ELISHA MITCHELL SCIENTIFIC SOCIETY. 79 



period of seventeen has only four known elements in it 

 and the fifth has only two. Of the five periods only 

 one is completely filled out. To say the least, this 

 shows a very imperfect knowledg-e of the elements, or 

 a great deal of guess work. In the table there are 

 sixt3^-four known elements and thirty-five blanks for 

 elements yet to be discovered. I hardly think it pos- 

 sible that the majority of chemists believe that after all 

 of our diligent search for the past centur}" less^ than 

 two-thirds of the elements have been discovered. 

 Where are the others in hiding? Will the}' be discov- 

 ered b}' the spectroscope in the rare earths? There is 

 certainly hope of finding some but the number to be 

 found is appalling. The average student thinks, in all 

 honesty, that the coincidences of the first part of the 

 table wmU scarcely justif}' such forcing and wholesale 

 interpolation. If our knowledge of the elements is as 

 imperfect as that, we have no right to force them into 

 periods. Some of them seem little inclined to fall into 

 these periods of their own accord. How do we know 

 that the remaining two-thirds ma}' not upset the entire 

 calculation? Certainly we are venturing a good deal 

 upon a very imperfect knowledge of the remainder. 

 Let us see how the matter stands. The periodic idea 

 may be true but we do not know enough about these 

 elements 3'et to be able to give this idea a very promi- 

 nent place in the natural law, and we ought to avoid 

 the assumption of so man}' unknown elements unless 

 absoluteh' necessar}'. 



As I do not intend to tear down without some effort 

 at re-building, I would, with much real diffidence, fori 

 realize that I may be looked upon as one who would 

 rush in where onh' the great masters of the science can 

 safely tread, offer the following table as a substitute: 



