Part 2 (2/2)
TRIBOLUMINESCENCE AND PIEZOLUMINESCENCE.--Under this head are grouped a number of light phenomena which at first sight may appear to be electrical in nature but in reality are not. The light is produced by shaking, rubbing, or crus.h.i.+ng crystals, and only crystalline bodies appear to show _triboluminescence_ or _piezoluminescence_. A striking case is that of uranium nitrate. Gentle agitation of the crystals is sufficient to give off sparks of light which much resemble the scintillations of dinoflagellates when sea-water containing these animals is agitated. If Romberg's phosphorus, which is fused CaCl_{2}, is rubbed on the sleeve, it glows with a greenish light. Lumps of cane sugar rubbed together will glow. Saccharin crystals will also light if shaken and Pope (1899) found that the bluish light of saccharin was bright enough to be visible in a room in daytime. It only appeared from impure crystals and freshly crystallized specimens. Other crystals, also, have been found to lose their power of lighting after a time.
Among biological substances, cane sugar, milk sugar, mannite, hippuric acid, asparagin, _r_-tartaric acid, _l_-malic acid, vanillin, cocaine, atropin, benzoic acid, and many others show triboluminescence. A long list is given by Tschugaeff (1901), by Trautz (1905), and by Gernez (1905). The spectrum is a short continuous one, the waves emitted depending on the kind of crystal. Thus the color of the light varies among different santonin derivatives from yellow to green. In saccharin it is blue.
Although the light produced by some living organisms resembles triboluminescence in that it may be evoked by rubbing or shaking the animals, it is in reality fundamentally different since it is dependent on the presence of oxygen whereas triboluminescence is not.
CRYSTALLOLUMINESCENCE.--Crystalloluminescence is observed when solutions crystallize. It was described by Bandrowski (1894, 1895) in a.r.s.enious oxide, in NaF, or if HCl or alcohol is added to hot saturated NaCl solution. A bluish light with sparkling points appeared. All well authenticated cases are exhibited by simple inorganic salts and these are also all triboluminescent. The reverse is not true, however; many triboluminescent substances are not crystalloluminescent.
Crystalloluminescence is much less widespread than triboluminescence.
Trautz (1905) has studied the matter in a number of compounds and comes to the conclusion that the light is really a special case of triboluminescence in which the growth of individual crystals causes them to rub together. The light becomes much brighter on stirring a ma.s.s of crystals which exhibit crystalloluminescence. While in some cases crystalloluminescence is unquestionably due to the triboluminescence of crystals rubbing against each other it is not in every case, as has been clearly shown by the work of Weiser (1918 _b_). He studied luminescence of saturated aqueous alkali halide solutions (NaCl, KCl, etc.,) upon addition of alcohol or of HCl. The salt crystallizes out under these conditions and Weiser found that the light is brightest when the conditions of concentration of alcohol or of HCl are such as to cause heaping up of Na and Cl ions. He believes that the bluish light which appears is due to the combination of ions in the reaction, Na^+ + Cl^- = NaCl. Only if this proceeds rapidly enough does luminescence occur.
Weiser studied also the crystalloluminescence and triboluminescence of AsCl_{3} and of K_{2}SO_{4}. By photographing the luminescence through color screens of different absorptive power (Weiser, 1918, _a_) a spectrum of the light could be obtained, and it was found to be identical in both the tribo- and crystalloluminescent light; in the case of AsCl_{3}, a band in the green-blue, blue and violet. Weiser believes the light in this case also to come from recombination of the ions, As^{+++} + 3Cl^- = AsCl_{3}, and that crystalloluminescence in general is due to rapid reformation of molecules from ions broken up by electrolytic dissociation while triboluminescence is due to rapid reformation of molecules from ions broken up by violent disruption of the crystal. Of course in triboluminescent organic crystals which do not dissociate into ions, some other reaction must be responsible for the light. One thing seems certain, that the two types of luminescence are similar. As Bigelow[1] remarks, ”It is altogether probable that the cause of this” (crystalloluminescence) ”whatever it may be, is the same as the cause of triboluminescence, whatever that may be.”
[1] Theoretical and Physical Chemistry, 1912, p. 516.
Crystals are not found in the luminous organs of animals with the exception of the fireflies. In these a layer of cells occurs (see Chapter IV) filled with minute crystals of one of the purine bodies (xanthin or uric acid). One might surmise that the light of the animal was a crystalloluminescence accompanying the formation of these crystals. It is easy to show, however, that the light comes not from the crystal layer but from another layer of cells containing large granules.
It is also dependent on the presence of oxygen while crystalloluminescence takes place in the absence of oxygen. The crystal layer possibly serves as a reflector. Its significance will be discussed in a later chapter.
[Ill.u.s.tration: FIG. 5.--Dubois's figures showing transformation of photogenic granules to crystals (_after Dubois_).]
The light of luminous organisms is quite generally a.s.sociated with granules. In one of the centipedes (_Orya barbarica_), which produces a luminous secretion, Dubois (1893) has described the transformation of these granules into crystals and at one time he supposed the light to be a crystalloluminescence. He later reversed this opinion and, certainly, examination of his drawings which are reproduced in Fig. 5 does not convince one of the actuality of crystal formation.
The phenomenon of _lyoluminescence_, described by Wiedemann and Schmidt (1895) as a light accompanying the solution of colored (from exposure to cathode rays) crystals of Li, Na, or K chlorides, is probably due to a triboluminescence from stirring of the crystals during solution.
CHEMILUMINESCENCE.--As the name implies, chemiluminescence is the production of light during a chemical reaction at low temperatures. This does not mean that the other types of luminescence are not connected with chemical reactions--using the word _reaction_ in a broad sense--for we have reason to believe that in some cases spectra are not characteristic of the element as such but are rather characteristic of a particular reaction in which the element takes part (dissociation into ions, changes from monovalent to bivalent condition, etc.) and that this is the reason one element may show various spectra under different conditions (Bancroft, 1913). The chemiluminescences are rather oxidation reactions involving the absorption of gaseous or dissolved oxygen and may be very easily distinguished from all the previously mentioned luminescences by this criterion. They should, perhaps, more properly be called _oxyluminescences_.
The glow of phosphorus is the best known case, recognized since phosphorus was first prepared by Brandt in 1669. It is interesting to note that when first prepared phosphorus was regarded as a peculiarly persistent type of phosphor, _i.e._, a material akin to the impure alkaline earth sulphides.
Fresh cut surfaces of Na and K metal will glow in the dark for some time, especially if warmed to 60-70 (Linnemann, 1858). A film of oxide is formed over the surface, showing definitely that oxidation has occurred. Ozone oxidizes organic matter with an accompanying glow (Fahrig, 1890; Otto, 1896). The light from ozone acting on pyrogallol solution is especially bright under certain conditions.
Radziszewski (1877, 1880) gives a long list of substances, chiefly essential oils, which luminesce if slowly oxidized in alcoholic solutions of alkalis. Formaldehyde, dioxymethylen, paraldehyde, metaldehyde, acrolen, disacryl, aldehydeammonia, acrylammonia, hydrobenzamid, lophin, hydroanisamid, anisidin, hydroc.u.minamid, hydrocinamid, besides waxes, and such biological substances as glucose, lecithin, cholesterin, cholic, taurocholic, and glycocholic acids, and cerebrin, all luminesce on oxidation. Radziszewski himself and many other authors have compared the light of organisms to this type of luminescence. Indeed the incorrect identification of granules found in the cells of practically all luminous tissues as oil droplets, is largely due to the influence of Radziszewski's work. Dubois (1901 _b_) has added esculin, and Trautz (1904-5) many aldehydes and phenol derivatives, including vanillin, papaverin, tannic and gallic acids, besides glycerol and mannite to the list of biological substances oxidizing with light production. Guinchant (1905) has described oxyluminescence of uric acid and asparagine, Weitlaner (1911) of substances in humus and McDermott (1913) of substances in urine and the anaerobic alkaline hydrolysis products of glue and Witte's peptone.
Pyrogallol is especially p.r.o.ne to luminesce, as was first noticed by Lenard and Wolf (1888) in developing a photographic plate with pyrogallol developer. Later the luminescence was studied in some detail by Trautz and Schloringin (1904-5) who developed the well-known luminescent mixture of pyrogallol, formaldehyde, K_{2}CO_{3} and H_{2}O_{2}. As I have shown, pyrogallol can be oxidized in a great many different ways, and some of these are of great interest, for they very closely imitate the mechanism for the production of light in organisms.
These are recorded in Table 3, which also includes various other types of oxyluminescence of general or biological interest.
TABLE 3
_Types of Oxyluminescent Reactions_
1. Oxidation in air spontaneously.
(_a_) At ordinary temperatures. [Phosphorus. Fresh-cut surfaces of Na or K. Thiophosgene and Thio-ethers (RCS.OR).]
(_b_) At melting or vaporizing points. (Fats, terpenes, sugars, resins, gums, ether, silk and others.)
2. Oxidation in aqueous or alcoholic alkalies. (Many organic substances.)
3. Oxidation in hypoiodites, hypobromites, or hypochlorites. (Many organic substances.)
4. Oxidation in peroxides (H_{2}O_{2} or Na_{2}O_{2}). (Many organic substances.)
5. Oxidation in ozone. (Many organic substances.)
<script>