between the Atomic Weights. 389 
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 PCls and PSs, but also 
PCls 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 NH: has basic properties, PHs, AsHs, and SbHs have 
not, he can be shown that the loss of basic properties in passing 
from NH; to PH; corresponds to a decrease in the strength of 
the affinity between the elements, and that if in PHs, SbHs, or 
AsHs, atoms of methyle, ethyle, or other organic radicals analo- 
gous to hydrogen, are substituted for the hydrogen atoms, and 
more stable compounds thus obtained, strong bases are the result. 
The other series would afford similar illustrations, and, from my 
own experience, I am confident that no teacher who has once 
used a classification of the elements like that here proposed, 
would ever think of attempting to teach chemistry without its aid. 
Classifications of the elements, more or less complete, have 
been given by many authors; 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 
fication into metals and metalloids, which separated phosphorus 
and arsenic, sulphur aud seleninm, because arsenic and selenium 
or a zodlogist to separate the ostrich from the class of birds 
catise it cannot fly, would not be more absurd, than it is for a 
chemist to separate two essentially allied elements, because one 
has a metallic lustre and the other has not. Yet it is surprising 
to see how persistently this classification is retained in every ele- 
Mentary work on the science; and if it is sometimes so far modi- 
as to transfer elements analogous to selenium and arsenic to 
the class of metalloids, this is only acknowledging the worthless- 
hess of the principle, without being willing to abandon it. I 
there were any fundamental property common to all the elements, 
the law of whose variation was known, this might serve as the 
is of a correct classification. Chemistry, however, does not 
as yet present us with such a property, and we must, t erefore, 
here, as in other sciences, base our classification on general analo- 
gies. The most fundamental of all chemical properties is, un- 
Seems, Vol. XVII, No. 51.—May, 1854. 50 
