On 6 Oct 1816 Jonas wrote a letter to his brother Philip who was at that time in Rocky Mount, Franklin, Virginia. This letter, dated at Allentown, is two legal size pages long. The first page starts off by chiding Philip for not writing since "25 august 1813 - which Letter was delivered to our Brother John Hartzell By the same man that you gave it to". There was no organized postal delivery service which could be depended upon to take a letter from Virginia to Pennsylvania. It then goes on to provide a chatty account of various items of family news, and says that their brother John will send Philip's share due from their mother "as sun as the Bank notes of the united States will be in Circulation".

The second page of Jonas' letter is almost entirely a genealogy of the family. Jonas mentions first the death dates of their parents, then gives the birth dates of all of the children of the family, with a calculation of the intervals between each birth. He then copies a "memorandom given By me Jonas Hartzell to my son Isaac & his Sons it Being from Jonas and philips grand father, down to our grand Children in the mail Blod down to the present fifth generation - to Jonas and philip Hartzels grand Children". He then lists "old george" and his son Jacob, both born in Germany, and gives the years of their birth and death. Then himself, "as god pleases yet Living in the year of our Lord 1816". Finally, he lists the dates of birth of his son Isaac and grandson Thomas.

At the end of the letter he says that he has sent this to Isaac to be "Entered in his Bible for our great grand Children", and requires that Philip must also send a copy "to your oldest son adam & then his sons if any". This provides the only documentary evidence we have that our ancestor Adam is the son of Johann Philip Hertzel.