60 POPULAR SCIENCE MONTHLY. 



ally. One form, dissolved in water, twisted a ray of polarized light 

 to the left, the other produced a rotation to the right, while the 

 crystals of the two acids, similar in all other respects, also showed 

 a right- and left-handedness in the arrangement of their planes. 

 The crystal of one variety resembled the other as would its reflec- 

 tion in a mirror — the same, but reversed. These differences, dis- 

 covered by Pasteur as long ago as 1848, the theory of valence could 

 not explain; to interpret them, and other similar cases, the arrange- 

 ment of the atoms in space had to be considered. 



In 1874 two chemists, Van t'Hoff and Lebel, working inde- 

 pendently, offered a solution of the problem, and stereochemistry, 

 the chemistry of molecular structure in three dimensions, was 

 founded. They proposed, in effect, to treat the carbon atom essen- 

 tially as a tetrahedron, the four angles corresponding to the four 

 units of valence or bonds of affinity. They then studied the link- 

 ing or union of such tetrahedra, and found that with their aid the 

 formulae for tartaric acid could be developed in different ways, 

 showing right- and left-handed atomic groupings. Other similar 

 compounds were equally explicable. Thus the definite conception 

 of a tridimensional, geometric atom led to a new development of 

 structural formulae, from which many discoveries have already 

 proceeded. The fruitfulness of the speculation vindicates its use, 

 but it is only the first step in a method of research which must in 

 time be applied to all of the chemical elements. Probably the 

 study of crystalline form will be connected with these chemico- 

 structural expressions, and from the union some greater generaliza- 

 tions will be born. From the geometry of the crystal to the geom- 

 etry of the molecule there must be some legitimate transition. 

 With all their utility, our present conceptions of chemical structure 

 are incomplete; they represent only portions or special phases of 

 some great general law, but so far as they go, properly used, they 

 are valid. 



But light is not the only physical force involved in chemical 

 changes; heat and electricity are far more important. Heat, in 

 particular, is essential to every chemical operation; it provokes 

 combination and effects decomposition; it appears in one reaction 

 and vanishes in another; apart from thermal phenomena the sci- 

 ence of chemistry could not exist. From the very beginnings 

 of chemistry this interdependence has been recognized, and its 

 study has led to notable discoveries and to great enlargements of 

 resource. In the theory of phlogiston the connection between heat 

 and chemical change was crudely stated, and when Lavoisier saw 

 that combustion was oxidation, thermochemistry began to exist. 



In every chemical change a definite amount of heat is either 



