  A decrease in microhardness along the (010) cleavage in potassium acid
phthalate single crystals by 15--18% after the application of a permanent
magnetic field was revealed for the first time. It is shown that the effect
revealed is of the volume character. The role of interlayer water in the
processes stimulated by a magnetic field is studied., Interlayer water plays
does not cause the observed changes it only plays the part of an indicator of
these changes in potassium acid phthalate crystals in a magnetic field. It is
established that microhardness in the (100) plane of the crystal in an applied
a magnetic field first increases by 12--15% and then remains constant in time
within the accuracy of the experiment. The possibility of varying the crystal
structure of potassium acid phthalate crystals by applying magnetic fields
inducing rearrangement in the system of hydrogen bonds or in the defect
structure is discussed.
