April, 1913. 



KNOWLEDGE. 



151 



image upon silver paper would only appear as a continuous 

 stain. There is, therefore, a great difference between a 

 printed-out image and one produced by development. Then, 

 again, the former is much more readily affected by external 

 agencies, even prolonged soaking in water being sufficient to 

 very considerably weaken them. Then, if the cause of fading 

 be due to sulphurisation, the product formed may be an 

 organic sulphur compound of silver, as pure silver sulphide 

 itself stands the action of reagents very well indeed, and 

 bromide prints toned by means of " Hypo " and alum, " which 

 is a sulphurisation process," are remarkably permanent. It 

 may be remarked also that some bromide prints tone much 

 more rapidly than others, and examination under the micro- 

 scope shows that those which tone quickly have smaller-sized 

 particles of silver than the others, and this appears to bear out 

 the statement with regard to the state of division of the silver 

 and its influence upon the permanence of the image. If the 

 presence of hydrogen sulphide in the atmosphere is the most 

 destructive agent upon silver prints, then perfect toning 

 with separate baths of either gold or platinum is to be 

 recommended, as a good deposit of these should go far 

 to ensure greater resistance to atmospheric influences. 

 As a test of permanency prints may be subjected to the 

 action of sulphuretted hydrogen for a short time and the action 

 noted by comparison with part of a print not so treated. It 

 will be found that any prints toned with the combined bath 

 will bleach first, then albumen prints, after which gelatino- 

 chloride and collodio-chloride, and, lastly, bromide. There is 

 still one point, however, which has not been touched upon, 

 and that is the influence of the mountant upon the permanence 

 of silver prints. If prints are kept in a damp atmosphere, the 

 mountant may, from decomposition, liberate hydrogen in its 

 nascent state, when, if any " hypo " remained in the print, 

 sulphuretted hydrogen would be formed, with the result that 

 the prints would be affected. This, no doubt, accounts for 

 many prints which have not been mounted lasting longer than 

 those which have, and is an argument in favour of employing 

 a method of mounting in which a material that is not affected 

 by moisture is used. In conclusion we may say, that whatever 

 may be the exact nature of fading in silver prints, sulphur in 

 some form appears to be the prime cause. That collodio- 

 chloride appears less liable to change than some other forms, 

 is the conclusion arrived at from experience, although there 

 appears no theoretical reason why this should be, as the 

 medium itself cannot be considered as making much difference. 



PHYSICS. 



By Alfred C. G. Egerton, B.Sc. 



RECENT LECTURES.— The results of physical investiga- 

 tions have been expounded recently by their chief pioneers. 

 It seems the time of year is fitted not only to the planning of 

 new investigations, but the clearing up of past work. Among 

 the most interesting summaries of work accomplished which 

 have lately been given, the lecture by Professor Strutt, at the 

 Royal Institution, on February 28th, finds place. He showed 

 numerous experiments illustrating the phenomena caused by 

 active nitrogen. Active nitrogen is almost certainly nitrogen 

 in the atomic instead of the molecular condition ; when the 

 atoms re-combine to form molecules, then ordinary nitrogen is 

 again formed. The transformation of active into ordinary 

 nitrogen takes place with evolution of light of a yellow colour. 

 Such a transformation, contrary to usual chemical action, 

 takes place more rapidly at low temperatures, as was shown 

 in the lecture, by plunging a tube containing active nitrogen 

 into liquid air. 



The nitrogen is made active by being submitted to an 

 electric discharge from Leyden jars or similar condensers : 

 ordinary pure nitrogen, freed from oxygen by passage over 

 phosphorus, is sucked through a discharge tube at a pressure 

 of about one millimetre of mercury ; the nitrogen, as it leaves 

 the discharge tube, passes into a wider tube, where it can be 

 viewed undergoing its transformation into ordinary form by the 

 yellow light emitted. When other gases, such as chloroform 

 or carbon bisulphide, are passed in together with the active 



nitrogen, cyanogen is formed and gives out the characteristic 

 pinkish light of the cyanogen flame. The nitrogen, unlike 

 ordinary nitrogen, combines readily with most substances. If 

 the metals are heated gently in the active gas, they give out 

 their characteristic line spectra and form nitrides. Thus 

 mercury was shown to give a brilliant mercury arc effect and 

 form a very unstable mercury nitride, which decomposes 

 explosively on heating. 



Now ozone and nitric oxide, when they combine under 

 similar conditions, also give out light, but of a much greener 

 hue, and repetition of experiments on active nitrogen in 

 Germany by certain investigators have been ascribed to the 

 production of ozone or nitric oxide from minute quantities of 

 oxygen contained in the nitrogen. But it must be allowed 

 Professor Strutt has freed his nitrogen from oxygen quite 

 satisfactorily. The active modification of nitrogen can be 

 destroyed by the catalytic action of a surface coated with 

 oxide of copper. A copper wire, coated with oxide, dropped 

 into a tube containing active nitrogen immediately extinguishes 

 the glow. It is necessary for the nitrogen atoms to collide up 

 to seven hundred and eighty times before recombination with 

 another atom occurs, in presence of a copper oxide surface ; 

 whereas only one collision of a molecule of ozone against a 

 silver surface is necessary to cause it to transform to an 

 ordinary oxygen molecule. Ordinary nitrogen can be con- 

 verted into active nitrogen by means of the ring discharge. 

 If the inner coatings of Leyden jars are connected to an 

 induction coil, and the outer to a coil of thirty or forty turns 

 of wire, electrical oscillations will be set up in the coil of wire, 

 which will ionise and induce currents in gases enclosed in 

 tubes at low pressure placed within the coil. Therefore, all 

 that is necessary is to fill a tube with pure nitrogen, at about 

 one or two millimetres pressure, and place it within such a 

 coil, in order to obtain it in the active condition and view the 

 glow given by it. 



Another lecturer has gathered together Mr. C. T. R. Wilson's 

 work, which has rendered visible the paths of ionising 

 radiations. When Rontgen rays or the rays from radioactive 

 substances pass through air, they cause ionisation of the air in 

 their path : that is to say, air particles carrying positive and 

 negative charges are formed. Moisture will deposit from air 

 .overcharged or supersaturated with water vapour on either 

 dust particles or electrified ions, such as these charged 

 particles are called. Thus, if the air through which rays are 

 passed is supersaturated with moisture, the paths of those 

 rays will be marked by the condensation of moisture on the 

 ions produced in those paths. That is the principle of Mr. 

 Wilson's method of making visible the paths of such rays. 

 However, it is not easy to record such effects photographically: 

 much ingenuity has been displayed in doing this satisfactorily. 

 Mr. Wilson allows the base of the ionisation chamber, con- 

 taining the moist air through which the rays from radium or 

 the X-rays pass, to drop slightly whereby sufficient expansion 

 occurs to cool the air to the necessary point to cause the 

 moisture to be capable of condensing out on any ions present ; 

 at the same time as the expansion chamber base drops a ball 

 is released, which passes between a spark gap, and the spark 

 through a mercury arc gives sufficient light to illuminate the 

 condensed particles, and allow them to be recorded on a 

 photographic plate which has previously been focused on the 

 expansion chamber. The photographs obtained are very 

 remarkable ; the paths of the a-rays of radium are very distinct 

 and perfectly straight to within a millimetre or so of their 

 termination, when they often branch off for a short distance 

 at another angle. Presumably, the a-rays have been so 

 reduced in velocity by their collisions with air molecules, that 

 they get deflected out of their path. The results with p-rays 

 — the negatively-charged rays from radioactive substances 

 and the rays produced when X-rays strike against substances 

 — have much more irregular and meandering paths, and the 

 streaks on the photographic plates are much less dense than 

 those due to the a-rays, because the number of ions produced 

 are much fewer. It is indeed wonderful that the actual 

 paths of single particles, of which there are more than a 

 million million million in one cubic centimetre, can be made 

 visible and recorded on a photographic plate ; and it is a very 



