A capsule (1) for insertion into a bore of a porous element to apply treatment substance to the interior of the element and having a container (2), with at least one internal cavity (3) for receiving treatment substance and an outer stop flange (8), and a plug (4) received in and fixed to the container through a common interconnecting land (5). A first container shank portion (6) that extends from a first insertion end (1A) of the capsule and up to the area of the interconnecting land has a uniform, generally cylindrical shape, a second container shank portion (7) extending away from said container first shank portion, from the area of the inter- connecting land towards the outer stop flange, at least at an inner surface (7B) thereof has an outwardly slightly diverging, conical shape and at an outer surface (7 A) thereof, separated a short distance from the outer stop flange has a first circumferential beveled edge (9) serving as a transition to an outer increased diameter section (7C). A method for applying
