81. If the owner of two or more properties mortgages them to one person and then mortgages one or more of the properties to another person, the subsequent mortgagee is, in the absence of a contract to the contrary, entitled to have the prior mortgage-debt satisfied out of the property or properties not mortgaged to him, so far as the same will extend, but not so as to prejudice the rights of the prior mortgagee or of any other person who has for consideration acquired an interest in any of the properties.