Complex sentence: The exterior algebra of Hermann Grassmann, from the middle of the nineteenth century, is itself a tensor theory, and highly geometric, but it was some time before it was seen, with the theory of differential forms, as naturally unified with tensor calculus.
Split into simpler sentences: The exterior algebra of Hermanne Grassmann, from the middle of the nineteenth century, is itself a tensor theory, and highly geometric. It was quite some time before it was seen, with the theory of differential forms, as naturally unified with tensor calculus.