684 THE POPULAR SCIENCE MONTHLY. 



the trituration. Sulphuric acid, cortibined with distilled water, 

 disengages a pungent odor resembling that of musk. The odor 

 of musk is brought out in a great many reactions. The nitrate 

 derivatives of aromatic substances smell of it ; artificial musk and 

 natural musk have no chemical resemblance. So alcohols chemi- 

 cally identical, but of different derivation, do not behave alike 

 with essential oils. As odor is thus in a great measure inde- 

 pendent of the chemical constitution, it must depend upon the 

 disposition of the particles, a property which it is evidently im- 

 possible to discover by any known chemical processes. 



A few eminent chemists, following Dalton, Avogadro, and Am- 

 pere, have tried to make up for this impossibility by hypothesis, 

 and have taken up the great problem of predicting and explain- 

 ing chemical combinations and isomerics. Their theories, called 

 atomic, have been adopted in most of the original memoirs and 

 taught in most of the text-books. Whatever may be their scien- 

 tific value, the aids they give him in retaining and recollecting 

 the formulas present incontestable advantages to the student. 

 The applications of them to the study of the aromatic series are 

 famous. 



The radical of the hydrocarbons of this series and of all the 

 other compounds is benzene, a body composed of six atoms of car- 

 bon and six atoms of hydrogen ; when it is attacked by a reagent, 

 and we substitute for an atom of hydrogen another simple body 

 or a group of atoms, whichever of the atoms of hydrogen the sub- 

 stitution may bear upon, we obtain a single product ; whence it is 

 concluded that each atom of carbon is united to an atom of hydro- 

 gen, and that a symmetrical exchange can take place of the atoms 

 of carbon among their valencies. A German chemist, Herr Ke- 

 kule", has tried to express these peculiarities by a hexagonal scheme 

 which has still some lack of symmetry, and M. Ladenbourg has 

 substituted a prismatic scheme for it. In this figure the six 

 atoms of carbon of the benzene occupy the summits of a trian- 

 gular prism, each one being united with an atom of hydrogen and 

 exchanging the three valencies that are left it with the three next 

 atoms of carbon by the three edges which meet at the summit. 

 The perfect symmetry of this scheme is well expressed in the sim- 

 ple construction of the figure. But usually, for greater conven- 

 ience, the hexagonal construction is adopted, and the reciprocal 

 relations of the atoms of carbon and hydrogen are represented 

 by figures in which the more or less complex lateral chains are 

 joined, and which offer the remarkable characteristic of being 

 closed chains that is, of always returning to their starting-point. 

 What the atomic theories have taught us concerning odor is lim- 

 ited to this singular and so far unfruitful representation; it is 

 evident that they are still mute concerning the real structure of 



