BETWEEN THE ATOMIC -WEIGHTS. 237 



told, that, if in these symbols of the Nitrogen compounds he replaces N by P, As, or 

 Sb, he will obtain symbols of similar compounds of Phosphorus, Arsenic, and Anti- 

 mony ; for he thus learns, once for all, the mode of combination of all four elements, 

 so that when he comes to study the properties, in turn, of Phosphorus, Arsenic, and 

 Antimony, he has not to learn with each an entirely new set of facts, but finds the 

 same repeated with only a few variations. Moreover, these very variations he will 

 learn to predict, if he is shown that the elements are arranged in the series according 

 to the strength of their electro-negative properties, or, in other words, that their affin- 

 ities for Oxygen, Chlorine, Sulphur, etc. increase, while those for Hydrogen decrease, 

 as we descend. He wUl then readily see why it is that, though Nitrogen forms NO^ 

 and NO5, it forms only NCI3 and NS3, and that this reason is correct he will be pleased 

 to find confirmed when he learns that Phosphorus, which is more electro-positive 

 than Nitrogen, and has, therefore, a stronger affinity both for Chlorine and Sulphur, 

 forms not only PCI3 and PS3, but also PCI3 and PS^. Again, he will not be surprised, 

 after seeing the affinity of the elements for Hydrogen growing constantly weaker as 

 he descends in the series, to learn that a compound of Bismuth and Hydrogen is not 

 certainly known. Should he inquire why, though NH3 has basic properties, PH3, 

 AsHg, and SbHg have not, he can be shown that the loss of basic properties in passing 

 from NH3 to PH3 corresponds to a decrease in the strength of the affinity between the 

 elements, and that if in PH3, SbH3, or AsHs, atoms of Methyle, Ethyle, or other organic 

 radicals analogous to Hydrogen, are substituted for the Hydrogen atoms, and more 

 stable compounds thus obtained, strong bases are the result. The other series Avould 

 afford similar illustrations, and, from my own experience, I am confident that no teach- 

 er who will once use the classification of the elements here proposed, or one similar 

 to it, will ever think of attempting to teach Chemistry without its aid. 



Classifications of the elements, more or less complete, have been given by many au- 

 thors ; but the fact that no one has been generally received, is sufficient to prove that 

 they are all liable to objections, and would, indeed, also seem to show that a strictly 

 scientific classification is hardly possible in the present state of the science. The 

 difficulty with most of the classifications is, undoubtedly, that they are too one-sided, 

 based upon one set of properties to the exclusion of others, and often on seeming, 

 rather than real resemblances. This is the difficulty with the old classification into 

 metals and metalloids, which separated Phosphorus and Arsenic, Sulphur and 

 Selenium, because Arsenic and Selenium have a metallic lustre, while Phosphorus 

 and Sulphur have not, though there could hardly be found another point of diflference. 

 For a zoologist to separate the ostrich from the class of birds because it cannot fly. 



