LECTURE IV. CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. Chemical Effects. 31. Where intense radiation is intercepted by a body chemical action may result by the heat energy into which the radiation is converted. This, however, is not a direct chemical effect of radiation but an indirect effect, resulting from the energy of the radiation. Direct chemical effects of radiation are frequent. It is such an effect on which photography is based : the dissociating action of radiation on silver salts, the chloride in ordinary photographic paper, the bromide and iodide in the negative plate and the quick printing papers. This chemical action is greatest in the violet and ultra-violet and decreases with increasing wave length, hence is less in the green, small in the yellow, and almost absent in the red and ultra-red, so that the short waves, blue, violet and ultra-violet, have sometimes been called " chemical rays." This, however, is a misnomer, just as the term "heat rays" sometimes applied to red and ultra-red rays. In so far as when intercepted they are converted into heat, all rays are heat rays, but neither the ultra-red nor any other radiation is heat, but it may become heat when it ceases to be radiation. Thus all radiations are chemical rays, that is, produce chemical action, if they strike a body which is responsive to them. The chemical action of radiation is specific to its frequency and seems to be some kind of a resonance effect. We may picture to ourselves that the frequency of vibration of a silver atom is that of violet or ultra-violet light, and therefore, when struck by a wave of this frequency, is set in vibration by resonance, just as a tuning fork is set in vibration by a sound wave of the frequency with which it can vibrate, and if the vibration of the silver atom, in response to the frequency of radiation, becomes sufficiently intense, it breaks away from the atom with which it is chemically 64 RADIATION, LIGHT, AND ILLUMINATION. combined in the compound, the silver bromide, etc., and this compound thus splits up, dissociates. The phenomenon, how- ever, must be more complex, as a simple resonance vibration would be especially pronounced at one definite frequency, the frequency of complete resonance, and rapidly decrease for higher and for lower frequencies. The chemical action of radiation on silver compounds, however, does not show such a response to any definite frequency, but, while strongest in the ultra-violet, ex- tends over the entire range from the frequency of green light beyond the ultra-violet and up to the highest frequencies of X-rays. That the chemical activity of radiation is some form of resonance, is, however, made very probable by the relation which exists between the active frequency range and the weight of the atom or molecule which responds to the radiation. Thus, while the fairly heavy silver atom (atomic weight 108) responds to rays near the violet end of the visible spectrum, the much lighter oxygen atom (atomic weight 16) responds only to much higher frequencies, to those of the physiologically most destructive rays, about one to two octaves beyond the visible spectrum. These very short radiations energetically produce ozone 03,from oxygen 02, probably by dissociating oxygen molecules 02,into free atoms, and these free atoms then join existing molecules: 0 + 02 = 03, thus forming ozone. Possibly their destructive physiological action is due to this ability to cause resonance with the oxygen atom and thereby destroy molecular structures. 32. Response to the long waves of red and ultra-red light thus may be expected from atoms or groups of atoms which are very much heavier than the silver atom, and this indeed seems to be the case in the action of radiation on the life of the plants. There the response is not by atoms, but by the much heavier groups of atoms, radicals of carbon compounds, which separate and recom- bine in response to radiations and thus produce in vegetable organisms the metabolism which we call life. The action of radiation on plant life thus seems to be a chemi- cal action, and this would be the most important chemical action, as on it depends the life of the vegetation and thereby also the existence of animal life and, thus, our own. This action by which the vegetation converts the energy of radiation into chemical energy is related to the presence of chlorophyl, a green body which exhibits a red fluorescence. I show you here a solution CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. 65 thereof in alcohol. This use of the energy of radiation occurs only in those parts of the plant in which chlorophyl is present, usually shown by its green color, that is, in the leaves and young stems. In those plants in which the leaves have lost their chloro- phyl in taking up other functions — as the function of protection against attack by conversion into spines in the cacti — the stems and trunks have acquired the function of energy supply from radiation, and show the green color of chlorophyl. When the leaves die in the fall their chlorophyl disappears and they change to yellow or red color. Those parts of the plants which contain chlorophyl, mainly the leaves, take carbon dioxide (C02) from the air through breathing openings (stomata), absorb the radia- tion, and convert its energy into chemical energy, and use this energy in splitting up or dissociating the C02, exhausting the oxygen 02 and using the carbon in producing the complex carbon compounds of their structure: fiber (cellulose), starch, proto- plasm, etc. The energy of plant life thus is derived from radia- tion and their work is constructive or synthetic, that is, they produce complex chemical compounds from simple ones: the carbon dioxide of the air, the nitrates and phosphates of the soil, etc. Inversely, the animal organism is analytic, it converts the chemical energy of complex compounds into mechanical and heat energy by splitting them into simpler compounds, burning them in the lungs or gills. For the supply of mechanical energy which maintains the life, the animal organism thus depends upon the synthetic work of the vegetation by consuming as food the complex compounds constructed by the plants from the energy of radiation, either directly (vegetarians), or indirectly, by eating other animals, which in their turn live on the vegetation. Thus, while the plants take in from the air carbon dioxide C02, exhaust the oxygen 02, and convert the C into complex compounds, the animal takes in oxygen 02, by it burns up the complex carbon compounds derived from the plants, and exhausts C02 as product of combustion, but in its ultimate result, all life on the earth de- pends for its energy on radiation, which is made available in the plants by conversion to chemical energy and used as such by the animals. The radiations which supply the energy of plant life, probably are the long waves of yellow, red and ultra-red light, while the short waves of blue, violet and ultra-violet cannot be used by the 66 RADIATION, LIGHT, AND ILLUMINATION. plant, but are harmful, kill the vegetation. This can easily be understood : to the long waves of red and yellow light the atoms do not respond, but only the much heavier groups of atoms or car- bon radicals, and these thus separate and recombine and thereby constitute what we call life. To very short waves, that is, high frequencies, these heavy groups of atoms cannot respond, but single atoms would respond thereto and thus by their separation break up and destroy the atomic groups. That is, the resonant dissociation produced by low frequency of radiation extends only to the groups of atoms and thereby results in their separation and recombination to heavier molecules : life, while the resonant dis- sociation produced by high frequencies extends to the atom and thereby splits up and destroys the molecules of the living organ- ism, that is, death. Therefore the short waves of radiation, green, blue, etc., which are more or less harmful to plants, are not used but are reflected by the chlorophyl; hence the green color. To some extent violet radiation is absorbed by chloro- phyl, but it is questionable whether the energy of violet light directly contributes to the chemical action, and it is rather probable that the violet radiation is converted into red light by fluorescence — chlorophyl fluoresces red — and used as red light. Excessive violet radiation seems to be harmful. Physical Effects. 33. Some of the most interesting physical effects of radiation are those by which it is converted into another form of radiation : fluorescence and phosphorescence. Many substances have the property of converting some of the radiation which is absorbed by them into radiation of a different wave length, that is, act as frequency converter of radiation, fluorescence. Many bodies when exposed to radiation store some of the energy of radiation in such a manner as to give it out again afterwards and thus, after exposure to light, glow in the darkness with gradually decreasing intensity, phosphorescence. These phenomena probably belong to the least understood effects of radiation. They are very common, but phosphorescence usually lasts such a short time that it can be observed only by special apparatus, although a few bodies continue to phos- phoresce for hours and even days. Fluorescence also is usually CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. 67 so weak as to escape notice, although in a few bodies it is very strong. The change of frequency in fluorescence always seems to be a lowering of the frequency, that is, an increase of wave length, and in phosphorescence also the light given out seems always to be of lower frequency than the light absorbed and indeed, fluores- cence and phosphorescence seem to be essentially the same phenomenon, radiation is absorbed and its energy given out again as radiation of lower frequency and that part of the returned radiation which appears during the absorption we call fluores- cence, that part which appears later, phosphorescence. There is, however, frequently a change of the color of the light between fluorescence and phosphorescence and also between phosphores- cence immediately after exposure to light and some time after- wards. For instance, some calcite (calcium carbonate or lime- stone) fluoresces crimson, but phosphoresces dark red. The phosphorescence of calcium sulphide changes from blue in the beginning to nearly white some time after, etc. Due to the change of frequency to longer waves the longest visible rays, red, orange and yellow, produce no fluorescence or very little thereof, as their fluorescent and phosphorescent radia- tion would usually be beyond the red, in the invisible ultra-red. Blue, violet and ultra-violet light produce the most intense effects, as a lowering in frequency of these radiations brings them well within the visible range. Ultra-violet light is best suited for studying fluorescence as it is not visible, and thus only the fluorescent light is visible; white light, for instance, does not show the same marked effect, since the direct white light is superimposed upon the light of fluorescence. Most brilliant effects, however, are produced by using a source of light which is deficient in the frequencies given by fluorescence and then looking at the fluorescent body through a glass having the same color as that given by fluorescence. Thus the least traces of red fluorescence can be discovered by looking at the body through a red glass, in the illumination given by the mer- cury lamp. As the mercury lamp contains practically no red rays, seen through a red glass everything appears nearly black or invisible except red fluorescent bodies, which appear self-lumi- nous, glowing in a light of their own, and appear like red hot bodies. 68 RADIATION, LIGHT, AND ILLUMINATION. In the illumination given by the mercury lamp I here drop a few drops of a solution of rhodamine 6 G, rhodamine R and uranine (aniline dyes) into a large beaker of water. As you see, when sinking down and gradually spreading, they appear — especially against a dark background — as brilliant luminous clouds of orange, red and green, and seen through a red glass they appear like clouds of fire. I change to the illumination given by the incandescent lamp and all the brilliancy disappears, fluorescence ceases and we have a dull red colored solution. I show you here the sample card of a silk store of different colored silks. Looking at it through a red glass, in the mercury light all disappear except a few, which you can pick out by their lumi- nosity: they are different colors, pinks, reds, heliotrope, etc., but all containing the same red fluorescent aniline dye, rhodamine. A glass plate coated with a thick layer of transparent varnish, colored by rhodamine, appears like a sheet of red hot iron in the mercury light, especially through a red glass, while in the light of the incandescent lamp it loses all its brilliancy. This solution of rhodamine 6 G in alcohol, fluoresces a glaring orange in the mercury light, in the light of a carbon arc lamp (or in daylight) it fluoresces green and less brilliant. Thus you see that the color of the fluorescent light is not always the same, but depends to some extent on the frequency of radiation which causes the fluorescence. Here I have a sheet of paper covered with calcium sulphide and a lump of willemite (zinc silicate) and some pieces of calcite. As you see, none of them show any appreciable fluorescence in the mercury light. But if I turn off the mercury light, the calcium sulphide phosphoresces brightly in a blue glow, the others do not. Now I show you all three under the ultra-violet rays of the condenser discharge between iron terminals, or ultra-violet lamp (Fig. 11) and you see all three fluoresce brilliantly, in blue, green and red. Turning off the light all three continue to glow with about the same color, that is, phosphoresce, but the red fluorescence of the calcite very rapidly decreases, the green glow of the willemite a little slower, but the blue glow of the calcium sulphide screen persists, decreasing very little. I now hold my hand back of it and close to it and you see the picture of the hand appear on the screen by an increase of the luminosity where by contact with the hand the temperature of the screen was slightly CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. 69 raised, thus showing the effect of the temperature rise in increas- ing phosphorescence. These substances which I show you, calcium sulphide, cal- cium carbonate (calcite), zinc silicate (willemite), are not fluo- rescent or phosphorescent themselves, but their luminescence is due to a small percentage of some impurities contained in them. Chemically pure substances and concentrated solutions of the aniline dyes, or these dyes in their solid form, do not show the luminescence, but only when in very diluted solutions; that is, luminescence as fluorescence and phosphorescence seems to be the property of very diluted solutions of some substances in others. Thus a sheet of paper or cardboard colored red by rhodamine does not fluoresce, but if a small quantity of rhoda- mine is added to some transparent varnish and the paper colored red by a heavy layer of this varnish it fluoresces brightly red. To show you the fluorescent spectrum, I have here a mercury lamp surrounded by a very diluted solution of rhodamine 6 G, and some rhodamine R, contained between two concentric glass cylinders. As you see, through the spectroscope a broad band appears in the red and the green light has faded considerably. You also notice that the light of this lamp, while still different from white light, does not give anything like the ghastly effect of human faces, as the plain mercury lamp, but contains considerable red rays, though not yet enough. I also show you a mercury lamp surrounded by a screen of a very dilute solution of uranine: you see, its light is bright greenish yellow, but much less ghastly than the plain mercury light and the spectroscope shows the mercury lines on a fluorescent spectrum, which extends as a con- tinuous luminous band from the green to and beyond the red. You also see that with this uranine screen the mercury lamp gives more light than without it : considerable of its ultra-violet and violet light is converted to yellow and thereby made visible or more effective.