December 23, 1922] 



NA TURE 



849 



should study, immediately and as thoroughly as pos- 

 sible, those which are within our reach and are dis- 

 closed by Kohlhorster's experiment. The ionisation of 

 gases " in a sealed vessel has been measured in our 

 atmosphere up to an altitude of 9000 metres ; but it is 

 necessary to repeat the experiment at several places on 

 the earth, and to extend it up to the greatest altitudes 

 reached in exploring balloons. The undertaking, it is 

 true, will be costly ; it devolves especially on the 



countries which have the greatest resources. I pro- 

 posed, at the International Astronomical Congress, 

 which met at Rome in May last, that there should be 

 international co-operation for the complete study of 

 the electrical phenomena of our atmosphere at great 

 altitudes. The determination of the exact origin of 

 these penetrating radiations is one of the most im- 

 portant problems confronting physical astronomy at 

 the present time. 



The Desensitising of Silver Bromide-Gelatin Plates. 

 Bv Dr. T. Slater Price. 



IT is well known that the more sensitive a photo- 

 graphic plate is, the greater the care that has to 

 be taken with respect to the actinic value of the light 

 used in the dark room during the operation of develop- 

 ment. The less the amount of light used, the more 

 difficult it becomes to control the result ; and it is 

 therefore not to be wondered at that attempts have 

 been made to modify the course of procedure in such 

 a way that the exposed plate could be developed in a 

 fairly good light. During the last few years various 

 so-called " desensitisers " have been put on the market ; 

 when the exposed plate is either treated with a solution 

 of these before development, or when some of the de- 

 sensitiser is added to the developer, the plate can 

 safely be developed in a light which would otherwise 

 give rise to very bad fogging. 



At the recent Deuxieme Congres de la Chimie In- 

 dustrielle, M. A. Seyewetz gave an interesting account 

 of the subject, and his paper has been published in 

 Chimie el Industrie, 1922, 8, 308-311. 



A. and L. Lumi^re and Seyewetz, in 1907, were the 

 first to notice that a silver bromide-gelatin plate 

 becomes less sensitive when bathed in a solution of 

 a developer such as diaminophenol, quinol, or pyro- 

 gallol. The loss in sensitivity varied slightly in different 

 regions of the spectrum, but was most marked in the 

 yellow and green. At a much later date, in 1920, 

 Liippo-Cramer noticed that the desensitising action 

 was much increased when sulphite was omitted from 

 the developing solution, that is, when the developer 

 was used in such a condition that it readily oxidised in 

 the air. After immersion for a minute in a 0-05 per 

 cent, solution of the developer the plate could be 

 developed in yellow light without fogging. Such a 

 method of desensitisation was insufficient, however, 

 for orthochromatic and panchromatic plates, and 

 moreover, the solutions underwent rapid alteration in 

 the absence of sulphite. 



Desensitisation only became a practical proposition 

 when Liippo-Cramer, in 1921, discovered the pro- 

 nounced desensitising action of the azine dye, Pheno- 

 safranine, and also of other dyes belonging to the 

 same class, on ordinary and panchromatic plates. 

 Contrary to what one would at first suppose to be the 

 case, these dyes do not owe their action to functioning 

 as colour screens : solutions of Phenosafranine transmit 

 red and violet light, and yet they desensitise plates for 

 these regions of the spectrum. Also, the violet safranines 

 desensitise just as do the red safranines, although their 

 absorption spectra are very different. These facts 

 are very similar to those observed with sensitisers, and 



NO. 2773, VOL. I IO] 



Liippo-Cramer has shown that certain optical sensi- 

 tisers for one haloid salt of silver may act as desensi- 

 tisers for other salts. For example, Erythrosin, Rhod- 

 amine B, Pinachrome, and Pinacyanol, which are the 

 best sensitisers for -chloride and bromide of silver, 

 when used in very dilute solutions (1 : 20000) diminish 

 the sensitivity of silver iodide-gelatin plates from 

 6 to 16 times ; Phenosafranine gives a reduction in 

 sensitivity of about 40 times. 



Lumiere and Seyewetz have made investigations to 

 see if there is any relation between the desensitising 

 power of a substance and its chemical constitution. 

 They have found, with the safranines, that the 

 presence of the characteristic phenazine grouping, 



ch'A 



( cJh 



is insufficient in itself, and that amino-groups sub- 

 stituted in the benzene nuclei must also be present. 

 Thus Neutral Red, which is a Eurhoidine having the 

 formula 



X(HC1). 

 XMe, . C 6 H 3 / >C 6 H 2 Me . \H, 



y 



has desensitising properties approximating to those of 

 Phenosafranine, which is 



/ N(C 6 H 5 C1) 

 NH 2 . C 6 H 3 j^giij 



\n= 



\C S H S .NH,. 



Safranines in which one of the amino-groups has been 

 eliminated, as in aposafranine, or in which this group is 

 replaced by oxygen, as in the safranones, are notably 

 less active as desensitisers. If both of the amino- 

 groups are replaced by or OH, as in safranol, there is 

 no longer any desensitising action. The acetylation of 

 the amino-group, or its diazotisation and copulation 

 with a phenol, destroys the desensitising properties 

 of the safranine, while the replacement by ethyl of 

 the phenyl group attached to the nitrogen has no 

 effect. The Indulines, which are near cousins to the 

 safranines, as also the thiazines and the oxazines, do 

 not act as desensitisers. On the other hand, other 

 colouring matters which have very different constitu- 

 tions from that of safranine as, for example, picric acid, 

 Indian Yellow, Chrysoidine, etc., are weak desensi- 

 tisers ; Aurantia (x : 1000) desensitises as actively as 

 safranine for the blue rays, but is noticeably less active 

 towards other parts of the spectrum. 



It follows from the above that there does not seem 



