The Progress of the Century

by Unknown

H H H H
C——C C C
/ \ /
N CH and furfurane, O == ,
\ / \
C==C C C
H H H H

and so on. By means of conceptions such as these many interesting compounds have been built up out of the elements which they contain; e.g., urea and uric acid, constituents of urine; theobromine and caffeine, the essential principles of cocoa and tea; alizarine and indigo, valuable dyestuffs; and several of the alkaloids, bitter principles contained in plants, of great medicinal value.

They have led, too, to the discovery of many brilliant colors, now almost universally employed, to the exclusion of those less brilliant, because less pure, derived from plants, and in one or two cases from animals; the manufacture of gun-cotton, dynamite, and similar high explosives; and to the development of the candle industry; the sugar manufacture; to improvement in tanning, in brewing, and in the preparation of gas and oils for illuminating purposes. In short, it may be said that the industrial progress of the latter half of the century has been due to the theoretical views of which a short sketch has just been given.

Such formulas, however, can evidently not represent the true constitution of matter, inasmuch as the atoms are imagined to lie on a plane, whereas it is evident that they must occupy space of three dimensions and possess the attributes of solidity. The conception which led to the formulation of such views was due first to Pasteur, in his later years director of the institute known by his name at Paris, and more directly to LeBel and Van’t Hoff, now professor at Berlin, independently of each other. In 1848 Pasteur discovered that it was possible to separate the two varieties of tartaric acid from each other; and that that one which rotated the plane of polarized light to the right gave crystals with an extra face, unsymmetrically disposed with regard to the other faces of the crystal. The variety, the solution of which in water was capable of producing left-handed rotation, also possessed a similar face, but so placed that its reflection in a mirror reproduced the right-handed variety. Pasteur also showed that a mixture of these acids gave crystals not characterized by an unsymmetrically placed face; and also that the solution was without action on polarized light. These observations remained unexplained, until LeBel and Van’t Hoff, in 1874, simultaneously and independently devised a theory which has, up till now, stood the test of research. It is briefly this: Imagine two regular tetrahedra, or three-sided pyramids, standing each on its triangular base. An idea can best be got by a model, easily made by laying on a table three lucifer matches so as to form an equilateral triangle, and erecting a tripod with three other matches, so that each leg of the tripod stands on one corner of the triangle. At the centre of such a tetrahedron, an atom of carbon is supposed to be placed. Marsh gas, CH₄, is supposed to have such a structure, each corner, or solid angle of the structure (of which there are four), being occupied by an atom of hydrogen. This represents the solid or stereochemical formula of methane or marsh gas. Now, suppose one of the atoms of hydrogen in each of these structures to be replaced by chlorine, the group (OH), or any other monovalent element or group. It is evident that if not exactly similar (owing to the replacement not having been made at similar corners in each), the two structures can be made similar by turning one of them round, until the position of the substituting atom or group (which we will term X) coincides in position with X in the stationary one. If two such replacements be made, say, with X and Y in each, coincidence can again be made to take place; but the same is not the case if X, Y, and Z replace three atoms of hydrogen in the structure; for there is one way of replacement which is the optical image of the other, and represents the other’s reflection in a mirror.

(Tetrahedron XYZ) and (Tetrahedron XZY)

Now, it is found that when the four corners of such a structure are occupied by four separate atoms or groups, or when (as the expression goes) the body contains an “asymmetrical carbon atom,” if the substance or one of its derivations can be obtained in a crystalline form, the crystals are also asymmetric, i.e., arc develops a face which is the mirror-reflection of a similar face developed on the other variety; and if a beam of polarized light be passed through the solution of the substance, its plane is rotated to the left if one variety be used, and, if the other, to the right. This hypothesis of LeBel’s and Van’t Hoff’s has had an enormous influence on the progress of organic chemistry. By its means Fischer, now professor at Berlin, has explained the reason of the existence of the enormous number of bodies analogous to grape and cane sugar, and has prepared many new varieties; and it appears likely that the terpenes, a class of bodies allied to turpentine, and comprising most of the substances to which the odor of flowers is due, may thereby find their explanation. It may be mentioned in passing that Pasteur, having found that ordinary mould destroyed one variety of tartaric acid rather than the other in a mixture of the two, and made use of this observation in order to prepare the unattached variety in a state of purity, was led to study the action of organisms more or less resembling mould; and that this has led to the development of the science of bacteriology, which has had an enormous influence on our views regarding fermentation in general, and guides the work of our physicians, our surgeons (witness Lister’s antiseptic treatment), our sanitary engineers in their estimate of the purity of drinking-water and of the disposal of sewage, of our manufacturers of beer and spirits, of wine-growers, and more recently of farmers. All these processes depend upon the action of organisms in producing chemical changes, whether in the tissues of the body, causing or curing disease, or in the production of flavored alcohol from sugar, or in the manufacture of butter and cheese, or in preparing the land for the reception of crops. We also owe to the genius of Van’t Hoff the most important advance of recent times in the region of physical chemistry. It has been observed by Raoult, professor at Grenoble, that the freezing-point of a solvent as a general rule is lowered to the same extent if there be dissolved in it quantities of substances proportional to their molecular weights. Thus, supposing 1.80 grams of grape-sugar be dissolved in 100 grams of water and the solution cooled below 0° with constant stirring, ice separates suddenly in thin spicules, and the temperature rises to −0.185°. If 3.42 grams of cane-sugar be similarly dissolved in 100 grams of water, the freezing-point of the solution is again −0.185°. Now, 1.80 and 3.42 are respectively the hundredth part of the molecular weights of grape-sugar (C₆H₁₂O₆) and cane-sugar (C₁₂H₂₂O₁₁). Similarly, Raoult found that quantities proportional to molecular weights dissolved in a solvent depress the vapor pressure of that solvent equally, or, what comes to the same thing, raise its boiling-point by an equal number of degrees. But ordinary salts, such as sodium chloride, potassium nitrate, etc., dissolved in water, give too great a depression of the freezing-point and too high a boiling-point. Next, it has been observed by botanists, Devries, Pfeffer, and others, who had examined the ascent of sap in plants, that if a vessel of unglazed porcelain, so treated as to cause a film of cupric ferrocyanide (a slimy red compound) to deposit in the pores of its walls, be filled with a weak (about 1 per cent.) solution of sugar or similar substance, and plunged in a vessel of pure water, water entered through the pores. By attaching a monometer to the porous vessel the pressure exerted by the entering water could be measured. Such pressure was termed “osmotic pressure,” referring to the “osmosis” or passage through the walls of the vessel. Such prepared walls are permeable freely to water, but not to sugar or similar bodies. Van’t Hoff pointed out that the total pressure registered is proportional to the amount of substance in solution, and that it is proportional to the absolute temperature, and he showed, besides, that the pressure exerted by the sugar molecules is the same as that which would be exerted at the same temperature were an equal number of molecules of hydrogen to occupy the same volume as the sugar solution. This may be expressed by stating that when in dilute solution sugar molecules behave as if they were present in the gaseous state. Here again, however, it was noticed that salts tended to give a higher pressure; it was difficult to construct a semi-permeable diaphragm, however, which would resist the passage of salt molecules, while allowing those of water to pass freely. Lastly, Arrhenius, of Stockholm, had shown that the conductivity of salt solutions for electricity may be explained on the assumption that when a salt, such as KNO₃ is dissolved in water, it dissociates into portions similar in number and kind to those it would yield if electrolyzed (and if no secondary reactions were to take place). Such portions (K and NO₃, for example) had been named ions by Faraday. The conductivity of such solutions becomes greater, per unit of dissolved salt, the weaker the solution, until finally a limit is reached, after which further dilution no longer increases conductivity. Now Van’t Hoff united all these isolated observations and showed their bearing on each other. Stated shortly, the hypothesis is as follows: When a substance is dissolved in a large quantity of a solvent, its molecules are separated from each other to a distance comparable with that which obtains in gases. They are, therefore, capable of independent action; and when placed in a vessel the walls of which are permeable to the solvent, but not to the dissolved substance (“semi-permeable membrane”), the imprisoned molecules of the latter exert pressure on the interior surface of these walls as if they were gaseous. Van’t Hoff showed the intimate connection between this phenomenon and the depression of freezing-point and the use of vapor pressure already alluded to. He pointed out further that the exceptions to this behavior, noticed in the case of dissolved salts, are due to their “electric dissociation,” or “ionization,” as it is now termed; and that in a sufficiently dilute solution of potassium nitrate, for example, the osmotic pressure, and the correlated depression of freezing-point and rise of boiling-point, are practically equal to what would be produced were the salt to be split into its ions, K and NO₃. These views were vigorously advocated by Ostwald, professor at Leipzig, in his Zeitschrift für physikalische Chemie, and he and his pupils have done much to gather together facts in confirmation of this theory, and in extending its scope.

It must be understood that the ions K and NO₃ are not, strictly speaking, atoms; they are charged atoms; the K retains a +, and the NO₃ a − charge. On immersing into the solution the poles of a battery, one charged + and the other −, the + K atoms are attracted to the − pole, and are there discharged; as soon as they lose their charge they are free to act on the water, when they liberate their equivalent of hydrogen. Similarly, the − NO₃ groups are discharged at the + pole, and abstract hydrogen from the water, liberating an equivalent quantity of oxygen. Thus the phenomenon of electrolysis, so long a mysterious process, finds a simple explanation. The course of ordinary chemical reactions is also readily realized when viewed in the light of this theory. Take, for example, the ordinary equation:

AgNO₃.Aq + NaCl.Ag = AgCl + NaNO₃.Aq;

i.e., solutions of silver nitrate and sodium chloride give a precipitate of silver chloride, leaving sodium nitrate in solution. By the new views, such an equation must be written: