306 MODERN SCIENCE READER 



found, and from these, in turn, rare elements have been 

 obtained. Altogether, between seventy and eighty ele- 

 ments have been found. Additions are made to the list 

 from time to time ; and, occasionally, one of the substances 

 supposed to be an element is found to be capable of decom- 

 position, and it therefore becomes necessary to strike it 

 from the list of elements. 



Out of these simplest forms of matter everything that 

 we see or feel, or are in any way cognizant of, is made up. 

 But now arises the deep question: What is an element? 

 To this question chemists are not able to give an answer. 

 The relations of the elements to one another form one of 

 the unsolved problems of chemistry. It may be that they 

 are not related at all, but that each one is an independent 

 form of matter. There are, however, indications of family 

 relationships between them that have long been the subject 

 of investigation. The elements fall into groups, the mem- 

 bers of which resemble one another very closely in some 

 respects. Thus, for example, phosphorus and arsenic con- 

 duct themselves, in general, alike toward other elements. 

 They combine with them to form compounds that are very 

 much alike so much so that in some cases it is difficult 

 to tell them apart. These elements are said to belong to 

 the same family. The family traits are easily recognized 

 in them. Similar relationships are met with throughout 

 the entire list of elements. This subject has been beauti- 

 fully worked out by the Russian chemist Mendel eef and 

 the German, Lothar Meyer. The former, indeed, pointed 

 out, thirty years ago, that some of the families are not 

 complete. There were a number of vacant chairs. He 

 was able to predict the discovery of some of these missing 

 members and to describe them in detail. Three of these 

 have since been discovered, and they have been found to 

 answer the description given by Mendeleef before their 

 discovery. Now that the way has been pointed out, it is a 

 comparatively simple thing to predict the discovery of 

 other elements, The vacant chairs are there, but though 



