>tAY. i9i: 



KXOWLEDCxE. 



195 



Hooded Crow, between ^ach of which there was an inter- their behaviour in all respects was similar to cathode rays, 

 n.ediate form possibly a hybrid .' If a luie were drawn from which consist of streams of negatively-charged partic es 

 (.lasgow to the Adriatic Sea the Black Crow would bo fonnd projected with ...eat velocity. In hononr of the discoverer 



these rays have been called 



on the east of it and the Hooded 

 Crow on the west. No answer had 

 yet been found for their differentia- 

 tion. More piu^ling still was that 

 birds of exactly the same species 

 were found in the northern half of 

 India and not sighted again till 

 Ceylon was reached, a slightly 

 different species separating the 

 two. The variants never passed 

 over the border line, and yet were 

 within ■■ crowing distance " of each 

 nther. The Tree Sparrow and the 

 House Sparrow differed in their 

 markings, and the male of one kind 

 could be distinguished from the 

 female of the other kind only by 

 dissection. In America there were 

 to be found male birds which 

 moulted to the colour of the female. 

 These differences and distinctions 

 .-.till await reasoned explanations. 



PHOTOGRAPHY. 



By Edgar Senior. 



RADIO-ACTIVE BODIES.— 

 Interesting as the results brought 

 ribout by the action of ultra-violet 

 rays are. especially in the light of 

 their practical bearing upon everv- 

 day work, there are other instances 

 of photographic effects resulting 

 from invisible radiations, which 

 may claim our attention equally 

 well. First of importance among 

 these stands the discovery made by M. Henri Becquerel 

 in 1896. that the double sulphate of uranium and potassium, 

 and also the metal uranium 

 itself, emitted rays that were 

 capable of forming im- 

 pressions upon photographic 

 plates in total darkness. It 

 was at first thought that the 

 cause might be due to phos- 

 phorescence, as uranic salts 

 possess this property when 

 exposed to ultra-violet light 

 • although only for a ver\ 

 limited periodi but as urarous 

 salts " which are non-phos- 

 phorescent " were found to 

 be equally active the above 

 explanat ion became untenable. 

 That the action was not due 

 to stored up energy from 

 previous exposure to sunlight 

 was proved by the fact that 

 crystals deposited from solu- 

 tions in darkness possessed 

 exactly the same properties 

 as those previously experi- 

 mented with, and which had 

 been exposed to light. 

 Further experiments pointing 

 to the absence of evidence 

 of either reflection, refraction 

 or polarization, went to show 



I-IGL'KL 227. Part of an incandescent i 

 mantle, photographed in total darkness. 



Becquerel rays, but the term radio- 

 active is now generally applied to 

 such bodies which belong to a class 

 "f substances of which uranium, 

 ihorium, radium and their com- 

 pounds form part, and which 

 possess the property of spontan- 

 lously emitting radiations which 

 ire capable of passing through 

 substances opaque to ordinary light. 

 >nch as metal plates, and the still 

 further characteristic of being able 

 to impress a photographic plate in 

 the dark, and of discharging 

 electrified bodies. It is also found 

 that radium, which is a strongly 

 radio-active body, is able to cause 

 marked fluorescence and phos- 

 phorescence in some bodies placed 

 near it. The remarkable property 

 of radioactive bodies is their 

 power of being able to continuously 

 radiate energy, and at a constant 

 rate, without " so far as is known " 

 any external exciting cause. As 

 already stated thorium belongs to 

 this class, and in Figure 227 is 

 shown a portion of an incandescent 

 gas mantle the photograph of which 

 was taken in total darkness by 

 means of the radiations given off 

 by the thorium, which was present 

 in small quantity in the mantle. 

 The photographic plate from which 

 the print was made was enclosed 

 in an orange paper envelope, the 

 mantle was laid upon this, and the whole enclosed in a 

 tin box which was placed away in a cupboard for three 

 weeks. The image was 



then developed, with 

 result shown. 



the 



FiGURU 226. .\ Fungus i Pciiicillunii glaKcnnij on a 

 gelatine photographic film, magnified. 



IHi: EFFECTS OF 

 1 ) .\ M P ON P H O T O . 

 UKAPHS. — It has lately 

 been found that a good 

 many photographs and 

 mounts are affected with 

 mildew, although kept under 

 the same conditions under 

 which they had previously 

 shown no signs of any such 

 trouble. The cause is no 

 doubt due to the excessive 

 amount of wet weather ex- 

 perienced some time back. 

 The form of the fungus (for 

 such it isl is of the nature of 

 that which produces common 

 mould and is known by the 

 name of Petiicilliuiii nUiii- 

 cum. This plant may be 

 found on the surface of jellies 

 and preserves, and consists 

 of a mass of filaments serv- 

 ing as its base, and from the 

 surface of which rise up thin 



stems bearing at their extre- 

 f f r K -rv, mity a number of minute cells which are the spores or repro- 



nature ot light. Ihe posing problem was, however, finally dnction organs. These plant like threads thrive in damp 

 ^° „'f„„'„'!-u1°„n'. ''"^L '|'^'=°;'«=„'"^d that those rays which act atmospheres of cellars or rooms generally, and gelatine forms 



^^^g^^^j^ ^^j ^1^^^ a good medium for their propagation. They form beautiful 



that the effects could not be due to ether waves of the 



photographically can be deflected bv 



