250 PROGRESS 11^ CHEMISTRY. 



caffeine, the essential principles of cocoa and tea; alizarine and indigo, 

 valuable dyestufl's, and several of the alkaloids, bitter principles con- 

 tained in plants, of g-reat medicinal value. 



THE EFFECT ON INDUSTRIAL PROGRESS. 



They have led, too, to the discovery of man}'- 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 formulae, however, can evidently not represent the true con- 

 stitution of matter, inasmuch as the atoms are imagined to lie on a 

 plane, whereas it is evident that they must occupy space of thi'ee 

 dimensions and possess the attributes of solidity. The conception 

 which led to the formulation of such views w^as due first to Pasteur, in 

 his later years director of the institute known by his name at Paris, 

 and more directly to LeBel (Bulletin de la Societe Chimique de Paris, 

 1874) and Van't Hoff (Voorstel tot Uitbreiding der Structuur-Formules 

 in de Kuimte, lS7-i), 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 crj^stal. 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 show^ed 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, simidtaneously 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 tri- 

 angular base. An idea can best be got by a model, easily made by 

 laying on a table three Inciter 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 center 

 of such a tetrahedron an atom of carbon is supposed to be placed. 

 Marsh gas, CH^, is supposed to have such a structure, each corner, or 

 solid angle of the structure (of which there are four), being occupied 



