rig the pulsations of water result in each slug of water impacting the coal face 104 to effect a momentum transfer which compresses the coal face locally with the rebound from this compression causing failure in tension in the form of mechanical spalling which results in coal separation from the mine face and comminution of the coal the same result can be accomplished by the explosion of a drop or pellet at the mine face the rate of injection of the droplets 110 or other small explosive charge into the water stream can be timed to equal the natural frequency of vibration of the coal face 104 so that larger vibrations of the coal face are produced to more easily separate the coal the detection of the natural frequency of the vibration of the coal face can be accomplished by a variety of means such as by maximizing the amount of coal comminuted by the apparatus and pumped to the surface the impact of each slug of water on the mine face or explosion thereat also helps to separate the coal from the mine face by driving pressured water into microcracks 134 in the coal the pressured water in the microcracks tends to break up the coal immediately after the pressured water slug has hit the coal face and the outside pressure has dropped the use of liquid carbon dioxide results in an additional effect aiding in the break up of the coal carbon dioxide and water form a solvent which dissolves coal by driving gaseous co 2 gco 2 and water into microcracks of the coal face the fluid in the microcracks acts chemically on the coal face.