What is sodium hyposulphite, or thiosulphate, a solution of which is used as a bath to wash out the unchanged silver salts in a picture. [Colloq.] called?

hypo — Sodium hyposulphite, or thiosulphate, a solution of which is used as a bath to wash out the unchanged silver salts in a picture. [Colloq.]. (Webster's 1913.)

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