In order to characterize a morbid lesion,' Laennec explains, 'it is usually enough to describe its physical or perceptible characteristics, and to indicate the course it takes in its development and in its terminations'; at most, one has time to use certain `chemical reactions' only if they are very simple and intended to 'reveal certain physical characteristics'; thus one may heat a liver, or pour an acid onto a degenerescence of which one is not sure whether it is fatty or albuminous.s'