#OTD in 1803.
British scientist John Dalton begins using symbols to represent the atoms of different elements.
In his laboratory notebook there is a list in which he set out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.
https://en.wikipedia.org/wiki/John_Dalton#Atomic_theory
Books about John Dalton at PG:
https://www.gutenberg.org/ebooks/56648