The first person to prove the principle of the conservation of mass was the 10th century Andalusian Muslim scientist al-Majriti in his book "Rutbat Al-Hakim".

A comment in response of the post
"Can you somehow show us (at least) some of the original manuscripts?"

The reply
" "I took natural quivering mercury, free from impurity, and placed it in a glass vessel shaped like an egg. This I put inside another vessel like a cooking pot, and set the whole apparatus over an extremely gentle fire. The outer pot was then in such a degree of heat that I could bear my hand upon it. I heated the apparatus day and night for forty day, after which I opened it. I found that the mercury (the original weight of which was a quarter of a pound) had been completely converted into red powder, soft to touch, the weight remaining as it was originally."

http://en.m.wikipedia.org/wiki/Maslama_al-Majriti "