In case study 4.1 regarding the hamm kidnapping how were the fingerprints on the ransom note developed

The FBI used a technique which is called Latent Fingerprint Identification: a silver nitrate solution was used to cause a reaction of the unseen fingerprints perspiration left by the kidnappers all over the ransom notes. The reaction caused the perspiration to turn into silver chloride. This substance is white and it is also visible to the naked eye which means the fingerprints of the perpetrators were easily identifiable.