250 



NATURE 



[July 10, 1890 



are both the result of photo-chemical decomposition, and 

 there is great probability that they are closely related, if 

 not identical, chemically. It may turn out that there 

 are various kinds of invisible images, according to the 

 vehicle or halogen absorbent — in other words, according 

 to the sensitizer with which the silver haloid is associated. 

 The invisible image is revealed by the action of the 

 developer, into the function of which I do not propose to 

 enter. It will suffice to say that the final result of the 

 developing solution is to magnify the deposit of photosalt 

 by accumulating metallic silver thereon by accretion or 

 reduction. Owing to the circumstance that the image 

 is impressed with such remarkable rapidity, and that it is 

 invisible when formed, it has been maintained, and is still 

 held by many, that the first action of light on the film is 

 molecular or physical, and not chemical. The arguments 

 in favour of the chemical theory appear to me to be 

 tolerably conclusive, and I will venture to submit a few of 

 them. 



The action of reagents upon the photographic film is 

 quite similar to the action of the same reagents upon the 

 silver haloids when exposed to the point of visible 

 coloration. Reducing agents and halogen absorbents 

 increase the sensitiveness of the film : oxidizing and 

 halogenizing agents destroy its sensitiveness. It is difficult 

 to see on the physical theory why it should not be possible 

 to impress an image on a film, say of pure silver bromide, 

 as readily as on a film of the same haloid embedded in 

 gelatine. Everyone knows that this cannot be done. I 

 have myself been surprised at the extreme insensitiveness 

 of films of pure bromide prepared by exposing films of 

 silver deposited on glass to the action of bromine vapour. 

 On the chemical theory we know that gelatine is a 

 splendid sensitizer — i.e. bromine absorbent. There is 

 another proof which has been in our hands for nearly 

 thirty years, but I do not think it has been viewed in this 

 light before. It has been shown by Carey Lea, Eder, 

 and especially by Abney — who has investigated the 

 matter most thoroughly — that a shearing stress applied 

 mechanically to a sensitive film leaves an impression 

 which can be developed in just the same way as though 

 it had been produced by the action of light. [Pressure 

 marks on Eastman bromide paper developed by ferrous 

 oxalate.] Now that result cannot be produced on a 

 surface of the pure haloid : some halogen absorbent, such 

 as gelatine, must be associated with the haloid. We are 

 concerned here with a chemical change of that class so 

 ably investigated by Prof. Spring, of Liege, who has 

 shown that by mere mechanical pressure it is possible to 

 bring about chemical reaction between mixtures of finely 

 divided solids.^ Then again, mild reducing agents, too 

 feeble to reduce the silver haloids directly to the metallic 

 state, such as alkaline hypophosphites, glucose or lactose 

 and alkali, &c., form invisible images which can be 

 developed in precisely the same way as the photographic 

 image. All this looks like chemical change, and not 

 physical modification pure and simple. 



1 have in this discourse stoically resisted the tempting 

 opportunities for pictorial display which the subject 

 affords. My aim has been to summarize the position in 

 which we find ourselves with respect to the invisible 

 image after fifty years' practice of the art. This image 

 is, 1 venture to think, the property of the chemist, and by 

 him must the scientific foundation of photography be laid. 

 We may not be able to give the formula of the photosalt, 

 but if the solution of the problem has hitherto eluded 

 our grasp it is because of the intrinsic difficulties of the 

 investigation. The photographic image brings us face to 

 face — not with an ordinary, but with an extraordinary 

 class of chemical changes due entirely to the peculiar 



I The connection between the two phefnomena was suggested during a 

 course of lectures delivered^ by me two years ago (" Chemistry of Photo- 

 graphy," p. 191). 1 have since learnt that the same conclusion had been 

 arrived at independently by Mr. C. H. Botta nley, of the Yorkshire College, 

 Leeds. 



NO. 1080, VOL. 42] 



character of the silver salts. The material composing 

 the image is not of that definite nature with which 

 modern chemical methods are in the habit of dealing. 

 The stability of the photosalt is determined by some kind 

 of combination between the sub-haloid or oxyhaloid, or 

 whatever it may be, and the excess of unaltered haloid 

 which enters into its composition. The formation of the 

 coloured product presents certain analogies with the 

 formation of a saturated solution ; the product of photo- 

 chemical decomposition is formed under the influence of 

 light up to a certain percentage of the whole photosalt, 

 beyond which it cannot be increased— in other words, the 

 silver haloid is saturated by a very minute percentage of 

 its own product of photo-decomposition. The photosalt 

 belongs to a domain of chemistry — a no-man's land- 

 peopled by so-called " molecular compounds," into which 

 the pure chemist ventures but timidly. But these com- 

 pounds are more and more urging their claims for con- 

 sideration, and sooner or later they will have to be 

 reckoned with, even if they lack that definiteness which 

 the modern chemist regards as the essential criterion of 

 chemical individuality. The investigation may lead to 

 the recognition of a new order of chemical attraction, or 

 of the old chemical attraction in a different degree. The 

 chemist who discourses here upon this subject at the end 

 of the half-century of photography into which we have 

 now entered will no doubt know more about this aspect 

 of chemical affinity ; and if I may invoke the spirit of 

 prophecy in concluding, I should say that a study of 

 the photographic film with its invisible image will have 

 contributed materially to its advancement. 



THE VELOCITIES OF PROJECTILES} 



HTHE experimenters, whose work is recorded in the 

 ■*- papers noted below, have succeeded admirably in 

 their attempts to photograph projectiles while moving 

 with their ordinary velocities. At the same time, they 

 have obtained indications of the forms of the waves ex- 

 cited in the air by projectiles when moving with velocities 

 higher than the normal velocity of sound in the air. 



The first experiments were conducted by Mach and 

 Wentzel with velocities of the projectiles about 240 

 m.s. (787 f.s.), which were below the normal velocity of 

 sound, when they obtained only negative results. After 

 this, Mach and Salcher carried on experiments of the 

 same nature with three small arms, which respectively 

 gave muzzle velocities of 438 m.s. (1437 f.s.), 338 m.s. 

 (iioo f.s.), and 522 m.s. (1713 f.s.). The arrangements 

 were such that, when the projectile was in the focus of 

 the camera lens, it caused the discharge of a spark from 

 a Leyden jar at a point in the axis of the lens which was 

 more distant from the lens than the projectile. As the 

 illumination was necessarily of very short duration, the 

 instantaneous photographs were taken on a small scale. 

 These photographs showed a well-defined wave of con- 

 densation of the air in front of the projectile when the 

 velocity of the shot exceeded that of sound, or about 

 340 m.s. (11 16 f.s.). All the experiments of value were 



' Aus den Sitzungsherichten d. kais. Akademie d. Wisscnschaften in 

 Wien:— 



(i) " Photographische Fixirung der durch Projectile in der Luft einge- 

 leiteten Vorgange," von E. Mach und P. Salcher. 1887. 



(2) " Ueber die Fortpflanzungsgeschwindigkeit des durch scharfe Schiisse 

 erregten Schalles," von E, Mach. 1888. 



(3) "Ueber die in Pola und Meppen angestellten ballistischphotographi- 

 schen Versuche," von E. Mach und P. Salcher. 1889. 



(4) " Ueber die Schallgeschwindigkeit beim scharfen Schuss nach von 

 dem Krupp'schen Etablissement angestellten Versuchen," von E. Mach. 

 1889. 



(5) "Optische Untersuchung der Luftstrahlen," von E. Mach und P. 

 Salcher. 1889. 



(6) "Weitere ballistisch-photographische Versuche," von E. Mach und L. 

 Mach. 1889. 



(7) " Ueber longitudinale fortschreitende Wellen im Glase," von E. Mach 

 und L. Mach. 1889. 



(8) " Ueber die Interferenz der Schallwellen von grosser Excursion," von 

 E. Mach und L. Mach. 1889. 



