Sept. 5, 1878] 



NATURE 



491 



it in one of his memoirs, and his explanation seems to 

 account for it in a most perfect and philosophical manner. 



In the Daguerrotype process, it will be remembered, a 

 silvered plate is subjected to the vapour of iodine (or of 

 iodine and bromine), and thus receives a fine layer of a 

 compound which is sensitive to light. When a plate so 

 prepared is exposed to a lenticular image in the camera, 

 the light causes the iodide (or bromoiodide) of silver to 

 throw off iodine (or this together with bromine), which is 

 immediately seized by the silver beneath, and thus forms 

 a deeper layer of the sensitive salt. The depth, almost 

 immeasurable though it be, depends on the intensity 

 of, and length of exposure to, the light. (That this is the 

 case has been proved by the fact that, if the sensitive 

 layer be removed by a suitable solvent, the surface 

 beneath is shown by reflected light to be etched to a 

 greater or less degree.) The invisible image thus formed 

 is exposed to mercury vapour, and the dew condenses on 

 it proportionately to the depth of the layer. Quincke, in 

 his memoir " On the Edge Angle and Spread of Liquids 

 on Solid Bodies," ' shows that the edge angle of a drop 

 of liquid on a solid body varies from zero to a constant 

 quantity, according to the thickness of any fine layer of 

 impurity which may be on the latter. When this layer 

 attains a certain value then the edge angle of the drop 

 will remain constant. The thickness, or rather the thin- 

 ness, of the layer may be appreciated when it is stated 

 that it bears a relation to what is called "the radius of 

 the sphere of sensible action of molecular forces," and 

 is usually greater than "00005 millimetre. In this case 

 the sensitive plate is the solid body and the invisible 

 image forms different thicknesses of impurity. By this 

 difference in the edge angles of the mercury dew, con- 

 densed on different portions of the latent image, the light 

 is reflected in different ways, which gives rise to the visible 

 image. 



This explanation entirely does away with the necessity, 

 which previously seemed to exist, of the silver iodide (or 

 bromo-iodide) being reduced to the metalHc state, in order 

 to cause condensation, or — perhaps it might be said — to 

 cause the formation of an amalgam of mercury and silver. 



The next method of development speaks for itself ; the 

 metallic silver is deposited in fine granules and is 

 attracted by the salt which has been altered by the influ- 

 ence of light. Perhaps further investigation will show 

 that development is dependent on what is known as the 

 Brownian movement, or the rapid movement of small 

 suspended particles in a liquid. If this movement be 

 dependent on the electrical condition of the neighbouring 

 body, as has lately been supposed ; and if, as Dewar 

 has shown, the condition of an exposed sensitive salt is 

 electrical, then the deposition of the metallic particles of 

 silver on the image is accounted for in a satisfactory 

 manner. 



The last mode of development is principally employed 

 with silver bromide, and is known as the alkaline method. 

 When a film of collodion or gelatine holds a sensitive 

 salt on a plate, the portions exposed to light are reduced to 

 the metallic state by the application of an oxygen absorbent 

 such as alkaline pyrogaUic acid. Since the image is 

 invisible, it must be remembered that but a few molecules 

 of the sensitive salt are reduced by the action of light to the 

 less complex and developable form, we therefore must look 

 for some further action between the developer and the rest 

 of the unaltered compound. It has lately been proved that 

 silver bromide or silver chloride cannot exist in dose contact 

 with metallic silver. It invariably forms the developable 

 salt. Thus if we take a glass plate, silvered by any of the 

 well-known processes, and expose it to the fumes of 

 bromine or to hypobromous anhydride, it will be found 

 that it is impossible to secure a film of argentic bromide 

 until the last trace of silver has been attacked, after 

 which the true colour of argentous bromide gradually 



» Phil Mai^., May and June, 1878. 



gives way to the well-known colour of argentic bromide. 

 We may try the experiment with bromine water and the 

 same holds good. The action of chlorine on silver is the 

 same as of bromine, but the action of iodine seems to be 

 different, the fully saturated compound, argentic iodide, 

 being formed at first. In other words, this compound is 

 the more stable than argentous iodide. 



Now the alkaline developer, when mixed with a 

 soluble bromide of an alkali, has the property of much 

 more readily attacking the argentous than the argentic 

 bromide, presumably because the soluble bromide used 

 in development combines with the former, giving rise 

 to an apparently difficultly reducible compound, whilst 

 it refuses to combine with the argentous salt. It is 

 thus easy to see, if this property of the developing 

 solution be connected with what was stated in the 

 preceding paragraphs, how development takes place. 

 The developer is applied to the exposed film, and the 

 minute quantity of argentous compound is reduced to 

 the metallic state, and at once this particle of silver 

 which is in close contact with the unaltered compound 

 combines with it and forms new argentous bromide. 

 This is ready for a,ttack by the developer, and thus the 

 action spreads till the whole thickness of the sensitive 

 salt is reduced to the metallic state where the greatest 

 exposure has taken place. An interesting result i of this 

 action is afforded by the fact that, if a film of unexposed 

 argentic bromide be superposed over one that has been 

 exposed, the image impressed on the latter can be deve- 

 loped in the former so long as close contact is secured. 

 It has been said that this action is due to the solubility 

 of the silver-bromide used in the alkaline development ; 

 and, to some extent, this is true ; but it is evident that 

 this cannot be explanatory of the whole phenomenon, 

 since the same effect is produced by using, with the 

 pyrogaUic acid, potash as the alkali in which the silver- 

 bromide is absolutely insoluble. We have been thus 

 particular in showing the cause of this alkaline develop- 

 ment, as it explains some phenomena to which attention 

 will subsequently be called, and which otherwise would be 

 inexplicable, except by reversing usually-accepted physical 



laws. W. DE WiVELESLIE ABNEY 



{To be continued^ 



MILITAR V BALLOONING 



THE matter of ballooning for military purposes 

 appears to be once more attracting attention in this 

 country. In France they have now a properly organised 

 service under the command of a colonel of the National 

 Engineers, who considers all novelties and proposals as 

 they arise, and who sees, moreover, that the State has 

 always a body of skilled aeronauts at its disposal. At 

 the end of the Paris siege the Postal department, it may 

 be remembered, possessed a large number of balloons, 

 and these being handed over to the French war minister, 

 constituted the materiel necessary in the formation of 

 a military balloon service. Col. Laussedat, whose name 

 as an energetic officer of the French Topographical 

 Department, is well known, was placed in command, and 

 he at once secured the services of one of the Messrs. 

 Goddard to put the whole of the apparatus in a fit con- 

 dition for service. Since that day ballooning in France 

 has been considered as much a duty of the Engmeers of 

 the army as telegraphing and surveying, and classes 

 both for officers and men are held for instruction. 

 Lately, by the resignation of Col. Laussedat, the French 

 balloon service has lost its chief support ; but his place 

 has just been supplied by Gen. Farr, who will, no doubt, 

 take measures to maintain the high efficiency which has 

 been attained by his predecessor. 



In France, as in this country, the balloon is chiefly 



Phil. Mag., January, 1877. 



