Reversal type electroscopic developer powder

Claim 1

A REVERSAL TYPE DEVELOPER POWDER FOR ELECTROSTATIC PRINTING COMPRISING 50 TO 85 PARTS BY WEIGHT OF A PARTICULATE FRENCH PROCESS ZINC OXIDE HAVING A SURFACE PHOTOCONDUCTIVITY OF ABOUT 10-9 OHM-1/SQUARE/WATT/CM.2 AT A WAVELENGTH OF ABOUT 3900 A., 15 TO 50 PARTS BY WEIGHT OF AN ELECTROSCOPIC COATING ON THE PARTICLES OF SAID ZINC OXIDE CONSISTING OF A THERMOPLASTIC INSULATING MATERIAL HAVING A MELTING POINT SUBSTANTIALLY WITHIN A RANGE OF FROM 90*C. TO 250*C. AND A VISCOSITY SUBSTANTIALLY WITHIN A RANGE OF FROM 45 TO 10,000 CENTIPOISES AT A TEMPERATURE SLIGHTLY ABOVE SAID MELTING POINT, AND AN ADDITIONAL COATING ON SAID PARTICLES CONSISTING ESSENTIALLY OF 11/2 TO 9 PARTS BY WEIGHT OF A FINELY-DIVIDED COLLOIDAL SILICA, SAID THERMOPLASTIC, INSULATING MATERIAL HAVING A POSITIVE TRIBOELECTRIC RELATIONSHIP WITH RESPECT TO SAID COLLOIDAL SILICA.