If writers would agree to regard that as the defining relative pronoun, and which as the non-defining, there would be much gain both in lucidity and in ease.
Modern English Usage by H.W. Fowler.
The use of "that" to introduce a restrictive clause and "which" to introduce a nonrestrictive clause remains a device valuable for accurate statement....This distinction is often ignored by even careful writers, but it can be useful in sharpening precise statement in science.
Scientific Style and Format, Sixth Edition. Published by the Council of Biology Editors.