A “capillary valve” having no moving parts, and suited for use in corrosive media has been designed and developed. The valve permits the bleeding of a gas into a liquid region if a certain pressure difference is exceeded and also prevents the counterflow of liquid. The valve consists of a porous metallic disk or plug that has small-diameter interconnected pores that average between 0.1 and 5 μ and have the essential property of being wet by the liquid involved.