XOVKMBER, 1911. 



KNOWLEDGE. 



440 



differing in their physical properties, are one and the same 

 chemical substance thougli the atoms may. perhaps, be com- 

 pounded into different sized molecules in the various allotropic 

 forms. They differ in their electrical conductivity among other 

 physical properties. Selenium cells have been constructed 

 which are sensitive to light ; when exposed to light the 

 electrical conductivity decreases considerably. The selenium 

 in this sensitive state contains at least two of the allotropic 

 lorms. and the light changes one form into the other, the effect 

 being reversible when the light stimulus ceases. Metallic 

 selenides, in small quantity, seem to improve the sensitiveness 

 of the selenium. Selenium cells are constructed by com- 

 pounding films of selenium with mica sheets, or between wires, 

 or the selenium may be condensed on to a cold surface, in which 

 case it possesses very little inertia and attains its maximum 

 or minimum conducti%ity very (juicUly and without showing 

 inertia as in the case of ordinary fused selenium. Selenium 

 cells have been used as receivers for wireless telephony. 

 Riihmer succeeded in telephoning along a fluctuating light 

 beam over ten miles. A vibratin.g membrane controls the 

 beam of light and causes it to fluctuate in intensity according 

 to tlie intensity of the sound vibrations. The selenium at the 

 other station receives the beam of light and alters its con- 

 ductivity according to the intensity of the li.ght falling on it. A 

 telephone connected to the selenium cell is thus affected 

 according to the fluctuation and intensity of the current and 

 of the light beam and therefore of the voice transmitted. 



Kontgen rays affect selenium just as light does — yellow light 

 is most active. Some years back I made a few experiments 

 on this point hoping to get a very marked effect with Rontgen 

 rays owing to their penetration throughout the mass of the 

 selenium. The change in conductivity is considerable, but no 

 comparisons were made of the energy in the Rontgen beam 

 with that in a light beam producing equal change in con- 

 ductivity, and so the results were only of small value. The 

 idea was to try other allotropic substances — such as tin, 

 to see if they were influenced throughout their mass, liy X rays, 

 whether they would possess similar properties to selenium. 



Another substance of considerable interest as regards 

 electrical conductivity is silver sulphide which, as Faraday 

 suggested, possesses a negative coefficient of resistance, i.e., 

 its resistance decreases as the temperature rises. \Vhen first 

 heated the resistance decreases rapidly ; but as it cools, 

 though it does not return to its original hi.gh resistance, yet the 

 resistance is greater than when hot ; after considerable heating 

 and cooling the resistance does not alter much on further 

 subjection to change of temperature. The author, H. V. Hayes, 

 of recent work on this peculiar substance, seems to infer that 

 the temperature coefficient of resisLiUce of silver sulphide is 

 not really negative, but that the apparent effect is due to 

 imperfect contact at the junction of the leading-in wires and 

 the silver sulphide. 



RADIATIONS FROM LATFX OF EUl'IIORIilA 

 PEPLL'S. — Chapman and Petrie find that the latex of this 

 Australian spurge possesses the property of aft'ecting photo- 

 graphic plates through paper, or even through aluminium foil. 

 The latex seems to give off a penetrating radiation which must 

 be comparable in intensity with the radiation emitted by 

 uranium or thorium. It has been found that many substances 

 affect the photographic plate, the eftect in some cases being 

 due, perhaps, to hydrogen peroxide ; in other cases, and more 

 conunonly, to the production of ions in the neighbourhood of 

 the substance, either by radiations from the substance or by 

 a chemical change induced by moisture or other causes on the 

 substance. It has been found to be quite difficult to obtain a 

 paint to inscribe the numbers on the black paper in which 

 films are rolled owing to this phenomenon. This particular 

 latex, however, seems to possess the property to a remarkable 

 extent. 



R.'^DIATIONS. — When describing in one of our recent 

 issues the instrument by means of which Professor Wood 

 was able to isolate ultra-violet waves free from \ isible rays, I 

 mentioned that it was also a convenient method of isolating 

 long heat waves. Professor Rubens has been able by means 

 of such a quartz lens apparatus to isolate waves one-third of 



a millimetre in length Iron, the radiations emitted by an 

 intense mercury vapour lamp. Now visible rays at the 

 extreme red end of the spectrum are less than inVxr of a 

 millimetre in wave-length, while very short electro-magnetic 

 waves of only a few niillmietres in length have been produced 

 by Fessenden and others. The conijilctc range of ether 

 vibrations has thus been nearly brid.gcd experimentally. It 

 appears quartz is extremely transparent to these long heat 

 radiations. Diamoiid, selenium, amber, paraffin and vulcanite 

 are fairly transparent, much more so than to the one-tenth of 

 a millimetre waves, which can be isolated from the Auer-manlle 

 radiations. Such invisible radiations are detected by means 

 of a bolometer or radiomicrometer. The first is a small 

 black very thin strip of platinum or silvered platinum, which 

 alters its resistance when radiations are absorbed by it : two 

 such strips of equal resistance are placed close to each other 

 and an electric current is made to pass down each strip — their 

 resistance being the same, the current in each branch will be 

 the same ; if the resistance of one be altered, then the current 

 in one strip is greater than in the other, and so an efi'ect is 

 produced on a galvanometer connecting the two branches of the 

 circuit containing the two strips. The radiomicrometer is 

 an exceedingly delicate thermopile. When a junction of 

 cadmium-antimony and bismuth-antimony is heated an 

 electromotive force is set up which tends to drive a current 

 down a wire connected to the two. In the radiomicrometer. 

 a silver strip is situated between such a junction and the two 

 ends of a silver wire are connected to the two alloys and the 

 wire is looped so as to hang vertically from a quartz fibre 

 inside a copper tube between the poles of a permanent magnet. 

 When the radiations, say of a candle placed 10 metres away, 

 fall on the blackened strip of silver, a current passes down the 

 loop, and the magnetic action is such as to turn the coil, which 

 motion is registered by a spot of light mo\'ing over a scale. 



Langley, by means of the bolometer, mapped the infra-red 

 spectrum of the sun to nearly a wave-length of .jju millimetre, 

 but below this the radiations are absorbed by the atmosphere. 

 He was also able to estimate from an examination of the 

 moon's r.adiation that the temperature on its surface was less 

 than O'C. Sir William Abney succeeded in photographing, 

 the shorter infra-red rays. Bathed plates, to photograph 

 objects in infra-red light, can now be bought commercially, but 

 they do not record a wave-length of more than rij\,T_; millimetre. 

 Such plates are bathed in dyes (.-Vlizarin blue. Nigrosin, diazo 

 black, cyanin, and so on) which absorb red rays. The energy 

 of these rays go to change the constitution of the sensitive 

 emulsion, and a photographic record is the result. Now 

 .■\bney conceived the idea of making silver bromide absorb red 

 li.ght instead of blue light as is usually the case : he succeeded 

 in obtaining an enmlsion of silver bromide which transmitted 

 blue light and absorbed red, and the plates he thus- made he 

 employed to photograph waves of even sJrr millimetre. P. V. 

 Hevan has recently investigated the absorption and dispersion 

 of light by alkalinietals — that is, by potassium, rubidium, 

 lithium, caesium and sodium. The measurements are somewhat 

 difficult to carry out with such metals owing to the fact that 

 they attack most materials with which they come in contact. 

 Lithium vapour even attacks steel tubes. It appears from this 

 work that different specialised atoms are absorbing different 

 spectral lines, so that a certain percentage of atoms in the 

 vapour may be employed in absorbing one particular wave- 

 length of the incident light, while another is absorbing another 

 wave-length. This idea is distinct from the idea that within 

 each atom the electronic arrangement is such that certain 

 groups of electrons are responsible for the absorption of the 

 \arious incident radiations. .An electric current passed 

 through such vapours of metals under diminished pressure 

 gives rise to a varietj' of spectra akin to flame, arc and spark 

 spectra according to the electric conditions. The luminosity 

 of the vapour of sodium will mask the spectrum of helium 

 when present in such tubes. The masking effect of the 

 spectrum of one substance on that of another is a matter of 

 great interest. 



The salts of potassium .are radio-active ; they emit a 

 penetrating radiation, which appear to be of corpuscular 

 nature like part of the radiation (/^ rays) from radium. 



