222 Mr. J. F. Heyes on the 



valency of an element, and the so-called atomicity of atoms, 

 the ordinary language of chemical theory is used, it is not 

 because I have any belief in an ultimate solution of the 

 problems on the chemical side, but because, as the problems 

 now stand, the conditions of the great mystery of chemical 

 affinity can best be attacked, and perhaps by chemists can 

 only be attacked, by facing the problems of what has been 

 termed atomicity, valency, and quanti valence. 



There is so much confusion of language on this subject 

 that I must beg leave to make a very elementary start with 

 our three historic typical compounds — HC1, H 3 0, H 3 N. 



Let us most carefully recognize the facts conveyed in these 

 formulas first ; for these facts, if fully ascertained, must remain 

 good when our transitional conceptions of them " have faded 

 into the infinite azure of the past." 



(1) There are the facts of definite composition by weight 

 and volume. The three gas formulae weights occupy the same 

 volume. 



(2) Experience, aided by comparison of composition, has 

 led us to the laws and facts of definite and multiple pro- 

 portion. 



(3) The latter can only, as far as we can see, be explained 

 in terms of the old atomic theory. 



(4) Referring to this theory, Dalton showed us it was 

 possible to obtain the ratio-weights of the atoms. 



The symbol in our formulae has therefore more than one 

 interpretation. It stands for (1) oxygen stuff, (2) an atom 

 of oxygen, and (3) for a number. Here is the material for a 

 confusion of thought which is observable in the history of our 

 science. This has been increased by the natural coincidence 

 of our selection of the lightest stuff known (hydrogen) as the 

 unit to which we refer (1) densities (that is, the ratio-masses 

 of equal volumes), which are absolute facts, (2) atomic weights 

 (that is, the most probable ratio-masses of atoms whose volumes 

 are unknown), which are relative facts. Thus 



but 



A 16 



-r- = -y expresses pure fact ; 



tr = -j- expresses well-grounded belief, 



based upon the cumulative probability resulting from the 

 consideration of many physical and chemical data. It is to 

 be noted, moreover, that the question of the most probable 

 value = 15"96, accepted by many chemists, versus the exact 

 integer 16, does not, so far as I can see at present, affect con- 



