234 



NATURE 



[January 9, 1896 



cadmium at an air temperature of I5°C. and pressure of 760 mm., 

 two wholly independent determinations were made. From the 

 first a metre was found equal to 1553 162 7 wave-lengths ; from 

 the second, 1553164-3 wave-lengths, giving a mean of 1553163-5 

 the deviation of each result from the mean being very nearly one 

 part in two millions (" Travaux et Memoires du Bureau Inter- 

 nationale des Poids et Mesures," Tome xi. p. 84, 1894). A 

 determination by Benoit from the first series gave 1553163-6, 

 which differs but one-tenth of a wave-length from the mean of 

 Michelson's measurements. 



The direct comparison of the lengths of two metre bars, 

 though not easy, is a simple operation in comparison with the 

 indirect method just described, but does not surpass it in 

 accuracy. Every one knows that the metre is not an exact sub- 

 multiple of the earth's circumference, and that the determination 

 of its exact value from the seconds pendulum is full of difficulty. 

 It may perhaps be said that the optical method is no more 

 absolute than the pendulum method, for no human measure- 

 ments can be free from error ; that there is no possibility of the 

 destruction of the original metre and all certified copies of it ; 

 and that there is no proof or probability that molecular changes 

 are gradually producing modifications in standards of length. 

 Even if we should grant that for all practical purposes the labour 

 of determining the metre in terms of an unchanging optical 

 standard has been unnecessary, the achievement is a signal 

 .scientific triumph that ranks with the brilliant work of Arago, 

 Fresnel and Regnault. In preparation for it much new truth 

 has been elicited, and light waves have been shown to carry 

 possibilities of application that Fresnel never suspected. 



The physicist is nearly powerless without the aid of those who 

 possess the highest order of mechanical skill. The interferential 

 comparer could never have been ultilised for such work as 

 Michelson has done with it, had not Brashear made its optical 

 parts with such an approach to perfection that no error so great 

 as one-twentieth of a wave-length could be found upon the 

 reflecting surfaces ("Travaux et Memoires du Bureau Inter- 

 nationale des Poids et Mesures," Tome xi. p. 5, 1895), In 

 the conception, mechanical design and execution, the entire 

 work has been distinctly American. 



The interferential refractometer has been used with much skill 

 by Hallwachs ( Wiedemann' s Annalen, Band 47, p. 380, and 

 Band 53, p. i) for comparing the variation of refractive index of 

 dilute solutions with variation of concentration. The fact of so- 

 lution brings about a change of molecular constitution, affecting 

 both the electric conductivity and the refractive index ; and the 

 changes in optical density are measurable in terms of the 

 number of interference fringes which cross the field of view for a 

 given variation of dilution. 



Luminescence. 

 While all work on the visible spectrum is confessedly optical, 

 we can no longer make an arbitrary division point, and declare 

 that one part of the spectrum belongs to the domain of optics 

 and the other not. Since the days of Brewster and the elder 

 Becquerel fluorescent solutions have enabled us to bring within 

 the domain of optics many wave-lengths that were previously 

 invisible. Stokes's explanation of this, as a degradation of 

 energy quite analogous to the radiation of heat from a surface 

 on which sunlight is shining, has been generally accepted. But 

 whether the phenomena of fluorescence and phosphorescence 

 are in general physical or chemical, has for the most part 

 remained unknown or at least very uncertain. E. Wiedemann, 

 who suggested the term luminescence to include all such 

 phenomena, published in 1895 {Annalen der Physik und 

 Cheniie, p. 604, April 1895), ''^ conjunction with Schmidt, 

 a part of the outcome of an extended investigation under- 

 taken with a view to clearing up these uncertainties. He 

 has shown that it is often possible to distinguish between cases 

 in which the emission of light springs from physical processes 

 and those in which it is due to chemical action, or at least in- 

 variably accompanied by this. We have here, as in photo- 

 graphy, a transformation of radiant into chemical energy, to 

 which is superadded the retransformation of chemical into 

 radiant energy of longer period, and this either at the same time 

 or long after the action of the exciting rays. Indeed, between 

 this process and that of photography in colours, the analogy is 

 quite striking. What has generally been called phosphores- 

 cence is well known to be the effect of oxidation in the case of 

 phosphorus itself and in that of decaying wood or other organic 

 matter, which under certain conditions shines in the dark. 



NO. 1367, VOL. 53I 



Wiedemann has shown that the shining of Balmain's luminous- 

 paint, and generally of the sulphides of the alkaline earths, is- 

 accompanied with chemical action. A long period of luminosity 

 after the removal of the source renders highly probable the 

 existence of what he now calls chemi-luminescence. A large 

 number of substances, both inorganic and organic, have beei> 

 examined both by direct action of light and by the action or 

 kathode rays in a controllable vacuum tube through which 

 sparks from a powerful electric influence machine were passed. 

 Careful examination with appropriate reagents before and after 

 exposure was sufficient to determine whether any chemical 

 change had been produced. Thus the neutral chlorides of 

 sodium and potassium, after being rendered luminous by action 

 of kathode rays, are thereby reduced to the condition of sub- 

 chloride, so as to give a distinctly alkaline reaction. 



Many substances, moreover, which manifest no luminescence- 

 at ordinary temperatures after exposure, or which do so for only 

 a short time, become distinctly luminescent when warmed. This 

 striking phenomenon is sufficient to warrant the use of a special 

 name, thermo-luminescence. Among such substances may be 

 named the well-known sulphides of the alkaline earths, the 

 haloid salts of the alkali metals, a series of salts of the zinc and 

 alkaline earth groups, various compounds with aluminium, and 

 various kinds of glass. Some of these after exposure give intense 

 colours when heated, even after the lapse of days or weeks. 

 That the vibratory motion corresponding to the absorption of 

 luminous energy should maintain itself for so long a time as a 

 mere physical process is highly improbable if not unparalleled. 

 That it should become locked in, to be subsecjuently evoked by 

 warming, certainly indicates the storing of chemical energ)', just 

 as the storage battery constitutes a chemical accumulator oF 

 electrical energy. Other indications that luminescence is as- 

 much a chemical as a physical phenomenon are found in the 

 fact that the sudden solution of certain substances is accom- 

 panied by the manifestation of light, if they have been previously^ 

 subjected to luminous radiation, but not otherwi.se : that altera- 

 tion of colour is brought about by such exposure ; and that fric- 

 tion or crushing may cause momentary shining in .such bodies 

 as sugar. There is no conclusive direct evidence thus far that 

 such luminescence as vanishes instantly upon the withdrawal of 

 light is accompanied by chemical action. But Becquerel demon- 

 strated long ago with his phosphoroscope that there is a measur- 

 able duration of luminous effect when to the unaided eye the 

 disappearance seems instantaneous (Becquerel, Comptes rendus 

 96-121). Wiedemann now shows that when this duration is- 

 considerable there is generally chemical change. Since durationi 

 is only a relative term it seems highly probable that even cases- 

 of instantaneous lumine-scence, commonly called fluorescence,. 

 are accompanied with chemical action on a very minute scale, 

 and that all luminescence is therefore jointly physical and' 

 chemical in character. We have thus colour evoked by the 

 direct action of light, which disturbs the atomic equilibrium that 

 existed before exposure, and the manifestation of such colour 

 continues only until the cessation of the chemical action thus 

 brought into play. 



The influence of very low temperature upon luminescence and! 

 photographic action has been studied by Dewar ( Chemical Newsy 

 Ixx. p. 252, 1894). The effect of light upon a photographic 

 plate at the temperature of liquid air- 180° C. is reduced to only 

 a fifth of what it is at ordinary temperature ; and at - 200° the 

 reduction is still greater, while all other kinds of chemical action 

 cease. In like manner, at - 80° calcium sulphide ceases to be 

 luminescent ; but, if illuminated at this low temperature and 

 then warmed, it gives out light. At the temperature of liquid 

 air many substances manifest luminescence which ordinarily 

 seem almost incapable of it ; such are gelatine, ivory, and evea 

 pure water. A crystal of ammonium platinocyanide, on the 

 other hand, when immersed in liquid air and illuminated by the 

 electric light, shines faintly when this is withdrawn. If now the 

 liquid air be poured off so that the crystal rises rapidly in. 

 temperature, it glows brightly. 



Luminescence a.nd Photograi'hy. 



Photography, like luminescence, is a manifestation of the 

 transformation of energy, most frequently of initial short wave- 

 length. The production of colour by photography is nothing 

 new. It was noticed by Seebeck nearly a century ago that 

 silver chloride becomes tinted by exposure to ordinary light, 

 with accompanying chemical change ; that if then subjected a 

 long time to red light it assumes a dull red hue, or a duU 



