"After a struggle of twenty minutes... Branwell started convulsively, almost to his feet, and fell back dead into his father's arms..."

From the Treasure Trove: Branwell's funeral card.

"After a struggle of twenty minutes... Branwell started convulsively, almost to his feet, and fell back dead into his father's arms..."

(p.670, The Brontes, Juliet Barker) facebook/Bronte-Parsonage-Museum
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The cause of Branwell's death was stated in his death certificate as chronic bronchitis and marasmus (wasting of the body). Charlotte said of her brother "I seemed to receive an oppressive revelation of the feebleness of humanity; of the inadequacy of even genius to lead to true greatness if unaided by religion and principle. When the struggle was over.....all his errors, all his vices, seemed nothing to me in that moment....he is at rest, and that comforts us all. Long before he quitted this world, life had no happiness for him." bronte