They may be roughly divided into two great classes, the technical and the popular.
The former are indispensable to the scholar, or the expert, and in the rapid advancement of science are the only real sources of information.
Text-books or treatises are out of date before published; therefore for a correct present view, or a complete history of the development of any science, the technical reviews and society transactions must be consulted.

These will be the principal part of a scientific library, and should be in the large public and college libraries in order to cover advanced study.
They have, on the other hand, little place in small libraries--they would seldom be of use, and are very expensive.
"But the popular periodicals every library needs.

In the better class of these reviews it is possible, if we know where to look, to find several articles on both sides of almost any subject.

Furthermore, these are often written by the foremost authors or scientists, and are in a language intelligible to all.

The amateur cannot give the time or patience to wade two-volume deep in the subject his club wishes him to treat in half an hour's speech.