Heat-sensitive copy-sheet

Claim 1

A HEAT-SENSITIVE COPY-SHEET ADAPTED FOR MAKING A CLEAR AND SHARP REPRODUCTION OF A GRAPHIC ORIGINAL BY A DRY THERMOGRAPHIC PROCESS INVOLVING BRIEF APPLICATION OF A HEAT-PATTERN CORRESPONDING TO SAID ORIGINAL, SAID COPYSHEET BEING VISIBLY STABLE UNDER NORMAL STORAGE CONDITIONS AND BEING RAPIDLY PERMANENTLY VISIBLY CHANGED ON HEATING TO A CONVERSION TEMPERATURE WITHIN THE APPROXIMATE RANGE OF 90-150*C., SAID COPY-SHEET INCLUDING A VISIBLY HEAT-SENSITIVE LAYER CONTAINING, IN INTIMATE ASSOCIATION, (1) WEAKLY COLORED, NORMALLY SOLID, ESSENTIALLY NON-HYGROSCOPIC AND NON-VOLATILE, INTER-REACTANT PROGENITORS OF INTENSELY COLORED ORGANIC COLOR-BODIES CONTAINING DOUBLY BONDED NITROGEN AS AN ESSENTIAL CHROMOPHORE, ONE OF SAID PROGENITORS BEING ACID-CONVERTIBLE TO A MODIFICATION WHICH UNDER ACIDIC CONDITIONS IS RAPIDLY VISIBLY IRREVERSIBLY INTER-REACTIVE WITH ANOTHER OF SAID PROGENITORS, AND (2) ACID-PROGENITOR MEANS, STABLE UNDER SAID NORMAL STORAGE CONDITIONS AND LIBERATING A LEWIS ACID ON BEING MOMENTARILY HEATED AT SAID CONVERSION TEMPERATURE.