100 to about 1000 ohm cm and a sonic velocity from about 1500 to about 1700 meters sec electrolytes can be added to adjust the specific resistivity of the ink usable electrolytes include dimethylamine hydrochloride sodium propanate and sodium acetate it is desirable that the microspheres not settle out for this reason the ink vehicle in which microspheres are suspended must have a specific gravity about equal to or greater than the specific gravity of the microspheres a specific gravity adjusting agent such as glycerol can be used to increase the specific gravity of the carrier vehicle if it is necessary to do so the ink formulations of the present invention are not limited to opaque white but colored inks are also included colorants may be added to the formulation such as diarylide yellow acid yellow 13 solvent yellow 13 acid green 73 solvent red 125 acid green 73 solvent red 25 acid yellow 166 acid blue 260 acid blue 229 and acid black 52 the ink is capable of printing on porous and non porous substrates with good opacity and adhesion ink compositions were formulated by combining the ingredients set forth below the specific gravities of the inks are also set forth below and are compared to the specific gravity of the vehicle alone and the specific gravity of the hollow microspheres ______________________________________ parts by.