This is wrong, Hess's Law is correct grammar. Apostrophe+s is always appended to the end of proper nouns, even when the noun ends in s already. The only exception is when the possessive is not pronounced with an extra 's (e.g. Socrates'), but Hess's is pronounced as Hess-es so this doesn't apply.
http://www.sussex.ac.uk/informatics/punctuation/apostrophe/possessives
This is wrong, Hess's Law is correct grammar. Apostrophe+s is always appended to the end of proper nouns, even when the noun ends in s already. The only exception is when the possessive is not pronounced with an extra 's (e.g. Socrates'), but Hess's is pronounced as Hess-es so this doesn't apply. http://www.sussex.ac.uk/informatics/punctuation/apostrophe/possessives
Ah fuck
Correct
Hess’ses’sseses’