STEKEO-CHEMISTRY. 557 STEREORNITHES. (see Carbon Compounds) leads to a clear ex- planation of cases of isomerism which, again, cannot be explained by the doctrine of atomic linking alone. In the simplest class of un- saturated compounds one carbon atom is united by tico valencies to anotlicr carl)on atom. In stcreo-cliemistry this means that two carbon tctrahedra have an edge in common, the four atomic groups held by the two carbons being situ- ated at the four free corners of the tetrahedra. Take, for example, the case of two carbon atoms luiited by a doulile bond and holding two hydrogen atoms aiid two carboxyl (COOH) groups. In this case the doctrine of the linking of atoms recognizes the possibility of two isomeric modifications, viz. : H (COOH)
/
C =C / H (COOH) Meth.vlene-raalonic acid H H
/
and C=C / (COOH) (COOH) Maleie and funiaric acidts Stereo-ehemical doctrine recognizes that with the second of these modes of linking two differ- ent arrangements in space are possible, as may be seen from the accompanying diagram, and H (COOH) (COOH)/<?X H, (COOH) (COOH) Maleic acid Fumaric acid better still bv the use of models. As a matter of fact, three unsaturated acids C^jH,) (COOH)^ are known, viz. methylene-malonic acid char- acterized by the first of the above modes of linking, and two acids — maleic and fumaric — both characterized by the second mode of atomic linking, and differentiated from each other by the different configuration of their atoms. Opti- cal activity is of course out of question, because none of the molecules under consideration con- tains an asymmetric carbon atom. But corre- sponding to the difference in configuration are certain dilTercnces in the chemical behavior of maleic and fumaric acids, although the close relationship between the two is indicated by the great readiness with which they are transformed into each other. The most important difference between the two consists in the fact that maleic acid readily forms an anhydride, its two car- boxyl groups losing the elements of water and becoming joined together, thus: H H / / C C
COOH COOH / C
H — H,0=| Maleic acid CO CO ./ c o
/
H Maleic anhydride Fumaric acid, on the contrary, has no anhydride of its own. This indicates that in maleic acid the two carboxyl groups nuist be nearer together than in fumaric acid, and therefore the first of the above two stereochemical fornndas is as- signed to maleic, the second to fumaric acid. See Fumaric and JIai.nic Actus. As to compounds with triple bonds in their molecules (see CARnoN CoMroi nds), tlic verdict of stereo-chemistry is that no more isomers are possible than can be foreseen by the doctrine of atomic linking. This, too, may be best demon- strated by means of models. Hr.STOKY. In conclusion it niay be mentioned that the optical rotary power of a substance in the non-crystalline state (sugar in solution) was first observed by Biot in 1 S 1 5. The opticallj' isomeric tartaric acids were thoroughly investigated by Pasteur, who in 1860 discovered the three known metliods of separating such isomers (see above). Ten years later Wisliccnus studied the isomeric lactic acids, and in a memoir published in 1871 expressed the view that the doctrine of atomic linking was no longer sufficient for the purposes of organic chemistry. Wislicemis's remark at- tracted the attention of Van 't Hoff, who ad- vanced the stereo-chemical theory of optical isomerism in 1874. Meanwhile Le Bel, in Paris, liad undertaken to continue the work of Pasteur, and soon independently arrived at the same con- clusions as Van 't Hoff. Remarkably enough, Le Bel's and Van 't Hofl"'s memoirs appeared within one month. The theory of geometrical isomerism was worked out b.y Van 't Hoff about 1878. At present stereo-chemistry forms quite an exten- sive science, and its theories, to which no ex- ception has yet been found in the case of carbon compounds, are being gradually extended also to the compounds of nitrogen. Consult: Wislicenus, Ueber die raumlichc An- ordnung der Atome, etc. (Leipzig, 1887) ; Hantzsch, Grundriss der Stereochemie (Breslau, 189.3; French trans., Paris, 1896); Van 't Hoff', D!e Lagerung der Atome im Raiime (Bruns- wick, 1894) ; Bischoff, Handbuch der Stereoehe- iiiie ( Frankfort-am- JIain, 1893). Consult also the works on theoretical and organic chemistry recommended in the article Chemistry. See Chemistry: Carbon Compounds: Lactic Acid; Tartauic Acid; Fumaric and Maleic Acids; Valency. STEREOCHROM Y, ster'e-ok'rci-mi ( from Gk. iTTepedi, stereos, solid + xP^I^"-' ehrOma, color). A process of wall-painting, invented in 1896 by Prof. J. N. von Fuchs, of JIunich, and first practiced by the painter Joseph Schlott- hauer. Upon a thin coating of mortar forming a part of the wall, mineral water colors mixed with soluble glass are applied, thus admitting of any part of the picture being retouched, as in the case of oil paintings. A coating of liquid glass is then added to protect it from the effects of the atmosphere. Experience has shown, how- ever, that the surface of the painting cracks, and the process is no longer practiced. STEREORNITHES, ster'e-Or'nithez (Neo- Lat. nom. jil., from Gk. (rrepeSs, stereos, solid + Spws, nrnis, bird). A group of extinct ratite birds of gigantic size whose remains are found in the Miocene strata of Patagonia. The pro- priety of this name as indicative of a distinct group has been criticised, and it is believed that.
