The Electromagnetic Spectrum as an Analvtical Tool DAVID M . GANS Interchemical Corporation, New York City
I.
T is frequently essential in the course of a research . .
investlgabon to obtain information about a substance which the methods of chemical analysis cannot provide. Such information may be the identification of a trace of some element; the determination of the crystal structure of a chemical compound; or the recording of some attribute of a sample, such as its color. These properties can be determined by physical measurements, for which a variety of complicated instruments has been devised. While each of these instruments has a highly specialized use, there is one function connecting all of them: they deal with the electromagnetic spectrum. ARRANGED IN OCTAVES In this diagram, the diagonally shaded regions show the range It is a common experience to find that objects in a of the data obtainable with each of the instruments listed. room will vibrate when sound waves of certain fre- The dotted regions give the wave-length range of the radiation quencies, as from a piano, strike them. In an analo- employed to obtain these data, as discussed in the text. For the rpecuographs and the spwtrophotomcter, the range of the gous manner, the physicist exposes an unknown sub- &arz and of thr radiation cmployerl tn.dy be considered the same. stance to vibrations in the electromagnetic scale or Fur x-rav diffraction. the source i. mnnochrornat~c and its spectrum and observes the responses of its atoms and wave length is in the range of the x-ray distances spanned by With the electron microscope,the equivalent wave length molecules. Alternatively, the atoms and molecules data. of the electron beam used falls outside of the theoretical range may be induced to emit radiation, and from the de- of distances ex~lorablewith the inetniment. In the combar&ve scales at the top of the diagram, the te&ination of the points in the electromagnetic specmarked 0 shows the electromagnetic spectrum in octaves, where the radiation falls, it is possible to identify scale starting with the visible as the middle octave; the scale marked elements or to establish chemical structure. mm eives - distances in millimeters: thatmarkpd -- - wave leneth and To lend direction to our discussion, let us start with r. in microns; and"that marked A, in angstr6m units. The equipment represented here, from top to bottom, is: the infrared and up the 'petrock-salt prism spectrograph; Bausch and Lomb Littrow Quartz trum, stopping along the way to introduce examples Spectrograph; Beckman Quartz Spectrophotometer; General and to describe some of the instruments with which Electric Recording Spectrophotometer; light microscope; R.C.A. Electron Microscope; General Electric X-Ray DiEracresearch laboratories obtain the illustrative data. tion Unit. Equivalent instruments by other manufacturers are An electric charge under displacement creates a dis- in most instancs available. turbance in which there is both an electric component and a magnetic component. This disturbance forms which is most useful in this respect extends from the a wavelike electromagnetic vibration. The whole infrared, seven octaves beIow the visible region, through gamut of such vibrations, arranged in the order of their the visible and the ultraviolet, into the x-ray region, oscillation frequency, constitutes the electromagnetic 14 octaves above the visible, as depicted in Figure 1. spectrum. In comparison with the piano scale of about seven Part of the electromagnetic spectrum is very familiar octaves, this range, therefore, represents a spectral to us because i t forms what our eyes recognize as visible expanse of over 21 octaves, an exploratory range equivalight. In one respect this visible spectrum is more lent to a row of three consecutively tuned pianos. restricted in scope than its audible analog. While the RESPONSES OF ATOMS AND MOLECULES piano covers seven octaves and the entire audible scale includes a range of about nine octaves, the visible Why should matter respond to electromagnetic spectrum is only one octave broad. But electro- radiation? The answer is that matter is electromagnetic vibrations, like vibrations in the air, extend magnetic in nature. If we exclude such esoteric beyond the range in which they are detected by human phenomena as gravitation and nuclear disintegration, sensory organs. However, not all of the electro- an atom of matter can communicate with the outside magnetic scale, from the extremely high-frequency world only through electromagnetic radiation. Each radiation associated with transformations inside the atom is actually both a sending and a receiving set for nuclei of atoms to the longest, low frequency radio these waves. The wave lengths at which it is able waves, is useful in chemical industry. The portion to broadcast and receive depend upon its chemical 421
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nature. Generally speaking, matter is always ready oscillate like an itching octopus in an animated cartoon. to receive or absorb passing radiation whose frequency The extent of the distortion in each of the bonds would coincides with the characteristic frequencies of the depend upon the vigor with which the system was being atoms and atom groups of which i t is composed, and excited. Yet the bond fixing a hydrogen atom to a it can be induced to emit the same characteristic radia- carbon atom, for example, is not capable of twistings tions if supplied with the necessary energy. The and stretchings a t all conceivable frequencies, but, like frequendes shown in the reception spectrum and the a violin string, emits or sympatheticauy absorbs only broadcast spectrum can thus be used to identify atomic radiation of a characteristic frequency and of such species in the molecules constituting the sample as weU overtones as may be associated with the system. The complex organic molecule is therefore much like as the groups of atoms in it that may collectively be capable of acting as units in the emission or absorption a choir in which the presence of various individuals (here carbon-to-hydrogen, carbon-to-carbon, and carof radiation. Each atom of matter consists of an electrically bon-to-oxygen bonds) is revealed by the detection of charged nucleus, in which the mass of the atom resides, their voices in the group. The carbon-to-hydrogen and of an electrically compensating swarm of surround- bond, for example, sings a t a wave length of 3 microns ing electrons. When two atoms unite chemically, (0.003 millimeter), the carbon-to-carbon bond, such as an inter-penetration and redistribution of the dectron is present in saturated hydrocarbons, vocalizes a t a swarms about the two atoms take place. Under suit- wave length of 10 microns, and the unsaturated ethylable conditions electromagnetic responses may, there- ene type of bond emits a t 6 microns. If a sample is fore, be expected, not only from the electron swarm shown to absorb a t these three wave lengths, i t may about a single atom but also from the redistributed be deduced that its molecules contain these three types of linkage. Such information is invaluable in organic electron cloud that constitutes a chemical.bond. The chemical bond may be represented by the time- chemical research. It happens that the energy necessary to modify the honored physical analog of a coil spring. A complex organic molecule would thereby be equivalent to a twistings and stretchings of the inter-atomic bonds, group of correctly weighted balls connected to each when translated into electromagnetic radiation, is of other by springs of the proper elasticity. A hydrogen such magnitude as to show up in the near infrared poratom would be connected to a carbon atom by a single tion of the spectrum, which includes the wave lengths spring. A saturated carbon atom would be joined to 3, 6, and 10 microns mentioned above. The infrared four other atoms such as carbon, hydrogen, nitrogen, portion of the spectrum, which does not &ect the oxygen, or sulfur by four such springs. This wobbly human eye as light, corresponds to the piano bass. model of a molecule, when suspended in space, would Beyond the infrared, a t even longer wave lengths (or lower frequencies) lie the radio waves, but these fall outside the present discussion since they have as yet had no extensive application in the field of chemical analysis. INFRARED SPECTROSCOPY
Let us first consider the infrared, where, broadly speaking, information concerning chemical bonds is obtainable, because that is where, in the nature of things, the characteristic frequencies associated with these chemical bonds happen to fall. To acquire this knowledge, an instrument capable of analyzing infrared radiation is required. A small number of such instruments are currently in use in industrial research laboratories ( I ) , primarily in those of the petroleum companies. Each of these instruments employs a large triangular prism of rock salt as the means for breaking the radiation into its prismatic components. Rock salt, chemically identical with table salt, is chosen for the prism because it is transparent to infrared radiation in the range under investigation. The instrument is therefore called a rock salt prism spectrograph. For special purposes prisms of material other than rock salt are sometimes employed. Details of the construction of one spectrograph are shown schematically in Figure 2. Infrared radiation from source A, such as an electrically heated element, falls upon a slit, B. The radiation contains energy
of all wave lengths. The portion passing through slit
B is focused by a parabolic mirror, C, on the rock salt prism D (5 inches on a side and 3 inches high), through which it passes, and is reflected back in its tracks by flat mirror E, thereby utilizing the prism twice and doubling its ability to spread the original radiation into its spectral components. The returning radiation is refocused by mirror C upon the small flat mirror F which deflects i t a t right angles to its original path through the second slit G onto the elliptical mirror H. This mirror concentrates the energy upon a very sensitive, specially constructed thermocouple, J. The heat effect occasioned by the absorption of theradiant energy by the thermocouple, which is sometimes as little as one bundred-thousandth of a degree, is converted into an electrical current that is electronically amplified and recorded automatically on a sheet of paper. The recording mechanism is sensitive to thermocouple currents of a small fraction of one-millionth of a volt. Only a narrow band of radiation passes through the second slit G, and the wave length of this narrow band which so passes is varied by rotating the minor E, placed behind the prism D. As the mirror is rotated, the infrared spectrum sweeps over slit G and the radiation which slips through the slit activates the recording mechanism in proportion to the intensity of the radiation of that wave length. If, now, a sample such as an organic liquid is interposed between the source A and the slit B, the sample depletes the initial radiation of those components corresponding to the characteristic frequencies of the molecules composing the sample. Less radiation of that wave length will therefore strike slit G as the spectrum sweeps over it and the depletion is thus revealed as a drop in the curve traced by the recording mechanism. A typical tracing obtained with an infrared spectrograph is shown in Figure 3. The two dips a t 2.9 and 3.0 microns for aniline arise from the two N-H bonds of this compound. The dip a t 3.0 microns for di-n-propylamine originates in the single N-H bond of this amine. Primary and secondary amines may thus be differentiated. VISIBLE AND ULTRAVIOLET SPECTROSCOPY
We have seen that the infrared spectrograph uncovers information concerning the contractual relationship, so to speak, of the atoms that form a chemical partnership. What about the atoms as individuals? They may be identified by inducing them to talk. The third degree is applied electronically by placing them between the electrodes of\ an arc or spark. The electrons in the arc or spark then impinge on the electron swarms around the atomic nuclei and impart additional energy to them. The atoms release this additional energy by radiating a t their often numerous characteristic frequencies. It takes a more powerful punch to affect the electron swarm of an atom than it does to distort the bond between two atoms. As a result, the atomic radiation comes higher in energy along the electromagnetic spec-
30 35 40 NFRARfD WWA&L~NCTH/N fl/C@NS
2.5
F I G ~3.-TYPICAL E TRACIUGS OBTAINED WITH AN INPR ARED SPECTROCRA~H (AFTERBARNES,LIDDEL,AND W I L L I A ~1 S11) trum than the infrared radiation associated with bonds. The atomic radiation appears in the visible and ultraviolet and is experimentally detected in the region between 200 and 700 millimicrons (0.2 and 0.7 microns). A prism spectrograph designed for this general purpose must therefore employ a prism that is quite transparent in this region. Crystal quartz is ideal for this purpose. One quartz spectrograph of this type as shown in Figures 4 and 5 consists primarily of a set of electrodes, A , between which the sample, which may be quite minute, is energized; a slit, B, through which the resulting radiation is passed; a small quartz prism, C, that acts as a mirror to divert the beam of radiation through a right angle; a quartz lens, D ; a quartz halfprism, E, which the light traverses twice since its back is silvered to reflect the beam and spread i t into its spectral components; and a photographic plate, F, on
R o m , 4.-THE BAUSCH AND LOMELARGE LITTROWQUARTZ
SPECTROGRAPH
which the spectrum is recorded. The distance from
E to F is about five feet.
Each atomic species will register its own characteristic spectrum on the photographic plate. While all atoms may be induced to talk by similar electrical methods, the language of one atomic species is entirely different from that of any other. The languages of some atoms, such as sodium and magnesium, consist of relatively few words or spectral lines, while the languages of others like iron and chromium have rich vocabularies. It is a simple matter to determine the wave length of a few of these spectral lines and, from existing tables, to ascertain the corresponding chemical element. The relative intensity of its lines also gives a very sensitive method for determining the amount of the element present, particularly when minute traces, often difficult to assay otherwise, are involved ( 2 ) .
FIGDRE SPE ABSORPTION CURAS OBTAINEDWITH THE
BECKMAN
Curves A and B a r e both fur lioolric nchl, hut the two double I,onrl$ of thc u n s ~ t u m t e darid fdr I3 are wpnrated by several rarhon atoms in thccnrhon chain of the fattv aritl: A i~ the curve for the isomer of this acid in which the two double bonds are conjuaated, that is, on ndjoining carbon arums. The spcctrophotometer i, thus useful In conjugation studies directed toward investigating and improrirly drying oils. VISIBLE AND ULTRAVIOLET ABSORPTION SPECTROPHOTOMETRY
FIGURE6.-ULTRAVIOLETSPECTROGRAMS
A portion of a spectrogram in the neighborhood of 3100
angst& u!.its. Spc&um . I ir from rwnnlen.ially pure titanium dioxide of pigment grad*. Slwrtnm I3 ir fr.m a rirtlilar titanium diorklr orcviouslv treated with alumina to orodnce certain desirable o i m e n t &overties. Spectrum C is'for aluminum. A compa&oi of A,-B,ind C shows that nearly all of the lines in B which do not appear in A are those of aluminum. The existence Cornoarison of B of - - a trace of aluminum is thus established. with spectra from samples of pure titanium dioxide containing known added amounts of aluminum would show that B represents a content of 1per cent of aluminum. Spectrum D is that of the iron arc which is used as a wave-length standard. ~~
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The spectrogram of Figure 6 is an illustration of the application of spectrographic analysis of industrial importance.
The quartz spectrograph just described is not rerestricted to the analysis of emission spectra from samples placed in an arc or spark. This spectrograph may also be employed, with the addition of suitable accessories, in the study of absorption spectra, which is accomplished by erecting a source of radiation a t A of Figure 5 and interposing the sample to be examined between the source A and the slit B, as was done in the case of the infrared spectrograph of Figure 2. The sample may he a colored plastic, or perhaps a solution or an organic liquid in a quartz cell. From the photographic record, curves like those of Figure 3 may then be drawn. A small instrument designed specifically for this type of work is the Beckman Spectrophotometer. The construction of this instrument is a variation of the design shown in Figure 5 . In place of a photographic plate, i t utilizes a photo-electric cell. The instrument yields point readings, which are then plotted in sequence to give the ultraviolet absorption curve. The quartz prism and photocells with which the instrument is equipped give i t a spectral working range from 0.2 micron to 1.2 microns, so that i t supplements the infrared spectrograph (3). Ultraviolet absorption spectra are useful in the study of certain problems in the chemical structure of unsaturated organic compounds; an example is given in Figure 7. Peculiarities in their visible and ultraviolet
SEPTEMBER, 1944 absorption spectra may also be employed to identify dyes. In analytical chemistry, compounds may be assayed through the intensity of their characteristic absorption or that of suitable chemical derivatives. REFLECTANCE SPECTROPHOTOMETRY
Chemical constitution is not the only type of information that is of interest in the portion of the electromagnetic spectrum already covered, or that can be supplied by instruments designed to analyze the radiation from a specimen. The visible spectral range between 400 and 700 millimicrons is of particularly great importance from another standpoint, that of color and its measurement. While some pertinent data in this range are obtainable with the two instruments already described, another instrument has been specially designed (5) to serve these needs. Fhis spectrophotometer, whose external appearance is shown in Figure 8, is represented in a highly simplified schematic version in Figure 9. Light from an incandescent lamp a t A is spread into its component colors by the succession of lenses and prisms shown. From the spectrum which is thus formed, a slit a t B selects one practically single-colored band of light a t a time. On passing through a complicated optical assembly between B and C, this light is split into two equally intense beams. Each of these beams is not constant in brightness, but is instead made to fluctuate very rapidly from zero to the maximum value. The fluctuations are so timed, however, that when one beam is at zero brightness, the other is a t the maximum. Now, if they were both directed to fall on one spot, their fluctuations would so interlace that no flicker would show on this spot. However, they enter the integrating sphere D as two separate beams. One of these beams falls upon a standard white surface a t E and the other upon the sample to be measured a t F. When the sample a t F is as white as the standard a t E, there is no flicker in the light which the white walls of the integrating sphere pick up from E and F. Of course, samples are generally not white, which is the same as saying that they absorb light in some if not all parts of the visible spectrum. Such a sample a t F reflects less light into the integrating sphere a t those wave lengths where absorption occurs than the standard a t E, and a flicker consequently arises in the sphere. The photocell G, which peeks into the integrating sphere, detects this periodic variation and, through the proper optical arrangements based on polarized light and electronic devices, acts to diminish the intensity of the beam which strikes the white standard until the flicker is ironed out. The amount by which the intensity of the standard beam is thus diminished is translated mechanically by the instrument into the relative percentage reflectance of the sample for this monochromatic light, which value is recorded by suitable means on a rotating drum. By passing the entire spectrum, as if in review, into B, a curve is obtained on the paper of this drum that shows the percentage
of light which the sample reflects a t all wave lengths in the visible range. One type of data obtained with the spectrophotometer is shown in Figure 10. These curves are permanent records of the color--often itself all too impermanent-of the corresponding specimens and are consequently better color standards than are color chips. Peculiarities in curve shape frequently serve to identify the pigment or pigment mixture in a surface. Because of such circumstances, the spectrophotometer is indispensable in camouflage studies, for which purpose the range of the instrument must indeed be extended beyond the visible to about 900 millimicrons in the infrared be-
cause the photographic plates which are used to detect camouflageby aerial reconnaissance are sensitive there. By placing white surfaces a t both E and F of Figure 9 and by interposing a colored transparent sample in the lower of the two beams a t C it is obvious that the spectrophotometer may be converted into an instmment for transmittance measurements, similar in purpose to the Beckman Spectrophotometer. While these instruments overlap in range, the General Electric Spectrophotometer gives much more accurate data in the region where it can be used.
ELECTRON MICROSCOPY
Knowledge concerning minute sizes and shapes may be obtained in much greater detail with the electron microscope than with the ordinary light microscope. In fact, where the ordinary white light microscope can barely distinguish, as separate, two particles which are 0.2 micron apart, the electron microscope will, under ideal conditions, distinguish between two particles which are as little as 0.002 micron apart. As a consequence, much greater wealth of information is revealed by the electron microscope (7,8, 9). Its present resolution limit of approximately 0.001 micron or 10
Wave/engfi h Mi//im~crons
FIGURE 10.-SPECTROPHOTOMETERREFLECTANCE CURVES FOR R u r r ~ sFORM OP TITANIUM DIOXIDE( A ) , BARIUMLITHOL( B ) , AND
ULTRAMARINE BLUE (C) MICROSCOPY
There is one means of observation, often overlooked because i t is so commonplace, which is of prime importance in saentific work. This, is, of course, the human eye. What the sense of sight reveals, the chemist accepts and records. Only occasionally is the sense of smell so honored, less frequently the taste, and rarely the hearing. However, the unaided eye is unable to discern sharply details smaller than several thousandths of an inch, or about 50 microns. Here the ordinary microscope extends the range down to a fraction of a micron. Using ultraviolet light, an experienced microscopist can separate photographically two particles spaced 0.1 micron apart. Each photograph may, of course, be subsequently enlarged to mural dimensions, but no additional detail is revealed thereby, for the limit of the microscope has already been reached in the original photograph. The microscope (6) is the most familiar of the equipment considered here. Yet it is not as widespread in use as it merits, for i t should approach the analytical balance in distribution and application. However, beneficial as i t is to the amateur, only the trained and experienced microscopist can extract with it the maximum information.
FIGURE 11.-R. C. A. ELECTRON MKROSCOPE Theinstrument illustrated is equipped for both electron microscopy and electron diffractionexperiments. The same sample when properly mounted may be subjected to both types of examination. angstrom units in theory and about 40 angstrijm units in practice is already in the domain of molecular dimensions, and it is therefore capable of perceiving- large molecules. The electron microscope, pictured in Figure 11, is analoeous in structure to the ordinam lieht micro" scope. The former uses magnetic lenses and beams of electrons where the latter employs optical lenses and beams of light. In the theoretical treatment of the operation of the electron microscope the electron beam is considered a stream of waves rather than of particles. Physicists have discovered that electrons, cosmic rays, gamma rays, light, and so forth, carry on a Dr. Jekyll and Mr. Hyde existence--they behave both as waves
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and as particles. This is an experimental observation that may he explained theoretically on the basis of their wave nature and, alternatively, from the particle standpoint. The electrons which form the beam, accelerated by potentials between 30,000 and 60,000 volts, have equivalent wave lengths of 0.07 to 0.05 angstrom units, respectively. Ordinary light comes in waves 100,000 times as long. As probes for picking out the finest detail in a subject, the electron waves act like the sharpest needles compared to the blunt crowbars of ordinary light, 100,000 times as thick. It is no wonder, then, that the electron microscope reveals so much greater detail than the ordinary light microscope.
FIGURE ~~.-ELECTRON ~IICROORAPH OF TITANIUM DIOXIDE energy, x-rays come very high in our electromagnetic spectrum. They are produced in the x-ray tube by DISPER~ION The sample used here is the same as A of Figures 6. 10, and 14. electrons which are drawn out of a hot filament by T h e unit of lengh shown in the lower left-hand comer is one rnlcron.
Matter absorbs the electrons of the beam so readily that the specimens for electron microscopy must he extremely thin. The open areas in the specimen then permit passage of the beam and the result on the photographic plate which records the image is a shadowgraph of the specimen, enormously enlarged in all its detail. Such a shadowgraph is shown in Figure 12. One of the advantages which the electron microscope offers is its relatively great depth of focus. Pictures taken with the ordinary light microscope are often sharp only for the middle stratum of the sample but fuzzy above and below this layer. The electron microscope, on the other hand, shows images in sharp focus throughout a much greater depth, which greatly increases its usefulness. X-RAY DIRRRACTION
The microscopes give the size and shape of a particle; the spectrophotometers diagnose its dermatological aspects; and the spectrographs delve into its internal constitution. An instrument which gets into the vitals of a specimen is the x-ray diiraction apparatus, which, however, supplies data from a widely different viewpoint. In
accelerating potentials of many thousands of volts. These electrons plunge into the metal target of the x-ray tube, which they convert into a source of x-rays, a dozen octaves in energy above that of ordinary light. Each element has its own characteristic x-ray spectrum. When the target of the tube is copper, it becomes a source of x-rays of wave length 1.5 angstrom units (0.15 millimicron or 0.00015 micron) which is about one three-thousandth the wave length of blue light. When such a source of x-rays A of Figure 15 irradiates a sample, B, the sample scatters the x-rays which then d e c t the surrounding photographic film C. If the sample is truly amorphous, the x-rays are scattered without appreciable concentration in any one direction. But this is not true if even the minutest crystals exist in the sample. In crystalline matter the constituent atoms or atom groups occupy regular geometric positions in space, which are uniformly repeated throughout the crystal. Just as a large group of men conducting synchronized calisthenic exercises in the twodimensional space of a field seems to fall into rows when viewed from various directions, so the units in the threedimensional space of a crystal form planes. As a result, each crystal is equivalent, from a number of directions, to a bundle of flat mirrors. The spacings between these planes differ from one crystalline com-
pound to another and determine the direction and intensity of their x-ray reflections-more accurately diiractions. A comparison of the diffraction pattern obtained with a collection of patterns from known crystal species often permits identification of the sample (10). In more unusual cases, a mathematical analysis
of the pattern to be expected from an assumed crystal structure, as compared with the pattern actually obtained, throws light on the internal structure of the unknown material (11). An x-ray diffraction unit is shown in Figure 13. Typical radiograms obtained with i t are reproduced in Figure 14. ELECTRON DIFFRACTION
An electron beam, as well as an x-ray beam, can be used to study the structure of matter (12). To exploit this possibility, the R. C. A. engineers (13) have modified their commercial electron microscope to make it applicable also to electron diffraction. While x-rays can penetrate a sample of even considerable thickness,
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Coated with Alumina
Frome 1 6 . - E ~ ~ c m aD~ ~ ~ ~ n n c ra# ~ oTrmrruM x Ilmxroe
otherwise be obtained. It permits us, for example, to identify the films, as of oxides, on the surfaces of chemically reactive metals and to study surface diierences among several pigments which are chemically identical but which have undergone different surface treatments. Such information is of prime importance to manufacturers of film-forming materials, powders of all types, and dispersions, among other products. I n these, surface properties often outweigh their bulk characteristics. The electron microscope is shown in Figure 11, equipped with the electron diiraction accessory. In Figure 1 6 comparative electron diffraction patterns secured with it are presented. SUMMARY
only the thinnest films are transparent to electrons.
As a result electron diffraction is useful primarily to study surface layers only and the sample must either be prepared as a very thin film or, if bulky, must be so disposed that the electron beam strikes it a t a glancing angle, from which the diiracted beam is focused on the photographic plate that permanently records the dif- . fraction pattern. It is perhaps fortunate that the electron beam is such a sensitive nrobe for surfaces onlv. since it therebv becomes a fountain for information which could not
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When we consider that each of the topics discussed above constitutes in its own right a branch of physics, we cannot hope to gain here more than an introduction to the subject covered. We see, however, that as a group they form a clinic in which the patients are samples of matter in great variety. The specimen may be snbjected to an external examination: as when its shape and superficial details are viewed with the light microscope and the electron microscope; or its color characteristics determined with the spectrophotometer; or its surface analyzed by electron diffraction. The examination may delve somewhat more deeply into its constitution and supply information of aid in the determination of its molecular structure and chemical composition, by use of the several spectrographs, with absorption and emission methods. Furthermore, with the help of x-rays, the arrangements of atoms and atom clusters may be revealed. In this electromagnetic clinic, a diagnosis of considerable interest is the rule, and the prognosis is generally profitable, whether in the academic or industrial sense. REFERENCES
(1) BAFXES, LIDDEL,AND WILLIAMS,Ind. Eng. Chcm., Anal Ed., 15, 659-709 (1943).
BRODE, "Chemical Spectroscopy,"
2nd Edition, Tohn
chap. 12 in 'Trotectik and Decorative .~&tings;" Vol. 4, pp. 362405, Edited by J. J. Mattiello, John Wiley and Sons, Inc., New York, 1944. HARDY. lour. Optical Soc. Amer., 25, 305-11 (1935). CNAMOTAND MASON."Handbook of Chemical Microscopy," Vol. 1, 2nd Edition, John Wiley and Sons, Inc., New York, 1938. ANDERSON,"The Study of Colloids with the Electron Microscope," Chapter in "Advances in Colloid Science,"
Vol. 1, pp. 353-90, Edited by E. 0.Kraemer, Interscience Publisbleis, Inc., New York; 1942. GREENAND FULLAM, l o u r . APP. Phys., 14, 677-83 (1943). Interchemical Reuim. 1. 15-21 11942). FULLAM. HANAWALT. RINN.AND FRETEL, Ind.~ n- d~. h e k .Anal. . Ed.. 10,457-512 (1938). BuERcER, "X-Ray Crystallography." John Wiley and Sons, Inc., New York, 1942. THOMSON AND COCNRANE, "Theory and Practice of Electron Diffraction," Macmillan and Company, London. 1020
