LECTURE XV. 



THE FUNDAMENTAL CONCEPTIONS OF CHEMISTRY PHENOMENA OF 

 DISSOCIATION ABNORMAL VAPOUR DENSITIES -CONSTANT OR 

 VARIABLE VALENCY THE DOCTRINE OF VALENCY IN INORGANIC 

 CHEMISTRY THE PERIODIC LAW LATER DEVELOPMENT OF THE 

 DOCTRINE OF AFFINITY SPECTRUM ANALYSIS SYNTHESIS OF 

 MINERALS CONTINUITY OF MATTER IN THE LIQUID AND GASEOUS 

 STATES LIQUEFACTION OF THE SO-CALLED PERMANENT GASES 

 TIIERMO-CHEMISTR-Y ELECTRO-CHEMISTRY PHOTO-CHEMISTRY 

 MOLECULAR PHYSICS MORPHOTROPY. 



HAVING now followed organic chemistry in some of its more 

 recent discoveries, and having obtained a knowledge of its re- 

 markable progress under the influence of the theory of valency, 

 it is appropriate to suggest and to discuss the question whether 

 this theory is capable of serving as a fundamental principle in 

 mineral chemistry ; and also to recount some of the most 

 important results of investigations in general chemistry. 



Before passing on, however, to this part of our task, the 

 theories themselves must be subjected to a more minute con- 

 sideration and scrutiny. In describing how they have come 

 into existence we have not always been able to enter into the 

 exact significance of their fundamental conceptions. We shall 

 now turn our attention to this matter, although, naturally, it is 

 only possible to bring forward the most important points. For 

 the remainder, the reader is referred to the standard text-books 

 of theoretical and general chemistry. 



Our views rest essentially on the precise formulation and 

 distinction of the conceptions of atom, molecule, and equivalent. 



An atom is denned as the smallest indivisible quantity of 

 an element which exists under any circumstances ; and most 

 generally it only exists in combination with other atoms. 



