water, mercury, iodine, &c. So, also, if a polished plate of silver, or of 

 copper, be covered by, but not in contact with, a blackened glass plate, 

 with some characters scratched on it through the opaque black, and so 

 exposed for some days to the sunshine, the metal plate will be 

 impressed with the characters, which will make themselves visible by 

 exposure to vapour. Experiments of this kind may be varied to any 

 extent ; and no sooner did Moser call attention to them, than other 

 persons were ready with similar examples. Thus, Rauch, the sculptor, 

 stated that the glass used to protect a print after Raphael, although 

 not in contact with it, had received an impression of the print on its 

 inner surface. It was found on inquiry that this fact was well known 

 to persons accustomed to frame prints.* Breguet, the celebrated Parisian 

 watchmaker, stated a fact that had long been known in his workshop ; 

 namely, that the inscription and letters on the back of the inner cases 

 of his watches were found repeated on the inside of the outer cases, 

 although the two surfaces were not in contact. It was also noticed by 

 engineers, &c., that the parts of machines in contact or near together 

 easily and quickly impress themselves upon each other. 



It was also found that very perfect impressions of objects could be 

 produced by means of electricity. Thus, if a com be placed upon a 

 glass plate which is supported by a metal one, and a stream of sparks 

 from the knob of the prime conductor be thrown upon the coin, about 

 a hundred turns of the machine will suffice to impress the coin very 

 perfectly upon the glass. On throwing off the coin, nothing is visible 

 until the breath be projected upon the plate, when the image and 

 superscription, the milling, and other minute details of the coin 

 instantly appear. In producing these impressions on metal, it is 

 necessary to separate the coin from the metal by means of a thin plate 

 of mica, or of glass, or oiled paper. The impression is produced after 

 about fifteen or twenty turns of the machine, but by continuing the 

 action on a polished silver plate a permanent impression has been made 

 as distinctly as if it had been etched by aqua fortis. As many as one 

 thousand turns of the machine are required for this effect, and it would 

 be reasonable to suppose that nitric acid was actually formed, but 

 Karsten, who first obtained electrical breath figures, was not able to 

 detect the presence of any. 



Hoser explained the production of his figures, which could be formed 

 in total darkness aa well as in light, by referring them to the action of 

 the most refrangible dark rays of the spectrum, considering that all 



* While reading this proof we have received, from Professor George Wilson, 

 of Edinburgh, the following remarkable case : " In connexion with electric 

 images, I received yesterday two photographs, the peculiarities of which I shall 

 try to explain. The wax-paper negative of a ship was laid in the usual way 

 for printing, on a sheet of prepared paper, and below the latter intended to 

 receive the positive copy, a sheet of ordinary letter post was laid to act as a 

 packing In the printing-frame. The positive was duly taken, fixed, and laid 

 aside. Some time after another negative, namely, of a stoop, was arranged as 

 before, with a piece of positive paper next it, and for package, the same sheet of 

 letter post as before. When the poiitive was removed, it had the tloop on one 

 sidr, and the ship in addition on the other. In some strange way the letter 

 post had received a latent image, I suppose, of its first neighbour, the Mp, and 

 transferred this to the back of its second neighbour the tloop ! Fixing has 

 dimmed the sharpness, but there they both are." 



bodies radiate light even in the dark. Karsten's figures received a more 

 satisfactory explanation from the experiments of Riess, who many years 

 before had remarked that, on sending the discharge of an electric bat- 

 tery over the surface of glass or of mica, certain ramified figures were 

 produced, which became visible by breathing on the plate. Now, it was 

 observed by Riess, that different surfaces condense the breath in a very 

 different manner ; that a surface of mica which had been exposed for 

 some time to the air, did not allow the moisture to condense in a con- 

 tinuous film, but that it formed a congeries of dew-drops with many 

 breaks and interruptions ; whereas, by splitting open the mica so as to 

 obtain one of nature's clean surfaces, the vapour condensed in a con- 

 tinuous film or sheet of water. In the course of a short time, however, 

 the new and chemically clean surface becomes covered "with a film of 

 matter chiefly organic, arising from motes floating in the air, dust, and 

 moisture charged with the breath of animals sufficient to prevent thu 

 vapour of water from coming in contact with the surface, and explain- 

 ing why vapour condenses in drops and not in a continuous film. Now, 

 the action of the electricity is to bum off a portion of this film, which 

 it does more or less according as the parts of the coin are more or less 

 salient ; so that, on throwing off the coin and breathing on the plate, 

 the breath becomes condensed, and reflects the light differently in those 

 parts of the plate which were nearest to the coin compared with the 

 other and remoter portions. That this is the true explanation is evi- 

 dent from the fact, that the electrical breath figures cannot be produced 

 on glass if the surface be made chemically clean by being boiled in 

 nitric acid and washed in dilute ammonia ; nor on platinum if that 

 metal be made sufficiently clean to fire hydrogen gas. 



This simple explanation is not however sufficient to account for all 

 the varieties of roric figures. There must also be some molecular 

 change induced in the plate itself, which, though quite invisible to the 

 eye, nevertheless disposes the surface so as to make it catch the breath 

 differently. Karsten states that eight or ten thin plates of glass super- 

 posed, will all receive the impression of a coin placed on the top plate 

 and electrified ; the impressions become fainter and fainter in descend- 

 ing, and at a certain depth disappear. This seems to be a case of mole- 

 cular action in the lower plates not to be explained by the burning off 

 of a portion of the film in the upper one. So, also, if we cover a 

 polished surface, such as granite, with a perforated pattern or trellis- 

 work, and expose it to the sun for half an hour, the vapour of water, 

 or mercury, &c., will afterwards, when the perforated pattern is 

 removed, make out the pattern with perfect distinctness. This mole- 

 cular action is assisted by heat, as in the last experiment, and still 

 more strikingly by placing a sovereign on a glass plate on the warm 

 mantel-shelf, and leaving it there for half an hour. On throwing off 

 the sovereign and exposing the glass to the vapour of mercury, the 

 details of the coin are beautifully made out. In this case, the warmth 

 streaming up through the glass is intercepted by the metal and reflected 

 back again upon itself, and that differently by different parts of the 

 com, whereby a new arrangement of molecules is produced at the 

 surface, and the vapour of mercury is caught and held with 

 corresponding differences in different parts of the surface thus acted 

 on, so that the light reflected by the vapour informs the eye of 

 these differences, and thus makes out the image of the coin. We should 

 be disposed to refer the impression to the action of radiant heat when 

 the coin is not in contact with the plate, but suspended at a short dis- 

 tance over it. It requires in such case a longer time to produce the 

 impression, but it is nevertheless made. 



De la Rive and others, who contend for the electrical origin of roric 

 figures, imagine that the surfaces, though not in contact, have electri- 

 city developed upon one of them at least, by the action of a moist 

 atmosphere. It is stated also that the metals must be of different 

 kinds, as in Mr. Hunt's experiment, in which a sovereign, a shilling, 

 and a penny were placed on a copper plate, when the two former gave 

 excellent images, and the penny a very slight one. It is stated that 

 roric images cannot be produced on plates of gold and of platinum, nor 

 on a surface that does not oxidise; also that in very pure water 

 deprived of air, and also in vacua, these images are formed either not 

 at all, or but slightly. Many of these statements are the result of 

 preconceived theory, in which there is an anxiety to explain the pro- 

 duction of all roric figures by electricity ; whereas, as there are many 

 modes of producing these figures, so there are, in the wide diversity 

 of nature's operations, several methods of explaining them. 



Some remarkable roric figures may be produced on the surface of 

 glass by any one who is in possession of a small electrical machine, a 

 Leyden jar, and a discharging rod. Plates of crown glass about four 

 inches square answer the purpose very well ; they should be washed in 

 soap and water, and carefully dried. The jar should be fully charged ; 

 and on discharging it, with a plate of glass interposed, the spark will 

 pass to one of the edges of the plate, and turning round will proceed 

 up the glass on the other side to the knob of the discharging rod. In 

 this way the electricity traces a figure on both sides of the glass, and 

 it becomes visible by breathing on the surface. The figure, on the 

 side of the glass presented to the jar, which we may call the positive 

 side, is much more developed than on the other, or negative side. The 

 following engraving is a specimen of the figures thus produced. The 

 various ramifications do not proceed from the trunk or course of the 

 discharge, but the trunk proceeds from them. In fact, these minute 

 and delicate branches are feelers, sent out to determine the line of 



