there is an asymmetrical atom, which can be either C, N, Si, &c. All of these cases of isomerism are well known and a few examples will suffice. Fig. 1 shows some typical cases of isomerism represented by structure symbols. (Can. Chem. J., 1918, ii., 135; Science, N.S., 1918, xlviii., 333; CHEMICAL NEWS, 1919, cxviii., 289. Although it has been said that the structure symbols where a radical change from our conventional symbols, the writer must confess that he sees in the structure symbols nothing fundamentally new or deviating from the accepted theories, but merely a shorter and accurate device to represent structure formulæ which undoubtedly possesses some advantages, which will well repay the initial inconvenience of understanding and using these structure symbols). a. Metamerism : -CH,ON2 30 d 31 m 32 d 33 Ꮎ 36 d 37 38 t 39 FIG. 2. Examples of Tautomerism. =3 = -CH2ON b. Desmotropism: -CHON, trans- 19 cis 13 Syn C. же же 15 FIG. I. Examples of Isomerism. a. Structure Isomerism : Butane (1), isobutane (2), and tetramethylmethane (3). Propylcarbinol (4), isopropylcarbinol (5), methylethylcarbinol (6), and trimethylcarbinol (7). b. Trans. Cis- Isomerism : Trans (8), and cis (9) forms of -CH=CH- radical. c. Stereo Isomerism : Dextro- (14), and lævo- (15) methyl-ethyl-propyl-methane. Tautomerism expresses itself in a difference in the type of compound, that is, the same number and kind of atoms in an organic molecule, which may or may not have the same relative position or linkage to each other, will by a change of position or linkage produce a different type of compound. To this class belong three categories: (a) metamerism, where the linkage remains the same, but the position is different; (b) desmotropism, where the position remains the same, but the linkage is different; and finally (c) tautomerism proper, where both, the position and linkage, is different. It is remarkable to find on examination of tautomeric cases that the majority belong to the desmotropic category, while the tautomeric category is comparatively rare. Desmotropism is a name little used, yet it describes aptly and excellently the phenomena as a "turn or change of the bonds or linkage" and thus characterises what actually happens. This turn or change is well illustrated in the structure symbols of Fig. 2, giving some examples of the three categories of tautomerism. -CH2ON =CHON -C2HO -CH,NO2 Tautomerism: amidoximes (20) and hydrazonic acids (21) acid hydrazides (22) and ureides (23). pseudo-ureides (24) and acid hydroxilimides (25) acid imides (27) and oximes (28). imidoethers (31) and ketoximes (32). hydrazonic acids (21) and acid hydrazides (22). ureides (23), and pseudo-ureides (24). = acid amides (26) and acid imides (27). oximes (28) and nitroso-methyl compds. (29). = acid amides (30) and imidoethers (31) ketoximes (32) and nitroso-methylen compds (33) ketones (34), and enoles (35). = nitro-cmpds. (36) and iso-nitro cmpds. (37). -OCN = cyanates (38) and isocyanates (39). Isologous series are formed by compounds of a similar structure, but with different atoms. This definition restricts Gerhardt's original conception of isologous series to a smaller group of compounds. Such a restriction seems justified and perhaps necessitated by the increase of knowledge which requires an ever-growing specialisation. Thus, e.g., isologes of pentacyclic compounds may be of the furan type-C,H,O, C.H ̧S, C.H,Se; or of the pyrrole type-C,H,NH, CH.PH, CH,ASH; or of the pentamethylen type-C,H,CH, (=C,H,), C,H,SiH,, &c. Some examples of isologous series of types of compounds are given in Fig. 4. R-methanes plumbyles (69). e. Isologes of carbinol (70): silicanes (67), stannyles (68), Tertiary alcohols (71), silicols (72), sulphines or sulphonium compounds (73), telluronium compounds (74). Similar are also Sn, Pb, &c. f. Isologes of formaldehyde (75): Ketones (76), silicon oxides (77), sulphoxides (78), tellurium oxides (79). Similar are Sn, Se, &c. g. Isologes of formic acid (80): Acids (81), siliconic acids (82), stannonic acids (83), alpha sulphinic acids (84). h. Isologes of ammonium hydroxide (85): Quaternary amines (86), phosphonium compounds (87), arsonium compounds (88), tungsten compounds (89), Isologous are astibonium compounds. 1. Isologes of tauto-nitric acid (90): Nitro compounds (91), phosphino compounds (92), arsino compounds (93), iodo compounds (94). Homologous series and polymerism are too well known to need illustration, their characteristics are already mentioned in the key to isomeric phenomena. San Francisco, Cal., College of Phys. and Surg. AGGREGATION AT THE MELTING POINT. By WILLIAM R. FIELDING, M.A., M.Sc. (Vict.), Senior Science Master at King Edward VII. School, Lytham. (Continued from vol. cxx., p. 255). Last Portion of the Curve (beyond 932°). OWING to the high melting points, the rapid changes in the values of specific heats (the specific heat of nickel rising from o'0575 to 01338 between 100 and 900° and the specific heat of diamond from o'0635 to 0 4589 between 223° and 1258°), the - greater difficulty of measuring the higher melting points with exactness, and the greater possibility of experimental error, the calculated values of R, melticular weights, and specific heats at the melting point may not be so accurate as those previously obtained. The curve has been calculated from Alse and Fes.. The latter value was obtained after frequent trial, e.g., the specific heat of Ag at 100° is 0.0467 and it rises to 0.0590 58 The melticular weight of silver is therefore 1955, its R about 18, and s (at 1233'5° A, its M.P.) =0'0603, not much different from the highest recorded value, viz., o'059 at 700°. The value of R for nickel, however, is only 456, whereas the value required to give it its highest recorded specific heat (0 1338 at 900°) is 57; but this discrepancy need not be seriously entertained at present as one expects exceptional properties for nickel owing to its anomalous position in the periodic system. The equation obtained from Alsa and Ni,, is TRUE Science has always been rightly accredited with a supreme honesty of purpose, a desire for absolute Truth, and an almost unhuman faculty of freely admitting mistakes. "To reveal, not to conceal" has been its watchword. But the latter propensity of admitting mistakes and fallacies, errors of judgment, and mal-observations, and of even courting correction is only a comparative modern acquirement of science. Noble and grand as was the progress of the Victorian giants of science, they were only human, and they were apt to become irritable and lay down dogmatic assertions when any intruding foot crossed their paths of work. The Victorian man of science was nothing if not material and concrete. The vague utterings and mystical speculations of countless generations of ritualistic alchemists and other "adepts" had had their effect, and towards the end of the eighteenth century a new type of person was found coming into prominence; an individual who like the legendary youth, "held a banner with a strange device"-Experiment ! It is therefore not surprising that as the nineteenth century grew in years, certain ideas became fixed in the collective mind of science-ideas which were firmly rooted there simply because no attempt had been made to question them. And one of these fixed ideas was the Boyle-Dalton conception of the Element and the Atom. Originally having its rise in ancient metaphysics, and centuries afterwards in empirical experiment, the conception of the Atom and the Element was raised to the pinnacle of supreme truth. The atom was regarded as an infinitely minute spherical body, and moreover it was impenetrable. Dalton really took Newton's hypothesis of the atom being a "solid, massy, hard, impenetrable" body, and gave to it a new significance which he developed into his atomic theory. And there the matter rested; it was final. The atom was regarded as being a fundamental and indivisible unit, and the existence of a state "beyond the atom" was undreamt of. On the conception of the Atom and Element there was gradually built up a wonderful system of qualitative and quantitative analysis. It was considered that by the act of analysis, a compound was separated into its fundamental constituents, and that even by the most delicate methods imaginable the compound could not be "analysed" any further. Founded on the fundamental idea of the element, a most remarkable classification of the known elements was gradually evolved. This table of elements, originated by the Englishman Newlands, and subsequently developed by Mendeléeff, has proved to be a wonderful stimulant for investigations into the relationship which certainly exists between the elements both in their free and combined state. Its true significance, however, still remains a mystery, and theories of the constitution of matter have been brought forward from time to time to explain it, but only to be found out of confirmation with the general facts. Still, however, the Daltonian idea of the elementary atom as a fundamental and ultimate unit held sway, practically unrestrained. Crookes Meta-Elements.-One of the first men to postulate that, whilst the atomic theory of matter was rational as far as it went, but that it did not go far enough, was the late Sir William Crookes. In 1887, Crookes was struck by the extreme closeness of the relationship between the rare earth metals, and he put forward the theory that these metals might possibly be modifications or variations of one parent element. He separated yttria into several different components, each possessing different phosphorescent spectra, but differing only minutely in chemical properties. The original yttrium was universally acknowledged to be a true element, having an unvarying atomic weight, and other definitely ascertained physical constants. Here there was an element which could be split up into a number of different parts each possessing different phosphorescent spectra, and yet having practically no chemical differences. For these different constituents of an element, Crookes introduced the term meta-elements. The properties of an element were supposed to be the average of the properties of its constituent metaelements, if any. Ordinary chemical analysis was able to separate the groups of meta-elements from one another, but it was powerless to effect a separation of the meta-elements themselves. The Evolution of the Theory of Isotopes.Crookes' theory of meta-elements did not meet with much success. Those who upheld it were at least considered to be irrational, and even actually eccentric. The old idea of the element as an entirely "pure" and homogeneous body was too deeply rooted to be disturbed by this hypothesis. In fact, probably only Crookes himself saw that in our much-prided analysis, we are not dealing with "pure" elements, but only with types of elements. Consequently, the theory was disclaimed and over-ridden. The discovery of radio-activity and the radioactive elements which occurred at the beginning of the present century forcibly brought home to chemists, and scientific philosophers generally, the necessity of revising their definitions of atoms and elements. Here, under their very eyes, was a body which satisfied all the demands made of an element, splitting itself up into other elements of lower atomic weight. This process of transmutation was as unaffected by any external conditions as it was ceaseless and unrelenting. radio-activity The varied phenomena of demanded immediate explanation, and the first general and comprehensive theory saw the light in the form of Soddy's Theory of Atomic Disintegration. The old idea of the atom and element was retained, in fact the discovery of radioactivity rather strengthened the idea of the atom, but its conception was retained in a different light. From henceforth the atom, although it was the unit of chemical combination, was not the fundamental unit of matter. An atom might contain a lilliputian atomic system itself. Soddy, by his theory of atomic disintegration, showed that during the spontaneous breaking down of a radioactive element, a, B, and y rays may be given out. The rays were found to consist of a stream of positively charged particles of matter, an a particle being no less than an atom of helium associated with two positive charges of electricity. The 8 rays are composed of negatively charged corpuscles, having a mass equal to only 1/2000 of the hydrogen atoms. The rays are considered to be similar, if not identical with the Röntgen or r rays. Their exact nature, however, is uncertain (Bragg, Phil. Mag., 1910, vi., 20, 385). At this point, however, it is necessary in order to obtain a clear impression of the development of the theory of isotopes, to turn for a moment to the theory of valency due to Abegg (Zeit. Anorg. Chem., 1904, xxxix., 330). According to this theory, every element possesses two kinds of valency, a normal and a contra valency. These valencies are electrically different, the normal valencies being positive in the metals, and negative in the case of the non-metals. The normal valency is usually the stronger of the two, and it corresponds to the generally accepted valency of the element, the contra valency being latent or dormant. According to Abegg, the sum of the normal and contra valencies is always eight, and is divided between the elements in the different groups of the periodic table, as shown in Table I. It was discovered that when a radioactive atom disintegrates with the expulsion of an a particle, it looses four units in mass. It also looses two units of positive electricity, and therefore two units of valency. Consequently, the new atom appears in the periodic table two places for the left of the original element. Thus when Radium (group II.) of at. wt. 226 looses an a particle, the resulting atom (Niton) appears in Group o, and has an atomic weight of 222. If, however, an atom of a radioactive element looses a 8 particle, the mass of the resulting atom is to all practical intents and purposes not affected, but the positive charge of the atom is increased by one unit, and also the valency of the atom is increased by one. Thus the new atom finds a place in the periodic table one place to the right of the position occupied by the parent element. In illustration of these views it will be interesting to follow the radioactive disintegration of the thorium atom. In this case, an a ray change occurs primarily, and is followed by two consecutive 8 ray changes. On the loss of an a particle by thorium, the new atom-Mesothorium I. appears in group II. of the periodic table and has an atomic weight of 228-four units lower than the atomic weight of thorium. On the expulsion of a 8 particle from mesothorium I., a new element-Mesothorium II. results. This possesses an atomic weight identical with meso-thorium I., and on the further expulsion of a particle, an element is obtained-Radio - thorium-which although possessing an atomic weight four units lower than thorium, appears in the same group as the latter. (Table II). Elements, such as thorium and radiothorium, which although possessing different atomic weights, fall in the same group in the periodic table were called by Soddy "Isotopes" (Isos, equal; topos, a place). As the mass of the B particle is negligible, it follows that the atomic weights of the new elements mesothorium II. and radiothorium will be identical with the atomic weight of mesothorium I. Thus in addition to the phenomenon of elements possessing different atomic weights, yet falling in the same group in the periodic table, we are confronted with the additional fact that it o (or VIIIC Transitional Group. I +6 +7 TABLE I.-Illustrating Abegg's Theory of Valency. This A year or two later, the atomic weight of lead was determined by several investigators, and it was found to vary slightly according to the source of the mineral from which the lead was obtained (M. Curie, Compt. Rend., 1914, clviii., 1676; Hönigschmid and Horovitz, ibid., 1796). variation in atomic weight was found to be between 206 40 and 20715. Allowing for experimental error, it was at once apparent that ordinary lead was a mixture of two different kinds of lead which were more or less chemically inseparable. The admirable work of Hönigschmid (Monatsh, 1912, xxxiii., 253) on the atomic weight of radium was such that the atomic weight of the element was accurately known to be 226. Allowing for the expulsion of five α particles from radium, it became evident that the element formed would fall into the same group in the periodic table as lead, and would have an atomic weight of 226, thus differing by 11 from the generally accepted value for the atomic weight of lead. Thus it was shown that the element lead was not composed of atoms of the same atomic weight, but that it contained at least two sorts of atoms, chemically identical, and differing from each other in mass. A third isotope of lead may also exist, for if allowances are made for the loss by thorium of six helium atoms, during its radioactive disintegration, the resultant element would find a place in the same group as lead, and would have an atomic weight of 208. Ordinary lead, therefore, is probably a mixture of three isotopes of atomic weight 206, 208, and 210, although according to some authorities, the end product of the disintegration of thorium is not lead, but bismuth. (To be continued). THE TRUE TANNING VALUE OF VEGETABLE TANNING MATERIALS.* By JOHN ARTHUR WILSON and ERWIN J. KERN. DURING the past century an enormous amount of energy has been expended in efforts to devise a method for determining the tannin content, or rather the true tanning value, of vegetable materials. Numerous methods have been proposed, but without any indication as to the correctness of the results obtained. (For a review of work done since 1803, see Procter's "Leather Industries' Laboratory Book" (Spon. 1908), pp. 168-176). In fact the methods now in general use, both here and in Europe, were made official without any knowledge as to their accuracy, but solely because they are of such nature that different analysts have comparatively little difficulty in concordant results. Since tanning materials are usually sold on a tannin basis, these methods have proved of very great value in enabling buyer and seller to agree as to price, but investigators who have blindly accepted the results as reliable have sometimes been led into serious error. In this paper we present what we believe to be the first successful method for determining the true tanning value of vegetable materials. Practical Definition of Tanning. Work on the chemistry of the tannins is still so far from complete that no rigid chemical definition of them as a class can be given, but their extensive use, especially in the leather industry, has necessitated defining them in terms of some property of practical value. It has therefore become customary to apply the name tannin to that portion of the water-soluble matter of certain vegetable materials which will precipitate gelatin from solution and which will form compounds with hide fibre which are resistant to washing. The remaining portion of the soluble matter is called non-tannin. New Method. Principle of the Method. The method aims, of course, to determine exactly what is called for in the definition. A convenient amount of the tanning material is shaken with a definite amount of purified hide powder until all tannin has been removed from solution. This point is determined by filtering off a portion of the residual liquor and adding drop by drop, avoiding a large excess, a solution containing 10 grms. of gelatin and 100 grms, of sodium chloride per litre; if the solution becomes turbid or a precipitate forms, it shows that all tannin has not been removed from solution, in which case the mixture must be discarded and the test repeated, using less of the tanning material or shaking for a longer time, until the solution after filtration gives no visible reaction with the gelatin-salt reagent. The tanned powder is then washed free from soluble matter, including the non-tannin removed from solution by the hide powder, which is responsible for the large errors in the methods now in use. It is then carefully dried and analysed for tannin as in the regular procedure for vegetable-tanned leathers, and from this figure the percentage of tannin in the original material may readily be calculated. * Presented before the Section of Leather Chemistry of the American Chemical Society, St. Louis, Mo., April 14, 1920. 1 |