This came out last year, and was discussed on HN. One of the things I said then, which I'll repeat now, concerns these lines: > But many of the subject areas

This came out last year, and was discussed on HN. One of the things I said then, which I'll repeat now, concerns these lines: > But many of the subject areas that experienced a decline in older citations were part of two broader areas: chemical and materials sciences, and engineering. > Consequently, these broad disciplines show almost no increase in old citations. Just why citation trends should differ in these disciplines isn’t clear. If you look at the early history of information retrieval you'll see that chemical documentation was tightly connected with it. Several of the IR pioneers (Luhn, Mooers, Taube) worked with the American Chemical Society or presented at ACS conferences. I like to point out that "information retrieval" as a term was first presented at an ACS meeting. By the 1960s, CAS (the Chemical Abstracts Service of the ACS), the Institute for Scientific Information, and other organizations provided pretty complete indexing of the chemical literature. While it's improved since then, I believe the old literature for chemistry has been searchable longer than for other fields, and in large part because the chemical structure give a uniquely indexable key (usually) for indexing the literature.