A heat exchanger has a plurality of intermediate plates arranged one atop the other to form a stack having first and second ends. For example, the press of a channel key allows selecting one of the areas and moving to the selected area a focus frame indicating a candidate image for selection, and then the presses of cursor keys allow moving the focus frame to a desired image to be selected. For marking solid substances or articles, the water-insoluble medium containing known nucleic acid taggants is spread on the target solid substances or articles.