Paragraph 6
No incombustible dust shall be used for the purposes of these regulations unless in tests carried out at intervals not exceeding ninety days it is found to comply with the following requirements, that is to say—
it is of such fineness that of the dry dust which passes through a 60-mesh sieve of a specification approved by the Minister not less than fifty per cent. by weight and not more than seventy-five per cent. by weight, or such larger percentage as may be authorised by an inspector by notice served on the manager, shall pass through a 240-mesh sieve of a specification so approved; and
it is of such character that it is readily dispersable into the air and, if in use in places where it is not directly wetted by water from the strata, it does not cake but is dispersed into the air when blown upon by the mouth or by a suitable appliance.