388 J.P. Cooke on the Numerical Relation 
phorus and sulphur, which are not chemically allied, are frequently 
placed consectively, while arsenic, avtimony, and bismuth in spite 
of their close analogies with phosphorus, are described in a differ- 
ent part of the book. This confusion, which arises in part from 
retaining the artificial classification of the elements into metals 
and metalloids, is a source of great difficulty to the learner, since 
it obliges him to retain in his memory a large number of appat-_ 
ently disconnected facts. In order to meet this difficulty, a classi- 
fication of the elements into six groups, differing but slightly 
from that given in the table accompanying this memoir, was made. 
The object of the classification was simply to facilitate the acqui- 
sition of chemistry, by bringing together such elements as were 
allied in their chemical relations considered collectively. As the 
classification has been in use for some time in the courses of lec- 
tures on chemistry given in Harvard University, I have had an 
opportunity for observing its value in teaching, and cannot but 
feel that the object for which it was made has been in a great 
easure attained. The series which is headed the Six Series 
will illustrate the advantage gained from the classification ina 
difficulty can, however, be in a great’measure removed, if, after 
he has been taught that nitrogen forms two important acids with 
oxygen, NO: and NOs, that it unites with sulphur and chlorine 
form NSz and NCls, and also with three equivalents of hydro- 
gen to form NHs, he is also 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 ant- 
mony ; for he thus learns, once for all, the mode o combination 
of all four elements, so that when he comes to study the proper 
ties, 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 varl- 
ations he will learn to predict, if he is shown that the elemen!s 
are arranged in the series according to the strength of thelr elec- 
tro-negative properties, or, in other words, that their affinities or 
oxygen, chlorine, sulphur, etc. increase, while those for hydroge? 
decrease, as we descend. He willjthen readily see why 1t Is that, 
though nitrogen forms NO; and NOs, it forms only NCI: a 
NS:s, and that this reason is correct he will be pleased to 
