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already been successfully made from California-grown guayule rubber leave no doubt as to its true nature. Physical Quality of Rubber
Although the physical quality of guayule rubber has appeared to be different from that of Hevea, it is impossible, a t present, to state whether this difference is an inherent characteristic of the hydrocarbon itself or is merely due to methods of treatment, which largely determines the physical character of any rubber, whether from Hevea latex or from guayule shrub. U’e have already in operation a process of “retting” of guayule shrub similar to the retting of flax, by means of which we have succeeded in profoundly changing the physical character of the rubber from guayule, making it in all respects more like the product from Heuea brasiliensis. By this simple means, involving the exposure of the shrub for about a meek t o the action of microorganisms and air, we are enabled to mill out a rubber which in its vulcanized state has the tensile elongation and other physical. properties of plantation rubber to a very marked degree. By this same operation, if desirable, we can also remove as much as 7 5 per cent of the acetone-soluble materials characteristic of the
Silk-A
Mexican guayule rubber of the past, producing by this means a crude rubber from guayule shrub having an acetone extract of about 5 per cent, in this respect again closely resembling plantation rubber. In the past there has been no particular object to prepare material of this character, for with the limited output available Mexican guayule rubber, with nearly 25 per cent of acetone-soluble impurities, has found a ready market a t a price within 80 per cent of that of plantation rubber. Furthermore, with a uniform plant product to begin with and a uniform method of handling and treatment throughout, we shall be able, in commercial production in this country, to obtain a uniformity of quality of product from guayule hitherto unknown, and suitable for use in all kinds of rubber goods. In regard to the cost of production, we believe that guayule rubber will be successfully produced in this country in competition with present sources of supply from the Far East. I n this belief, which is based on our first-hand knowledge of the “all in” cost of rubber production from our own estates in the Far East, we have prepared and are going ahead to set out an additional 2500 acres of guayule this spring and annually thereafter.
Field for Research’
Elbert M. Shelton and Treat B. Johnson RESEARCH
LABORATORY OF C H E S E Y
VER four t h o u s a n d years of practical application in industry a n d approximately one hundred years of occasional scientific inve4gation have given 115 a fair genwal acquaintance with silk. Biochemical Processes of the Silkworm
B R O T H E R S , S O U T H A ~ A S C I I E S T E R , C O X N . , .AND T H E
Received February 27, 1930
UNIVERSITY,
NEWHAVEN,C O N N .
While s p i n n i n g its cocoon, t h e silkworm ejects the contents of its two silk glands through minute spinnerets, one on each side of its head, t o form two threads which a r e c e m e n t e d t o gether with a layer of socalled “sericin,” similar in its elementary chemical comp o s i t i o n but differing dist i n c t l y in physical properties and amino acid content f r o m t h e central filaments of fibroin. There have been many speculations as to the origin of sericin, none of which seem sufficient to account for its presence or t o explain its origin. One of the most promising explanations assumes that the fibroin is secreted by the long tail-like portion of the silk gland, while the walls of the enlarged reservoir secrete a slimy fluid which is the sericin. As the fibroin is accumulated in the reservoir of the silk gland, it assumes a position in the central portion of the reservoir in such a way that as the contents of the gland are later ejected there passes out a filament which is similar in cross section to the contents of the gland-that is, which might be pictured as a core of fibroin surrounded and possibly lubricated in its passage through the spinneret by a thin shell or tube of sericin. To confirm this or any other theory for the origin of the sericin, the support of the biologist is needed, and there is much room for careful correlation of the actual anatomy of the silkworm with the nature of the products secreted from its silk glands. A report of a preliminary investigation during 1929 from the Biology Department of Oberlin College commented very favorably upon the silkworm as a subject for study.
A study of the characteristic properties of natural silk reveals this substance as an organic compound whose chemistry is not well understood. Not only is our knowledge of its fundamental chemistry very limited, but we are also woefully ignorant of the biochemistry of the biological changes involved in its synthesis. On account of the great commercial irnportance of the natural silk industry, it is essential that an attempt be made to fill in these gaps in our knowledge of this textile material. The purpose of this paper is to point out some of the lines of investigation whose promotion promises to lead to new data of immediate commercial and scientific importance.
The si1ki\-oriii, regardleis of differing pel,sonal opilliolls as to the hand,omeness of its appearance, is a most attractive subject for study. Here, during that period of the life cycle of the moth when in its larval stage it is storing up materials and energy to carry it through the remaining stages of its existence, there occurs the only true organic chemical synthesis which takes place in the manufacture of silk. The transformation of the coinpoundb of vegetable growth to the viscous solution in a form t o be qpun by the worm into a silk fiber is a liiochemical proces. the understanding of which challenges the efforts of all n-ho are interested in the origin and composition of proteins. Although the silkworm has become so highly domesticated that it is extremely sensitive to adverse conditions during its larval stage and subject to more than its share of ills. yet with the information available regarding the care of the larvae, evrn a beginner with reasonable personal attention may expect to obtain a crop of healthy mature silkworms in the course of six weeks after the eggs are allov-ed to hatch. In other words, we have here a biological process which can easily he appropriated by a chemist for obtaining material in sufficient quantity for study by chemic7al technic. 1
DEPARTMENT OF C H E M I S T R Y , T A L E
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY Contents of Silk Glands
Little real study of the nature of the silk in the silk gland of the worm has been reported in recent years. Silk in the liquid form as obtained from the gland of the silkworm resembles strained honey and seems ideal material for study of the chemical and physical nature of proteins. The semifluid contents of fresh silk glands have been found to contain approximately 30 per cent total solids-organic matter apparently in an unstable colloidal dispersion in aqueous medium. One is impressed in many ways with the parallelism between silk and rubber. Not only does the siIk in the gland suggest the latex, but the coagulation to the elastic fiber of commerce and further physical changes which accompany aging and deterioration of silk resemble in many ways the behavior of rubber. I n attacking problems of silk reference to the existing knowledge of rubber is frequently illuminating. The glands are Iarge and easily removed by dissection. Immersed in water the contents apparently retain their physical qualities for a short time, though disintegration of the gland wall and dispersion of its contents become evident in a few hours. Organic solvents, as far as is known, cause immediate coagulation of the contents of the gland. An interesting experiment is performed by momentarily immersing the fresh gland in dilute acetic acid and then carefully stretching it out. At first the silk substance appears to flow and is easily drawn out, but suddenly a change in physical structure takes place and the fiber becomes tough and elastic, resisting further permanent elongation. Simultaneously its transparency gives way to an opaque white appearance.
Vol. 22, No. 4
Elementary analyses showing the presence and relative proportions of carbon, nitrogen, hydrogen, and oxygen are of little value beyond indicating that the material corresponds in this respect with protein substances, except that analyses to date show sulfur to be lacking from silk. Not only is the absence of sulfur from a protein unusual, but if sulfur plays a key role in oxidation changes, as it appears to do in life processes, we must consider cautiously any theories which involve oxidation or reduction in the various fundamental changes which take place in silk. Early in the last century, when the conception of the molecule was new, calculations of empirical formulas for the silk molecule were frequently attempted. I n the light of our present conception of silk fibroin as a mixture of independent or loosely bound individual proteins, the calculation of a formula for, and the determination of the molecular weight of, such a mixture is, of course, of no value. Hydrolysis of Silk and Production of Amino Acids
Many determinations have been made of amino acids from fibroin from various sources. This work has been done largely by students of Fischer and Abderhalden in Germany, and while the total information so obtained is small in proportion t o the effort expended, it a t least indicates the attractiveness of silk fibroin as a material for research. There is undoubtedly a renewed interest in hydrolysis of silk, judging by the large number of recent requests for silk to be used for this purpose in various schools and research organizations. While in many cases the material is used in the laboratory for instruction in the technic of protein Coagulation of Silk Fibroin There has been much speculation as to the cause of the hydrolysis and amino acid preparation and analysis, there coagulation of the fibroin as it is ejected from the spinnerets. are a number of investigators preparing amino acids, notably serine, HOCH&H(HN,)COOH, for use in other fields of This coagulation must take place in a very short time, since the worm spins almost continuously during the production research. Comparatively little is known regarding this of from one-third to two-thirds of a mile of filament which amino acid, and a promising field is open for investigation, enters into the construction of its cocoon. Perfect silk particularly in comparison with cystine and cysteine in their threads for subsequent weaving would be impossible if these roles in nutrition and life processes. Sericin of silk is the threads remained in a semi-fluid stage long enough to flow most practical source yet known for yielding serine in quantity. together or to be broken by the movements of the worm The highest yields of amino acids yet reported fall conwithin the cocoon. During the spinning the nature of the siderably short of accounting for all of the fibroin hydrolyzed, sericin is such that it might be assumed to be hardened by and in the case of sericin only about half of the protein has simple evaporation of moisture. Fibroin, however, shows been recovered as amino acids. Here is an opportunity, such resistance to solution in water that its coagulation not only to obtain more quantitative yields of amino acid must correspond more nearly to clotting of blood. Possibly previously found in silk, but to demonstrate the presence it is promoted by an enzyme or by an oxidation process, of others and to search for units, such as carbohydrate types, or even by an oxidation process catalyzed by an enzyme. hitherto not suspected. Just as blood clotting has been mentioned as similar to coaguFollowing the isolation of single amino acids, Abderhalden lation of silk fibroin, is it not reasonable t o suppose that a reported a series of investigations on various polypeptides true explanation of the chemical and physical processes which he was able to isolate from silk fibroin by partial which take place during the coagulation of silk fibroin might hydrolysis. This work seems especially significant through be of value to chemico-medical research into causes for failure the possibility that i t may be correlated with the results of coagulation of blood serum? of the use of our latest tool for scrutiny of molecules and Because of its importance, the coagulation of silk fibroin atoms, the x-ray. has been discussed a t some length. For many reasons, X-Ray Studies an early attack should be made upon this problem. To arrest successfully the coagulation of fibroin would a t once Several papers have been published on the results of xfurnish evidence as to the mechanism of the coagulation ray analysis of silk fibroin, all of which point to the presence process, and make possible the storing in quantity of this in fibroin of relatively small units of molecular weights in material in a form suitable for a wide range of scientific the neighborhood of 300 or 400. These correspond to fairly studies. I n this connection we are inspired by the successful short chains of amino acids which may be in turn grouped control of the solidification of rubber latex, and the possibility that an enzyme functions in the hardening of silk together into larger aggregates by secondary valences. The difficulty of evaluating and interpreting x-ray data which cannot be overlooked. a t present should not be overlooked, especially in view of Composition and Structure of Silk Fibroin the recent publications of Svedberg and others in which Studies into the composition and structure of silk fibroin molecular weights from 100,000 to 380,000 are ascribed to have been undertaken intermittently over the last century. the milk protein, casein. On the other hand, proteins dis-
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
persed in salt solutions by the method of the Russian chemist, Von Weimarn, give indications that the lower molecular weights may be confirmed. Out of all this research may eventually come a better understanding of the changes which may take place in the states of aggregation of protein units, and definition which may make possible agreement as to molecular weights. Solubility Studies
review of this work Hawley and Johnson have found evidence that the determinations have depended largely upon the methods used, and have themselves reported results by several methods in the neighborhood of 2.1. Harris and Johnson, using dispersions of fibroin obtained by the method of Von Weimarn, have apparently confirmed this new value. Action of Enzymes
The industry is naturally concerned with the possibility of the production of a synthetic fiber of a composition similar to silk but spun mechanically from a solution of albuminoids in a way parallel to the production of rayon. Very creditable products have already been prepared, similar in physical qualities to cellulose rayon, and who can predict but that with increased knowledge of the fundamental nature of silk, a fiber may eventually be artificially spun with the strength and elasticity of natural silk. The scientific world will welcome any information obtained from a study of dispersions of silk because of the light it will shed upon the structure of other albuminoids less conveniently obtained for study.
There is no question but that enzymes play important parts in the life history of the silkworm and possibly in the subsequent experience of the silk fiber. Disregarding the behavior of digestive enzymes which prepare the mulberry leaf to serve as food for the worm, we have already mentioned the possibility that some enzyme may favor the coagulation of the fibroin immediately following the operation of spinning. Another case is the method by which the nature moth liberates itself from the cocoon by weakening the fibers in one end of the cocoon by a drop of liquid, the composition of which has never been satisfactorily described and has probably never been scientifically studied. A knowledge of the nature of this liquid would be of great interest, both from the point of view of contribution to our knowledge of substances which attack silk itself, and also to the field of digestive ferments if this liquid is found to contain such an enzyme, as it seems most probable that it does. Silk is remarkably free from attack by molds and bacteria, although not entirely so. This renders all the more significant any cases which may be found of enzymes which have a dissolving action on silk. A study of the behavior of enzymes, particularly pepsin, on sericin led to very definite evidence of the presence of two fractions of sericin which were designated as sericin A and sericin B. Sericin A was readily acted upon by pepsin, while sericin B continued unattacked even after long periods of digestion. It still remains to be demonstrated whether these two fractions exist separately as individuals or are loosely combined in the original sericin. Are sericins A and B simply decomposition products of the original protein? Work begun by Daft and Johnson a t Yale to differentiate these two fractions of sericin by determination of nitrogen distribution is incomplete, but has not seemed conclusive thus far. So far as is known, determinations of the amino acid contents of the two fractions of sericin have not been attempted. The fact that sericin is a trade waste should be an added incentive to investigate its composition and to discover, if possible, some by-product of use to science or industry.
Attempts to Distil Silk Fibroin
Action of Light on Silk
Appel, a t the U. S. Bureau of Standards, attempted to distil silk fibroin, using an evacuated system with the silk maintained a t elevated temperatures, while a short distance away a surface maintained in contact with liquid air was designed to condense any vapors. Failure to distil over any fibroin below temperatures a t which the silk was visibly decomposing indicates its low vapor pressure.
One of the common problems which most concerns us with regard to silk is the action of light. Light is damaging to all textile fibers, and particularly so when a fabric not especially designed for the purpose is used as a window hanging or otherwise exposed to continuous or intense light. Under direct and continued exposure to sunlight, silk gradually loses its elasticity and finally becomes brittle and strawlike, capable of being broken when folded sharply. Silk is more sensitive than other textile fibers in this respect because of the small size of its filaments, allowing the damaging rays to penetrate to the core of each fiber more readily. It has been claimed that this change in the physical qualities of silk is the result of an oxidation stimulated by ultraviolet light. We are handicapped in experimental investigation of this or any other theory by lack of satisfactory measures of changes in the physical qualities with which t o judge the degree of damage to the fiber. Here is a field where a physicist, who by suitable measurements of changing
The fractionation of silk proteins, as well as the detennination of molecular weights of either the original aggregate or of any obtainable fractions, is rendered especially difficult by the physical characteristics of proteins. The solubility of fibroin in ordinary solvents is negligible except under conditions which may be suspected of bringing about a decomposition of the molecule. The studies of Von Weimarn into the solubility of various substances in saturated salt solutions, in which he is confining much of his attention to the application of these salt solutions to silk, may demonstrate the separation of the building units in the silk molecule a t the points of combination through these secondary valences, and thereby yield the unit polypeptide combinations. Harris and Johnson, in studies about to be published from the Yale laboratories, have employed the technic of Von Weimarn for bringing fibroin into solution and have opened up a large field for investigation into physical and chemical properties of protein so prepared. In this and the preceding paragraphs the term “solution” has been used loosely, since the protein so prepared does not pass through a dialyzing membrane, and our ultimate decision as to whether this is a true or a colloidal solution will depend upon our final knowledge of molecular size and our definition of a colloid. Synthetic Silk Fiber
Isoelectric Point of Silk
The determination of the isoelectric point of :isubstance like silk, while it may afford little information as to the actual composition and structure of the molecule, is of interest in that it represents a summation of various potential forces in the molecule and may a t least serve as a tool for differentiation between various fractions which may be isolated from either fibroin or sericin. A number of workers have published results in which they obtained values for the isoelectric point of silk ranging from a pH of 3.8 to 5. I n a
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elasticity, plasticity, or tenacity of the fiber, should be able to furnish some measure of the quality of silk a t any given stage in such an exposure. Conclusion
It is thus evident that the sum total of our knowledge concerning silk is woefully little, and that each bit of information only points the way t o a whole field remaining to be explored.
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Not only can such varied tools as organic and physical chemistry, physics, and mathematics, and their composite forms of colloid chemistry, physiological chemistry, biochemistry, and biology add to our knowledge of the fundamental nature of silk, but in doing so these sciences should themselves be enriched. As a material which is attractive in its form and cleanliness for handling and one which is allied with so many fields of pure and applied science, we believe silk has no rival.
CHANDLER LECTURE The Chandler Lecture for 1929 was delivered a t Columbia University on December 13, 1929, by Irving Langmuir, assistant director of the Research Laboratory of the General Electric Company, and a t the time President of the AMERICANCHEMICAL SOCIETY. I n presenting the medal awarded a t the close of the lecture, Dean George B. Pegram, of Columbia, spoke in part as follows in praise of the medalist:
To try to state all his scientific researches and discoveries would take more time than we have. His experimental and theoretical investigations of the emission of electrons by hot bodies, of ionic conduction in high vacua, of chemical reactions a t very low pressures, of molecular shapes, of the structure of atoms and of the forces between them put him in the front rank of those who have contributed to knowledge in these fields. You have just heard how clearly and definitely he visualizes phenomena a t low pressures in terms of individual atoms. No one has made friends with atoms more successfully. You have seen that he does not crowd them, he gives them plenty of room to be themselves, he keeps them a t a temperature they like so that they are neither too sluggish nor too excited, and they perform for him with wonderful fidelity. But besides having this mastery of scientific research, Doctor Langmuir has applied his knowledge to practical invention with the greatest success. His invention of the concentrated filament gas-filled electric light doubled the efficiency of electric light. I t is said to save the people of this country a million dollars a nightthough they probably spend most of that same million for more light, which is a good thing. Then there are his numerous inventions applied to electron tubes for radio and other purposes, his high-vacuum pump that is now used in every laboratory, and others. His great chief, Doctor Whitney, himself an early recipient of a Chandler medal, has said of Langmuir: ”The modern prophet is a doer. No one can fix the best ratio between thinking and ......
doing. The pure thinker is apt to think too much, and the active man to be too active. I know of no one who seems to combine these two characteristics more effectively than Langmuir.” The Charles Frederick Chandler Foundation was established in 1910, when friends of Professor Chandler presented to the trustees of Columbia University a sum of money, and stipulated that the income was to be used to provide a lecture by an eminent chemist and also a medal to be presented to this lecturer in further recognition of his achievements in the chemical field. The previous lecturers and the titles of their lectures are as follows: 1914
L. H. Baekeland
1916
M’. F . Hillebrand
1920
W. R. LVhitney
1921 E‘. G. Hopkins
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E. F . Smith
1025
1023 R. E . Swain 1925
E. C . Kendall
1926
S.W.Pari
1927
Moses Gomberg
1925 J. A. Wilson
Some Aspects of Industrial Chemistry [Vol. 6 , 7 6 9 (19141 I Our Analytical Chemistry and I t s Future [Vol. 9, 170 (1917)l T h e Littlest Things in Chemistry [Vol. 12, 599 (1920) 1 Newer Aspects of t h e Nutrition Problem [Vol. 1 4 , 6 4 (192211 Samuel L a t h a m hIitchill-A Father in American Chemistry [Vol. 14, 556 (1922) I Atmospheric Pollution b y Industrial Wastes [Vol. 15,296 (192311 Influence of the Thyroid Gland on Oxidation in t h e Animal Organism [Vol. 17, 525 (1925) 1 T h e Constitution of Coal-Having Special Reference t o t h e Problems OF Carbonization [Vol. 18, 620 (1926)l Radicals in Chemistry, Past a n d Present IVol. 20, 159 (192811 Chemistry and Leather [Vol. 21, 150 (1929)!
Electrochemical Interactions of Tungsten, Thorium, Caesium, and Oxygen’ Irving Langmuir GEKERAL E L E C T R I C C O M P A N Y , S C H E N E C T A D Y , N. Y.
HE atomic theory of electricity, in the form of the electron theory, has done much to make Faraday’s laws of electrochemical action seem obvious t o us. The Laue-Rragg methods of determining the arrangement of atoms in crystals by using x-rays have proved that even in solid rock salt there are no molecules of KaC1, but the crystal consists of a space lattice of Na+ and C1- ions, and the electric attractions between the ions account for the cohesive and elastic properties of the solid. We qee that the salt dissolves in water primarily because the attractive force between the ions is decreased greatly in a medium of such high dielectric constant. Thus the electrolytic dissociation theory also becomes obvious.
T
1
Received February 14, 1930.
In a state of thermal equilibrium, at any temperature T , the molecules in a gas are moving with velocities obeying probability laws, giving a Maxwell’s distribution of velocities. In a field of force, such as a gravitational or electric field, the molecules or ions vary in concentration in accordance with Boltzniann’T law nl,fn? = e w p ( U ‘ / k T ) Soie-The
(1)
symbol e x p l r ) has t h e same meaning as ez.
where W is the work that must be expended in transferring a molecule or ion from a region 1 to a region 2, while nl and n2 are the concentrations in these two regions; k is the Boltzmann constant 1.37 x 10 16 erg. deg. 1 If we are dealing with ion? having charges e , W becomes equal to lie, where L7 is the electric potential difference between 2 and 1.
