October 18, 1895.] 



SCIENCE. 



507 



closely related and must be treated iu some 

 measure as we treat the hydrocarbons. 

 The degree and nature of the relationship 

 is as yet unkown. 



This idea of the inter-relationship of the 

 elements must be at the bottom of all 

 teaching of the science. I do not think it 

 possible in the present state of our knowl- 

 edge to lay very much stress upon tliat 

 which is called periodicity, nor yet upon 

 the degree of relationship as expressed by 

 the atomic weight differences. 



If too much stress is laid upon them in 

 their incomplete state they may bring into 

 doubt the great truths of the law. 



2. The old division into metals and non- 

 metals, or metalloids, is no longer permissi- 

 ble. It was always most arbitrary and 

 indefensible, except on the grounds of con- 

 venience. It is no longer convenient, and 

 being a false distinction it serves onlj- to 

 obscure the truth. I think some of the 

 hesitation in accepting the Periodic Law has 

 been due to the false ideas springing from 

 this old-time division. 



There is no such clear-cut division be- 

 tween the elements. 



It would be contrary to the fundamental 

 idea of their kinship. They must be taught 

 by groups and the gradation of positive to 

 negative tendencies pointed out along with 

 the change of atomic weight and of valence. 



3. It is clear that, if these elements are re- 

 lated and show a certain gradation in proper- 

 ties and the old idea of their separate and 

 distinct individuality is to be given up, then 

 the proj)er classification for the salts is 

 under the head of the acid which mainly 

 determines their nature and not, as in the 

 old way of teaching, under the head of each 

 metal. Mineralogists long ago seized upon 

 this as the simplest and most natural way 

 of classifying minerals, but chemists have 

 been slow to catch the idea. I can assure 

 my fellow teachers, from my own experi- 

 ence, that time is saved, the subject made 



clearer and the tax upon the memory les- 

 sened by this simple change of classification. 



4. Valence and the gradations in it must 

 be taught according to the natural arrange- 

 ment. We cannot stand back because con- 

 fronted with something which we cannot 

 explain. Many of the facts of the science 

 must be taught as facts leaving the explan- 

 ation to those who are to follow us and to 

 whom many of the things which are mys- 

 teries to us will be made plain. 



It is just as well for the chemist of to-day 

 to acknowledge that with all of the prog- 

 ress, of which he is justly proud, he is really 

 only on the threshold of his science and 

 that he is surrounded by the unknown on 

 every side. What does he know of chem- 

 ical force itself, of the nature of the atoms, 

 of the character of this wonderful relation- 

 ship, of valence and of many other prob- 

 lems? 



I do not think the beginner should be 

 tried too much with discussions of these 

 problems nor with attempted explanations. 



Such explanations are too subject to 

 change. As to periodicitj^, I question the 

 advisabilitj' of laying too much stress upon 

 this feature of the Mendeleeff System in 

 teaching Elementary Chemistry. It is true 

 that the author makes this the first one of 

 the eight conclusions drawn from his Sys- 

 tem but he speaks of it as an ' evident 

 periodicity.' Every chemist who has ex- 

 amined into the matter will admit evi- 

 dences of periodicity, but as the periods ai-e 

 irregular and not fully agreed upon, as the 

 character of the periodicity varies and is 

 unexplained, it is not wise, I repeat, to lay 

 too much stress upon this feature yet 

 awhile. The recurrence of elements of the 

 same properties, that is periodicity, must 

 be mentioned, but I would prefer to impress 

 all this in a general waj' as a dependence 

 of the properties upon the atomic weights. 

 Still, I think, this is largely a personal 

 matter. I do not like to teach with too 



