I,
u11I s ESCE s cE ___
BY E .
F. F . l R N A U
Introduetory The term luminescence is generally applied to that property possessed by a large number of substances of becoming luminous under the influence of light or other forms of radiation. \Then produced by light, the phenomenon is accompanied by absorption of the incident light of certain \\-avelengths, the luminescence consisting of light of \ r a ~ e - l e n g t h s difiering from those of the exciting light. T7:irious classifications ha\-e been made depending upon the conditions under Irhich the phe~iomenatake place. If the luminescence occurs oiily so long as the esciting light falls upon the substance, it is called fluorescence: if it persists after excitation is discontinued it is called phosplioresceiice. Often after the phosphorescence has decayed a t ordinary temperatures iintil it has become inr-isible it caii tie reyi\-ed for a time by heating the substance -this is knon.ii a s t h e r m o l ~ i r n i ~ i e s c e i i ~ Pro~~, duction of 1i;;ht by iracture o f crystals is called tribclluiiiii ceiice. by cathode ra)--, catl~otlo!iilnii~eSce~ice~ by solitliiication of a melt or precipitatioii from so!ritioii c r y s t ~ l l o I u r i ? i i i e s ~ e i ~ ~ c ~ ~ . In man)- of these cases the liimitiesccnce occiirs !!-it11 ahnost iniperceptible rise o f tenipera;tircl ; and ioi- this re;is;oii has beeii termed cold Iiyht." llariy chemical re;;ctioiib OccriI-. often ;it Ion. tempera tures. v,-it'n e\-oiutioii o f Ii;:lit, 111i5 is called clieniilririiiiiescence. 'There arc more or its. permanent chaiizes in the chemical properties of rubht:uic:es m:tde Iiimiiiesceiit -some shon-in? brit transitory changes, ij-liile in others \-isible chemical decomposition o c c i i n . I t is the purpose oi this research to determine the course of the cheniical reaction taking place and to correlate the cliii'erent kinds of lumiriesceiice under one substantially cotifirtnecl liypotliesis. Luminescence by Cathode Rays \T~iedemann anti Schmidt' were the first to make a "
'
I\-ied. .4nn., 54,
622
i i i g g ) : 56,
Zo?
( ~ b g j ) :64,
78 ( I S + ) .
systematic study of this kind of luminescence. They observed t h a t in many cases a t least, decomposition occurred, and decided that the luminescence was due to recombination of these decomposition products to form the original salts. Furthermore, it was claimed t h a t the color of the emitted light depended solely upon the metal, citing the case of cadmium and uranium salts. Goldstein' observed t h a t in the action of cathode rays upon alkaline halides the color change produced by the ray.; \vas hut temporary, heat or moist air restoring the salts to their original condition. The color change was accounted merely ;i physical change. TT7ilkinso:i' substantiated the hypothesis o f TYietlernaiin arid Schmidt hy comparisoti of the light emitted during the reaction of the probahle decompositiori products of the salt.; 17.-it h the c a t lio:c. o 1uniin e scence of t lie sa Its them se 1ve s 'I'11 is paper i~l-il!l ~ eilisc-tisscd under thc section 1,uminescence by Clieniicui iicai. tioii. ( iiiy to the iriiccrtniiiiy iii tlcpeiideiice t i p o i i aiiotliei-'h descripiioii ol cloi-ii\ \I eil as ;o posiihie variatioiis in cltia!ity of the luitiintsceuce ~11:e t o impiritics in the salts enipioyeci, TYilkiiiwii'L experir:ic:nts ere repe:itec\. -4 fcrx; aqreetneiit ivere ~dxei-\-ei!.a b is e\-ident from Table I. l ~ u t t h e w :ii?cratioiis iirereiy coliiiriri tlie h!-pothc.ii~;. ap]?i~ratu!-c.i?iploycd i o i - prmluction of cathotloeiwe coiisistetl of a cathoc!e tlibe se\-en inches lol~q. oiic aiic! oiie-lialf iiic.lies iii diameter, in tn-o parts a i upper carrying t h e disc catiiode aiid riiig anode, a n t 1 a lo\ver con.< taiiiiiiy the salt, 1 lie vacuum vas pro:iucctl Iiy a. Cieryk double cyiiiider oil piiinp niakiiiq :ihout thirty strokes per minute, run by :I H . 1'. induction motor hitched up to the wheel of the pump through a n iiitermediate reducing pulley. large induction coil. ivorkiiig a t a nieaii current of 5 amperes and a mean voltage of Go L-olt.; in the primary, and \;-hose secondary furnished under these conditions a four-inch spark. ,
"
"
) \ 3 ~
\ri
,j
A \
Ivied. .Inn..54, ,371 (1Xg4). Tour. Phys. Chcm.. 13, 691 ( 1 9 0 9 ) .
\\-asused to operate the cathode tube. continuously during excitation.
The pump w;ts lrorked
~
Salt
I~luoreseenee
n.
SaCl
whitish blue-white white Sa1 white KCl green KBr green KI Hg,Cl, orange Hg,Br? orange greenish HgC1, HgBr, orange __ CdCl, yellow-white CdBr, white CdI CdSO, yellow SaBr
I
Fluorescence
F
bluish white bluish greenish bluish white blue green orange orange greenish greenish green-blue green-blue ye11ow yellow
Color of residue
n-
,
Colur of residue
F
brown
b r o ivn
rose
brown brown dark L-iolet blue brown black brown brown brown brown brown b r on-n brown
brown dark violet blue bron n black brown brown i brown -
11r o wn
If cathodoluminescence is caused by a chemical reaction, rise of temperature should increase the luminescence for the same amount of decomposition because the reaction velocity would increase. The follon-ing experiments n-ere carried o u t to illustrate this point: -1sample of cadmium sulphate from Eimer 8r Xmend, showing b u t slight luminescence,' n-as heated to dull redness in p1a.tinum to remm-e moisture, and, after poi\-dering, \\-as heated in the cathode tube. The results \\-ere unsatisfactory, the vacuum dropping oft' markedly and hence reducing the amount of effecti\-e radiation. This is doubtless due either to a slow e\-olution of adsorbed gases and moisture a t the higher temperature, or to increased permeability of the glass to air. To obviate this difficulty, cooling the salt instead of heating v a s employed. ;111 of the tube sax-e about one-half a n inch of the bottom part ivas \napped loosely i v i t h cotton gauze and jacketed. The slightly luminescent -.
~
!
Ii'ilkins~in: Jour. Phys Chem., 13, 7 2 0 (1909).
pure salt [vas replaced by a conmiercial sample, fairly strongly luminescent. 1,iquid air was dropped upon the gauze until the luminescence reached a maximum indicating attainment of the best possible vacuum. The bottom of the tube was then immersed in liquid air in order to cool the salt; the fluorescence almost entirely disappeared. Loryering the temperature of the salt had decreased its luminescence by decreasing the rate of combination of its decomposition products. in confirmation of the theory. Discoloration of the salts in all the cases obserx-ed causes marked falliiig ofi of the fluorescence. I t \vas especially ohser\-ed Ivith cadmium iodide. This salt is discolored rapidiy utlder cathode rays, probably with separatiori of cadmium ant1 iodine, the emitted light a t the same time becoming extremely faint. If the tube is shaken, e s p o s i q fresh surfaces of the crystals, the fluorescence flashes up mohentarily, and then diminishes rapidly. The same difierence in light intensity is ohsen-ed [vith pme 11-liite crystals of cadmium bromide anti with t h e same salt .;lightly discolored by fusion in platinum. The efiect is cloubt!ess 1ar:;ely due lmth to decrease in the reflecting poi.ver oi the salt and i o absorption o f the cathode rays i ivhich actually possess but slight penetratiiig p v e r 1 by the discoloring sulxtance instead of by the salt i t d l ' . This latter esplanation is h r u e o u t in experiments ivith the iron arc in iq:hicli thi:i rapid decrease of liimiiiescence ivitli increasiny discoloration of the salt is not o b x r v e d , the ultraviolet light pelietratilig more deeply than do the cathode rays. , liie efiect of a.ddition of traces of other salts o n the luminescence of c d n i i t i m sulphate has been investigated. li7ilkinson' had found that smnl! arnounts of sodium, potassium, lithium, or zinc sulphate caused the excited salt to show luminescence. Some of the results of 11-aggoner' are at variance \vith these conclusions. Table I1 is from the latter paper. It 17;ill be noted t h a t in many cases metathetical reactions n-ith or ivithout the formation of a pre*?
Jour, Phys. Chem.. 13, ;19 ( I C ~ C , ) Phys. Rev.. 31, jj8 (1910).
cipitate n-ill occur, and the resulting substance n-ill not be homogeneous in these cases. IYaggoner is not specific in describing his method of preparation of these specimens and it \vould be difficult to decide from his results as to just n-hat chemical compound or solid solution the luminesceiice is clue. The experiments could be checked by comparinq the luminescence of each of the products of metathesis with that of the mixture. T i r j L E 11 -
-
I
I l a salt
added
SO, S,O,. OH, CO CrO,, CrlON O , . C10,
Phiisphorescence
S a salt added
Phosphorescence
yelloir
Si(),$,Br
blue
none
c1
green
,
faint j.elloir-
HPO,,R , 0 7 S a aluni
faint green
greeni.,h yellow
Luminescence by Canal Rays Under the influence of canal rays a number of salts luminesce, and the colclr of the light is in some cases markedly different from that of cathodolurniiiescence. Table I11 including the results of A4rriold’and of Schmidt’ is gi\-en helon-. On the basis of the present hypothesis, it must follon- that canal rays either cause a different kind of deconiposition o f the salt or a t least admit of a different kind of recombination of its dissociation products I t is interesting in this connection to refer to an observation of Schmidt, that carefully purified sodium chloride 011 exposure to canal rays luminesces a t first weakly bluish, but that the red-bronm or red-yellon- fluorescence fo1lon.s. This 11-ould indicate that the luminescence, and hence the chemical reaction characteristic of cathodo-excitation occiirs momentarily, h u t is succeeded by the usual anodic phenomena. -1rnold furthermore obserx-es that under the influence of canal rays sodium chloride, nitrate, sulphate, bromide, and
’ \Tied.
. I n n . . 6 1 , ,326 (IS,];). Drude’s A n n . . 9, ;oj ( 1 5 1 0 2 ) .
E. F. F a m a u
642
iodide all show the D line; and J . J. Thornson’ writes “ I f a layer of lithium chloride is placed on the plate, then when struck by Cn?ialst~aIalenit shines with a bright red light and the red lines of the lithium spectrum are very bright; if the direction of the discharge is reversed so that the lithium chloride is struck by the cathode rays, its color changes from bright red to steely blue, giving out a faint continuous spectrum b u t not the lithium lines.” =\gain, In some cases the Cnnalstiahlen excite the metallic lines more strongly in compounds of the metal than in the metal itself, thus if we bombard the surface of the liquid alloy of sodium and potassium m-ith CamlstiaIalen the specks of oxide floating on the surface shine out with a bright yellow light and show the D lines of sodium strongly; the clean parts of the surface on the contrary are hardly luminous a t all, and I have never been able to see the D lines on this part of the surface. The difference may be due in part to the sodium being much more \Tolatile than the oxide, so t h a t an atom of sodium struck by the Canalstralzlei~ may volatilize and get away from the surface, while a molecule of oxide would be fixed, thus the light might be much more concentrated when the surface struck is not volatile than when it is easily vaporized.” TABLE111 ”
Luminescence with Canal Rays
___
Substance
-~
Glass Sac1
reddish yellow reddish yellow
SaBr S a1 Sa2S0, Li salts R salt containing S a
reddish reddish reddish reddish red yellow
Sr salts Cd salts
rose-white yellow
XaSO I
~_~_______~
Fluorescence .%mold
yellow yellow yellow yellow
~~
Fluorescence Schmidt
-
firit bluish, then dish yellow reddish yellow reddish yellow reddiih yellow reddish yellom
red-
-
first weak bluish then reddish yellon-
yellow
“Conduction of Electricity through Gascs,” and Edition. h q ( 1 ~ 0 6 ) .
Lzmitzesceiise
6-43
Luminescence by Ultraviolet Light S o systematic study of luminescence of salts under ultrayiolet light has been attempted, previous workers restricting themselves chiefly to the study of minerals and phosphorescent sulphides. For this reason, experiments were made employing the :same salts as included in 1Vilkinson's list for comparison of the quality of the emitted light in the ttvo instances. -1s a source of ultraviolet light an iron arc \!-as constructed of tn-o machine steel terminals, each I ' , inches by inch by ' , inch fastened by means of machine scren-s arid nuts to a 6 inch hy 4 inch by , inch sheet of asbestos tile set 1-ertically on a board. The length of arc could be varied by turning the terminals. -In oil transformer connected through a suitable resistance n-ith a I I O ;I. C.\-olt 60 cycle supply and having six large Leyden jars of 1100 square inches total tin foil surface and ' , inch thickness of glass in parallel with the secondary, furnished an arc of about ' , inch length under these conditions. The salt tvas placet1 in a sniall crucible about one-half an inch belon- the arc. On account of the intense light of the arc, fluorescence could not be ohseri-ed. hut strong phosphorescence resulted i n all the salts n-hich according to \i'ilkinson exhibited this property iinder cathode excitation. Table 11- gives a summary of the restilts. i he coloration of the residue after about fifteen minutes' exposure to the light of the arc \]-as remarkable in tlit. case o f the potassium halides, potassium chloride after excitation sho\\-iiigby daylight a dark amethyst color, potassium bromide a blue as pronounced a.s that of ammonium copper sulphate, and potassium iodide resexhling nickel salts. The colors are doubtless due to free potassium modified by the respective: free halogen. The colors faded in about a minute leaving the salts pure white. 113th the moist halides no phosphorescence n-as obserx-ed; it n-as hoped that under this condition, the potassium and chlorine as decomposition products ~vould react in the presence of Ivater to yield hydrogen arid potas* 1
E. F. F a m a u
644
sium chlorate, b u t the amount formed-if any--was so slight that it could not be detected even by examination of the residue for anisotropic crystals of the chlorate under the microscope with crossed Nicols. T ~ E L 11E Luminescence n ith Ultraviolet Light __
Salt
Phosphorescence
X aC1
XaBr Sa1 KC1 KBr KI
Hg’ Yalt Hg” salt CdC1, CdBr, CdI, CdSij,
bluish white bluish white bluish white violet-blue intense blue green none
~ _ _ _ _ Color o i rrsidue
-
Thermoluminescence
hr o wn brown brown dark violet
bluish white bluish white
blue
blue green none
green
bluish white bluish white
none
black black
none
blue-green blue-green
hrown l jr o wii
blue-green blue-green
yellow >-ellow
Iirown
brown
\
ellon-
t elion.
Luminescence by Heating after Excitation If the salt be exposed to cathode rays or canal rays or to ultraviolet light. diid the excitation be discontinued. phosphorescence occurs a t first bright, then gradually decreasinq until the light is no lonqer visible In most cases, if the substance be non- heated, intense luminescence occurs for a time, and then the salt can be made luminescent only by another excitation. II-here the decomposition products of the salt are colored, thermoluminescence is accompanied by partial or complete restoration of the original color of the salt Thermoluminescence is most easily accounted for on the grounds of increased rate of diffusion and of increased rate of reaction of the decomposition products of the salt, brought about by raising the temperature. It is important to note that the quality of light emitted in all cases of thermoluminescence is the same as that of cathodoluminescence or of luminescence produced by ultraviolet light. Even the residue of sodium chloride excited by
Lzirninesieizce
645
canal rays emits bluish light' when heated. The important bearing of this on the theory will be stated in the section on " Luminescence by Chemical Reaction." The data on thermoluminescence are given in Table 11..
Luminescence by Trituration Crushing or grinding many crystalline substances will produce luminescence The light is in most cases extremely faint, and the following procedure \vas employed t o increase the e f e c t \]'here possible, the salts n-ere fused in a platinum dish and cast in a graphite mould in sticks of one inch diameter and t n o inches length These n-ere pressed against a rapidly revolving carborundurn wheel, and the luminescence \vas observed just beyond the point of contact of the salt and the wheel. The phenomenon of triboluminescence is probably due t o an actual separation of the constituents of the salt on breaking apart the crystals, and the reunion of these fragments of crystals permits recombination of the adherent decomposition products with production of light Table T' contains the observations made, not a great deal of reliance can be placed on them, owing to the extreme faintness of the light _ ~ _ _ _ _ _ _ _-~ _ ~
T I H L E1-
Salt ___
__--
~
-__
NaCl XaBr Sa1 KC1 KBr KI CdSO,
-
__
Triboluminescencc __ -
bluish white bluish white bluish white bluish white bluish greenish white yellowwhite
Luminescence by Cheniical Reaction (Chemiluminescence) Pringsheirn' substantiated t h e view t h a t Schmidt. Drude's AAnn.,0, 708 *IVicd. Ann., 45, 429 (1892).
(1902).
110
gas emits
646
E . F . Farizau
light save as a result of chemical action, and Armstrong' has ascribed luminosity and line spectra to the changes attending the formation of molecules from their atoms, or speaking generally, t h a t they are consequences of chemical changes. Wiedemann and Schmidt' concluded t h a t cathodoluminescence was due t o the reaction of the products of decomposition of the salts, and the experiments of Wilkinson3 confirm this view. The light emitted during chemical change varies in intensity with the rate of reaction, and in some cases under different conditions may suffer complete change in color. Miss Stevenson4 found t h a t quinine sulphate on dehydration by concentrated sulphuric acid emitted a strong bluish light, which decreased in intensity on employing more and more dilute sulphuric acid as dehydrating agent. If the salt be dried over concentrated sulphuric acid, the light emitted is so faint that a photographic plate is not affected. But in all cases irhere luminescence could be observed there was no change in the quality of the light. Sichols and Nerritt' observed that ivith varying Travelength of exciting light there v a s no shift in u-ave-length of the maxima of fluorescence bands. Trautz'] has investigated the luniinescence of a number of chemical reactions and has formulated the lan-s. The intensity of luminescence is about proportional to the velocity of the reaction, increasing enormously 11 ith the temperature; the color is dependent only upon the reacting system, being independent of the velocity of reaction or of the temperature. On the other hand, the results of Wilkinson' n-ould seem to indicate t h a t a \-ariation of color of the chemiluminesce~ice of some reactions is possible. These apparent anomalies Proc. Roy. Soc., 70, 99 (1902). TTied. *knn., 54, 6 2 2 ( 1 8 9 j ) ; 56, 2 0 3 (1895); 64, 7 8 (1898). 3 Jour. Phys. Chem., 13, 691 (1909). Ibid., 15, S j 4 (1911). j Phys. Rev., 19, 18 (1904). 6 Zeit. phys. Chem., 53, I ( I g O j ) ; Zeit. Elektrochem., 14, 4 j 3 (1908) Loc. cit.
I\ ill each receive its explanation as the indi\-idual ca5es are cited in the follon-ing sub-sections : C'hi~nzzccil Conzb2iiafi011-In those cases ivhere chemical reaction occurs Iritli considerable velocity a t ordinary tempera ture, the luminescence attending the chemical change can be ohsen-ed n-ithout much difficulty. Thus, lT3lkinson caused some reaction:; to occur slo~vlyand then rapidly and obserl-ed in some cases marked differences in the color of the emitted light. His results are tabulated in Table \-I.
Reactiori ! precipitant
u
I drniu -~
\-aC1 SaBr Nal KCl KBr
hluizh __ -
green (neak)
KI
-
bluihh \I hite hlui5h TT hite bluish white bluizh white bluish (weak) greenish iweak)
bluish green
1
,
bluih
TI -
-
-
-
1
1
hit€
;reen
bluish white
-
-
--
-_
ELtcti u l j s i s --Schluederberg’ noted that on passage of a n alternating current through dilute sulphuric acid, using lead electrodes. a greenish-white luminescence occurs a t the electrodes, and ’\ll‘ilkinson? carried out a number of experiments using various electrodes and solutions. The results are given inlTable X. TABLEX Luminescence with Electrolj sii _______
-
Electrodes and Iuns
I
-
I
I
-
+
--
Jour. Phys Chem , cit
* Loc
~-
Luminescence
intense yellow yellow
+ +
1
~~
-
Cd Br’ Cd - I’ Cd - SO,” Cd C1’ Zn SO,’’ P b - C1’ Pb - Br’ Pb - I’ P b - SO,’’ Hg - C1’ H g - Br’ Hg I’ Hg SO,”
___
~
12, 623
(1908)
-
greenish white faint bluish white yellow greenish white yellow orange brilliant orange orange greenish 11 hite
The discrepancy in the colors of the luminescence by electrolysis or cathodo-excitation on the one hand, and of chemical reaction of lead oxide and sulphur trioxide on the other, is accounted for by assuming the reaction of neutral lead into lead as ion, or of lead as ion to undissociated lead sulphate for the former. and the reaction lead oxide plus sulphur trioxide yielding lead sulphate for the latter ; according to the ti1-o formulations P b + P b + + ;P b + + + PbSO,; PbO
+ SO,,+ PbSO,.
The Color of' t h e Residue after Excitation The color of the exposed salts is in all cases cited easily explained in terms o f their decomposition products. H u t some observations of Tafel' suggest other factors. Zinc oxide after long exposure to canal rays darkens in color ivithout appreciable loss in weight. This 11-as ascribed to the lxxiibardment b y the c! particles, and this s-ien- n-as substantiated by submitting zinc oxide in a scren--press to a pressure of about j 0 , o O o atmospheres, and by grinding the oxide in a mortar n-hereby extremely high local pressures are obtained; in each of 1:hese cases. darkening of the zinc oxide occurred. A pressur12 of 500 atmospheres in a hydraulic press caused no color change. Furthermore, n-hile the colorless oxide shoIred strong p e e n anodoluminescence, none of the discolored s-arieties 11-ere actis-e, nor was the oxide 11-hen prepared by precipitation c)f zinc sulphate with :;odium carbonate, washing, and ignition a t the temperature of the Bunsen burner. Tafel Ivrites: "One must distinguish a t least three forms of zinc oxide, cine yellon--1x-on-nand tn-o white. The colored and one of the white forms have lost the ability to fluoresce under action of canal rays." a classification this is admirable; b u t UIIiortunately no explanation is offered, first, for the color change of the compressed oxide, unless tossing i t into the scrap-heap of ill-defined allotropic modifications constitutes -
.
~
~
_
_
_
Drude's .inn.,
_
_
11,
613 :1903).
E . F. Farnau an explanation; and, second, for the loss of ability to fluoresce under anodo-excitation It is true t h a t in the latter instance, Schmidt’s’ explanation t h a t zinc oxide shows anodoluminescence only when impurities are present is rejected by Tafel on the grounds t h a t the zinc oxide prepared by precipitation with sodium carbonate, n-hich surely contains adsorbed sodium salt, did not shon- fluorescence. This would seem to be a step backward, for i t is n-ell kno\rn t h a t n-hile traces of some impurities increase the luminescence of many salts, larger amounts either produce no further effect or actually cause a decrease until the luminescence has quite disappeared Doubtless the preparation of zinc oxide by precipitation \rith sodium carbonate beiongs to this latter class in t h a t it contains too much impurity, and had the zinc been precipitated lvith ammonium carbonate instead, a much purer product n-ould have been obtained lchich irould have shon-n anodoluminescence -A very simple and direct explanation of the color change of the oxide under pressure can be given hmorphous zinc oxide, I . c . , the supercooled liquid, when massive may be assumed t o be yellonish, b u t in finely divided condition, as obtained. for example, by combustion of zinc or ignition of one of the salts. is colorless The same thing is observed on powdering copper 5~ilphateor potassium dichromate or discolored rock-salt, each becoming lighter in color as the degree of fineness increases. until finally the substances are practically pure n-hite. The explanation in terms of optics is ob\-ious On the other hand, grinding may produce quite the opposite result; e . g , if on the one hand arsenic or antimony be rubbed in a mortar, the crystals are broken up and a state of greater subdivision obtains, b u t if lead or gold powder be similarly treated, the particles do not become finer, b u t are merely spread out, I e , burnished, and where they overlap Drude’s Ann.. 9, T O ; ( r y o z ) .
are n elded together, giving in each case the characteristic metallic luster. I n like manner, zinc oxide will show this effect of burnishing, save that here the substance, being transparent, will give the color of its transmitted light. The phenomenon is not restricted to zinc oxide but is shown equally well b y many amorphous substances, bismuth oxide, stannic oxide, zinc sulphide JVaentig' observed it in the case of phosphorescent alkaline-earth sulphides The decrease in fluorescence of discolored substances is of common obser\-ation It is probably due to absorption of the active radiation by the discoloring impurity. If in the case of zinc oxide tke discoloration under anodic excitation is solely due to a mere pressure efiect, and no decomposition occurs. it could be accounted for by decrease of the total surface of the substance, since the effect of anodic and cathodic excitation is only superficial I n order to imitate in the case of zinc oxide the crushing effect of grinding, the brittleness of the oxide [vas increased b y pouring liquid air upon it in a porcelain mortar If i t be ground under the liquid air no discoloration is observed. On the other hand some. discolored by grinding at ordinary temperature, \vas ground under liquid air, it became colorless These obserx-ations shed liqht 011 another set of phenomena, the chanqe of color of oxides when hot and cold Zinc oxide XT hen heated in a matrass in the Bunsen flame becomes yellon, b u t ret.1x-m to it5 original color \\ hen cold Bibmuth oxide and stannic oxide, similarly treated, become darker when hot and lighter in color when brought back to room temperature, b u t do not become white again Cooling of the heated oxides causes disintegration of the coalesced particles, and in their original state of fine subdivision the masses show more or less their original color. This disintegration is not complete in the case of bismuth and stannic oxides __ Zeit phys. Chcm
. 44, 499 (1903).
An experiment was made of heating the surface of zinc oxide contained in a crucible with the point of an oxy-hydrogen flame. -1good deal of volatilization of the material occurred, b u t on cooling, whereas the mass of material regained its original color, t h a t which had sintered together by immediate contact with the flame remained yellow on cooling. It was still yellon after several months. The analogy is evident between the results of this experiment and the making of a poor and a good joint in glass-blowing. ,According t o V'aentig, the activity of phosphorescent sulfides is increased by rapid cooling after ignition, and is decreased by grinding the substances in a mortar. -411 this is in line x i t h the present explanation. These results may seem a bit irrelevant to the matter a t hand. The experiments ivere instituted merely to account for discoloration efiects of pressure, and are not intended t o correlate such color chanqes x i t h those produced by anodoexcitation. Color-Photography of Luminescence Manifestly, apectrographic in\-estigation of the \-arious sorts of luminescelice is the ultimate criterion of the correctness of the hypothesis ascribing a common origin to luminescence, but lack of time and of the necessary apparatus prohibited this method. -1s 3 substitute for this more accurate method. color-photography TI as essayed. Dufay color-plate5 n-ere employed on account of their reputed speed, ease of manipulation. and correct color rendition. The apparatus for production of cathodoluminescence \vas that already described The plate \vas screened from X-rays by coveringwith lead foil all of the cathode tube save a small window just above the substance under excitation, the visible light being reflected by a mirror and focused by a lens onto the plate nhich n-as placed in a position not exposed to X-rays. In some preliminary experiments, successful results were obtained with the highly luminescent minerals, willemite
iyellon--green
and fluorite fluorescent sulfide. '
deel) blue I. anc! ivi th a red-
Summary Luminescence is due to chemical reactioii. Increase of the rate of reaction by increase of temperature or by addition of 3 catalytic agent iiicreases the litniinescence, 3 . The quality- of the luminescence is b u t slightly altered by change of temperature. 4. The quality c?f luminesceiice is b u t slightly altered by the nature of the catalytic agent. 5. The quality of the luniinescence is generally b u t slightly altered by the anion, being dependent almost wholly upon the nature of the cation; in a few cases a specific effect of the anion is noted. 6. The quality of the luminescence is independent of the method of production, 2'. e . , whether in a Bunsen flame, by electrolysis, precipitation, trituration, cathode rays, canal rays, ultraviolet light, chemical reaction, or as thermoluminescence; although in many cases it can be changed to another entirely different kind of light by particular modes of excitation, e . g., by rapid chemical reaction or by canal rays. 7 . The chemical reaction producing a given luminescence can in some cases be formulated; e . g., sometimes being betn-een molecular substances, in others passage from undissociated substances to ions, or rice ziersn. 8. Color-photogr,zphy offers a means of somewhat more exact determination of the quality of luminescence. 9 . -1s a minor point, the alteration of the color of compressed substances may be due, in some cases a t least, t o mere agglomeration of the substances, and not t o formation of allotropic modifications of them. I,
2.
This portion of the research n a s conducted in collaboration n i t h Llr J. LI Lohr. a n d has been published in a n original communication to the Eighth International Congress of Applied Chemistry, 20, 137. I n a later paper now ready for publication (private communication). ilk. Lohr has subslantiated the usefulness of the method in reproducing all save the faintest cathodolurninesccnce.
This research was suggested by Professor Bancroft and has been carried out under his supervision. I wish to express my sincere appreciation of his kindly criticism and encouragement during the progress of the n-ork. My thanks are also due Professor Merritt for his helpful advice and for placing so liberally a t my disposal the resources of the physical laboratory. Corizcll Cmicrsttj, JlrlJ,
I9IZ
