AKCHITECTUKE OF MOLECULES 227 



perhaps the most important result of all is the proof of the physical 

 existence of the atom as an independent entity. 



The atomic Theory of Dalton formulated about 1808 was 

 for half a century or more employed by the chemical world with 

 reservations, and so late as 1856 it was referred to in a prominent 

 English textbook as "at the best but a graceful, ingenious, and, 

 in its place, useful hypothesis." In 1869 Professor Williamson, 

 then President of the Chemical Society, thought it necessary to 

 lecture the Society on the position of the atomic theory at that 

 time. He pointed out " that on the one hand all chemists use 

 the atomic theory, and that, on the other hand, a considerable 

 number of them view it with distrust, some with positive dis- 

 like." And so lately as 1904 Professor Ostwald in the Faraday 

 lecture endeavoured to show that the theory of chemistry was 

 independent of the atomic theory. 



The attitude of chemists since the time of Williamson's lecture 

 has certainly been very different from that which he described. 

 Since 1872 the facts and theories connected with stereo-chemical 

 ideas have become so consolidated by experience that the 

 revolutionary notions introduced by Ostwald in 1904 came as 

 a surprise, and it may be confidently stated were never accept- 

 able to anyone. 



All wavering, uncertainty, and distrust must now disappear, 

 for, though it cannot be said that the separate atoms in a crystal 

 have been seen by human eye, their effects have been recorded 

 on a photographic plate, and their presence and separate physical 

 existence are as well established as the existence of the crystal 

 itself. 



Everything then proves that matter is made up of discrete 

 particles which cling together under the action of the various 

 forces cohesion, adhesion, and chemical attraction. The 

 separate particles are the atoms, which in a substance called an 

 element are all alike, while in a compound there are two or more 

 different kinds joined together in a certain order. As to the atoms 

 a hypothesis which has already been explained (Chapter X) has 

 been formed concerning their constitution. They are supposed 

 to consist of a large number of electrons embedded in or surround- 

 ing a nucleus of positive electricity. But outside this structure 

 there appear to be attached and detachable 1, 2, 3, 4, 5, or 6 

 (rarely 7 or 8 ?) electrons which correspond to the valency of 

 the atom, and hence its sphere of influence in forming chemical 



