1903.] Professor Wm. J. Pope on Stereochemistry. 301 



WEEKLY EVENING MEETING, 

 Friday May 1, 1*J03. 



Sir William Cbookes, F.R.S., Honorary Secretary and 

 Vice-President, in the Chair. 



Professor William J. Pope, F.R.S.j 



Recent Advances in Stereochemistry. 



In the year 1803, just a century ago, John Dalton delivered in the 

 Royal Institution a series of scientific lectures during the course of 

 which he doubtless laid before his audience a theory which he had 

 just devised for the purpose of connecting together the vast number 

 of isolated chemical facts known at the beginning of the nineteenth 

 century. This theory, of which the centenary is being celebrated 

 during the present month by the Manchester Literary and Philo- 

 sophical Society, is known as the Atomic Theory, and was destined 

 to form the foundation upon which the whole superstructure of 

 modern chemistry has been built up. For our present purpose, 

 Dalton's theory may be briefly stated in the form of the following 

 two principles : — (1) Every element is made up of homogeneous atoms 

 of which the mass is constant ; (2) Chemical compounds are formed 

 by the union of atoms of the various elements in simple numerical 

 proportions. In accordance with Dalton's hypothesis, chemical sub- 

 stances may be mentally pictured by imagining the atoms as small 

 spheres which have the power of aggregating themselves together 

 under suitable conditions to form complexes or ' molecules ' ; thus, 

 taking two similar spheres representing hydrogen atoms, in con- 

 junction with a sphere of a different kind representative of an atom of 

 oxygen, a chemical representation can be given of the compound 

 water, the molecule of which is composed of two atoms of hydrogen 

 and one of oxygen. 



The original atomic theory offers no explanation of the observed 

 fact that the atoms combine together in different proportions ; this 

 deficiency was remedied by the doctrine of ' Valency ' enunciated by 

 the late Sir Edward Frankland in 1852. Frankland supposed that the 

 atoms of certain elements, such as hydrogen and chlorine, are un- 

 able to combine with more than one atom of any other element ; these 

 elements are termed monovalent. Other atoms, such as those of 

 barium and zinc, can become directly attached to at most two other 

 atoms ; these are the divalent elements. Tri-, tetra-, penta-, hexa-, 

 hepta-, and octa-valent elements can be similarly distinguished, the 

 valency of hydrogen being taken as unity, in order to measure and 



