Popular Science Monthly/Volume 35/October 1889/The Chemist as a Constructor
|THE CHEMIST AS A CONSTRUCTOR.|
ONE of the most attractive branches of modern chemistry comprises the artificial preparation of compounds existing preformed in nature, or, in other words, the imitation of the works of creative power. Synthesis, as this section of chemical investigation is called, although it has already attained a considerable degree of success, is of but recent origin compared with analysis, or those researches by which we become acquainted with the composition of the products of nature, and of what we derive from them by industrial processes. It is an indispensable condition, before learning how to compound a body, to know what are its constituents, what their properties are, and by what agents they are most liable to be brought into combination with each other. Therefore, synthetical processes could only be founded upon the results of analytical investigations. It is chiefly to the thorough knowledge of the properties and affinities of the seventy so-called elements that we owe the innumerable discoveries which have raised chemistry to its present important position, together with the insight into the manifold changes and metamorphoses which terrestrial matter has undergone in past times, and which it still undergoes, and into the processes active in vegetable and animal organisms.
The events preceding the discovery of the composition of water afford a striking instance of how many difficulties had to be overcome from the very first observations on the chemical nature of this body—ubiquitous on the surface of the earth—to the ascertainment of its composition, and to our ability voluntarily to prepare it. In the middle ages the doctrine of Aristotle was predominant, that all matter consisted of four elements—air, fire, earth, and water—difference in properties being ascribed only to the varying proportions in which these elements were present. Not much more was known of its physical and chemical characters, but that it may be brought into a solid state by cold and volatilized by heat, and that it offers a good solvent for many substances. Paracelsus, a prominent physician and chemist of the sixteenth century, found that, on treating iron with sulphuric acid, a gas is given off. Boyle, in 1672, discovered this gas to be inflammable; thirty years later, its detonating properties in contact with air became known; but not until Cavendish, in 1766, devoted himself to the exact study of this gas was there any conjecture established on the relations existing between it and water. In 1787 Cavendish made the discovery that, by combustion of this gas in air, water is generated; but, prejudiced by the chemical theories then prevailing, he failed to explain the process in the right way. We are indebted to Lavoisier for a correct definition of the changes taking place in the combustion of hydrogen, which name he gave to the gas in question, signifying a body from which water may be generated by uniting it with oxygen. Thus Lavoisier, supported by the discovery of oxygen by Priestley and Scheele in 1774, became the originator of chemical synthesis. It is a trifling experiment nowadays to demonstrate the formation of water by placing an inverted glass over a jet of burning dry hydrogen, when a dew of water will cover the sides of the vessel and gradually gather into drops.
A rapid advance in synthetical knowledge took place during the third and fourth decades of this century, the artificial preparation of a long series of organic compounds becoming known; and it is a surprising fact, although the chemistry of the carbon compounds, or organic chemistry, was in an infantile state at that time, while most mineral bodies were pretty well known as to their composition and character, that the manufacturing of the former with all their physical and chemical properties was successfully performed, while the imitation of minerals in their peculiar structure and appearance frequently met with unsurpassable difficulties. Even our most modern expedients do not enable us to imitate more than a few well characterized and crystallized minerals regarding shape, luster, and other physical properties. We can build up the carbonates of calcium, iron, and manganese from their elements, but we lack the means to give them the rhombohedral form in which they are naturally found. It was only in the course of the last year that Krontschuff made known the first method of crystallizing silica in the hexagonal form of quartz; and that Fremy and Meunier succeeded in gaining real rubies and spinels by a melting process. It also required long years of incessant experimenting to find out a way of manufacturing the splendid blue coloring matter, ultramarine, as an approximative imitation of lapis lazuli. We should be at a loss, if requested to prepare crystallized manganic binoxide, or calcium triphosphate, or most other crystallized compounds spread throughout the rocky schists of the earth.
Asking for the reason of this insufficiency of our chemical faculties, we find it to be the impossibility of providing, through a sufficient length of time, those conditions of heat, pressure, and other circumstances which prevailed and were of influence when such compounds were separating from molten masses of mineral matter, or from saturated solutions, the composition of which will always remain concealed from us. Organic substances, on the contrary, of the most various kinds, are continually formed and decomposed in the bodies of plants and animals, very readily combining and separating under conditions which exist everywhere, or which may easily be induced. The extraordinary mutability of the compounds of carbon with hydrogen and oxygen is a feature particular to this element, not equaled by those of any other. Their liability to chemical changes enables us voluntarily to build up and to reconstruct carbon compounds occurring in organisms as well as those derived from them.
Alcohol, one of the best-known products of chemical industry, may serve as evidence to what degree of perfection the composition and decomposition of chemical compounds has been brought. As the chief constituent of intoxicating beverages, alcohol, together with carbonic acid, originates by fermentation from sugar; but this is not the only possible way to produce it. The brightness of electric lights, by which public places, roads, stores, etc., of our cities now are illuminated at night, is emitted by an electric current passing between two carbon points. When such a passage of electricity takes place in a glass balloon filled with hydrogen, the electric current causes this gas to unite with carbon, forming acetylene, a gaseous compound, which in contact with more hydrogen readily takes it up, forming a second gaseous compound— ethylene—which, is the chief light-giving constituent of illuminating gas. Ethylene, when brought into contact with sulphuric acid, forms a liquid combination, and this when treated with potassium hydrate is converted into alcohol. Having thus built up from its elements a substance formerly known only as a product of fermentation, we may proceed at once to decompose it again into its elements. We can easily regain the carbon which it contains, by heating alcohol with sulphuric acid, which again converts it into ethylene; and this gas, when mixed with chlorine gas and lighted, burns away, leaving carbon, which as a dense black smoke fills the vessel.
An event very encouraging and helpful to synthetical investigations was the artificial preparation of urea, a product of secretion in animal bodies, resulting from the decay of muscle, and one of the most important substances in animal exchange of matter. When Woehler, in 1828, found out that, by a chemical process, it can be composed with all its physical and chemical properties, this event gave a tremendous shock to the foundations of the doctrine formerly believed, that a "vital power" governed the functions of the organs of living animals, independently of physical as well as of chemical forces. The discovery of artificial urea was followed by others in an uninterrupted series, which, besides the practical interest they were entitled to claim, threw a new and clear light upon many processes in organic life. In glancing at some of them, we confine ourselves to cases of more general interest.
A conspicuous instance of the degree to which synthetical chemistry has enabled us to imitate nature in some of the processes going on in the bodies of plants and animals is represented by the changes which salicin undergoes. It is to this white and crystalline compound—belonging to the chemical group of glucosides—that the leaves of willow and poplar trees owe their bitter taste. Several species of Spirœa, while young, also contain salicin, which, during growth, is converted into a volatile oil of reddish color—salicylic aldehyde—an oil which, remarkably enough, is also produced from salicin in the body of the larvæ of Chrysomela populi, a beetle feeding on the leaves of poplar-trees. In Spirœa, as well as in other plants containing this oil, it is partly transformed into salicylic acid, which in its turn in Gaultheria procumbens and Betula lenta combines with methyl to form a product known as "wintergreen-oil." Now by synthesis we can artificially reproduce all these changes, though pursuing quite a different way from that which Nature follows. We can convert salicin into salicylic aldehyde; we can transform this into salicylic acid, and we can produce wintergreen-oil by combining this acid with methyl. We can even manage to prepare salicylic acid and wintergreen-oil from coal-tar, a substance which, as everybody may judge by the way of its production, is not likely to contain any ingredients found in living plants. The preparation of salicylic acid from the products of coal-tar was discovered by Kolbe about twenty years ago, inducing a more thorough study of the properties of this acid, from which it was found to be one of the most valuable remedies for rheumatic complaints and for gout. Thus one discovery often becomes the source of a whole series of new ones, and may prove a blessing to mankind in the most unexpected and various ways.
Few people know what xanthin is. The name, indeed, represents a body of neither commercial nor industrial significance. Scarcely anybody else but chemists and physicians knows that it is a substance which, in a small amount, is found in muscles, in the liver, brain, and certain other organs of the animal body. But little, therefore, does he who enjoys a cup of cocoa, coffee, or tea, fancy that the beneficent, animating effect of these beverages is due to the methyl compounds of xanthin, contained as theobromine in cocoa-beans and as caffeine, in coffee-beans and in the leaves of tea and several other plants. Both theobromine and caffeine can readily be prepared from xanthin, the products having exactly the same physiological effect as the natural compounds.
The line of products of organic life which have been built up artificially from their constituents includes representatives of many groups of compounds, although they are not equally numerous in all of them. A large number of vegetable acids may be synthetically prepared. The volatile oils of bitter almonds and mustard, as well as the coloring matters indigo and alizarin, besides being prepared from plants, are obtained from other sources by chemical processes; but, since their original production depends on fermentative actions, to which the material is subjected, they can not justly be classed among natural products. In some groups of natural organic compounds our efforts to obtain them by synthesis have hitherto almost utterly failed. Our knowledge of alkaloids, many of which, by their great physiological effects, are of prominent therapeutic importance, has advanced so far as to permit us to convert some of them into others—for instance, to transform morphine into codeine; but, with the exception of conine, which Ladenburg claims to have synthetically obtained, and conhydrine, prepared by Hoffmann, both of which are contained in hemlock (Conium maculatum), no success of consequence has been registered. Nevertheless, as the knowledge of their chemical structure has been cleared up to a very considerable degree, we may expect that, by continued researches, ways for their artificial manufacture will be found out.
Many chemical discoveries were made by accident; compounds of valuable properties were found by researches undertaken for other purposes; the knowledge of the making of china and of the separation of phosphorus resulted from experiments intended for producing gold. But the principal successes of modern science in general, and of chemistry in particular, were obtained in a speculative, inductive way, the only one to be considered as actually scientific. With positive surety the astronomer, from the movements of the stars, and from the attracting forces which they thereby manifest, can ascertain the presence of another star which has never been observed before. He may prophesy a solar eclipse to the accuracy of a minute, and many years before predict the return of a comet. In a similar way exact chemical knowledge often enables us to foretell the formation of certain compounds hitherto unknown, and to define the properties they may be expected to have. It was in this way that, the composition and chemical structure or arrangement of atoms in the molecule of conine and conhydrine having been explored, their preparation was likewise effected, the operators being guided by logical inferences. This scientific way of proceeding proved successful in numerous cases, and led to some surprising results in the course of the last year.
Not many years ago what was known regarding the source from which common plants draw their food consisted in the recognized fact that carbonic acid and water, both abundant in air and fertile soil, are taken up by the roots, converted into sugar by an unknown process, the sugar afterward being transformed into cellulose, the matter chiefly constituting the body of the plant, and into starch. It was also known that oxygen was set free in the course of these changes. In 1870 Baeyer promulgated a theory, explaining how assimilation of the mentioned substances might be effected. He demonstrated the possibility of "formaldehyde" being produced from carbonic acid and water, which is only possible, if—as is the case—oxygen is liberated. All plants in daylight exhale oxygen and absorb carbonic acid. Formaldehyde, a gaseous compound, is, as aldehydes in general are, very liable to condense to solid compounds by accumulating a greater number of atoms into one molecule. Baeyer expressed his belief that sugar, the composition of which agrees with that of formaldehyde multiplied by six, is the product of such a condensation.
The first signs that sugar might result from such a condensation, when conducted in the proper way, were observed by Butlerow, but since he claimed to have prepared a sugar-like compound from formaldehyde, all the experiments undertaken to the like purpose had proved futile. It was Loew who in 1886 succeeded in preparing a more concentrated solution of formaldehyde than could be made before. He found that the vapor of wood-spirit in contact with heated oxide of copper furnishes formaldehyde in abundant quantities. Moreover, he found that condensation of this aldehyde to sugar is easily achieved by digesting a solution of it with slaked lime. The product, to which he gave the name of formose, has exactly the composition of grape-sugar; it has a sweet taste, and acts on Fehling's solution as sugar does; the resemblance extends to several further properties; but still there are some slight points of difference, which have caused a few chemists to raise objections as to its classification among sugars. The question of the formation of sugar from aldehydes would perhaps have remained undecided for the present, had not recent experiments, made by Fischer and Tafel, confirmed the statements before mentioned by giving evidence of the formation of sugar by condensation from other aldehydes. Their statements were supported by Grimaux, who, by subjecting glycerin to the oxidizing influence of finely divided platinum, obtained a substance resembling grape-sugar in all its properties, which in contact with yeast even undergoes fermentation, producing alcohol and carbonic acid, and hereby manifesting the character of a true sugar.
These results not only enable us to prepare by a chemical process this substance, formerly only known to be produced by living plants, but they also afford important facts and proofs which justify us in expecting the synthetical formation of other compounds playing a part in the vegetation of plants, thereby acquiring an insight into those complicated phenomena of organic life which science hitherto has in vain tried to explain. By perfecting our comprehension of natural processes we become more and more enabled to utilize them for the advancement and the welfare of mankind—an attainment which constitutes the chief aim and purpose of natural science in general.
- The chemical changes in question are represented by the equations:
Carbonic acid and water = formaldehyde and oxygon
CO2 + H2O = CH2O + O2
Six formaldehydes, 6CH2O = C6H12O6, grape-sugar.