As before mentioned, the formalin is not added to the stock gelatin, but to what is used in snaking each preparation at the time of filling or imbedding. When once the gelatin is hardened by formalin it is insoluble in water and cannot be melted even at boiling temperature. The formalin, of course, also makes it antiseptic. If the formalin is permitted to evaporate from the gelatin the latter may again become fluid.