If filtering paper be dipped in nitric acid of 1.42 specific gravity, or, better, saturated with it and then washed with water, it becomes exceedingly tough without loosening any of its porosity. It permits to be washed and rubbed like a piece of linen, and exhibits a more than three-fold resistance against rupture. If preferred, only the centre part of the filter may be thus treated. The paper absorbs no nitrogen when undergoing this treatment; it parts with ash constituents, and becomes somewhat lighter and contracts.