Chemical substance identity of benzene, until the correct structure was described by Friedrich August Kekulé. Likewise, the idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers. An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it
CSX milepost prefixes in Waycross, Georgia, 587 miles south of Richmond on the branch to Montgomery, Alabama (which splits near milepost 548 in Jesup). Three-letter prefixes are used for minor branches of these branches, or minor branches off the mainline - for instance ANA is the northernmost branch of AN. Before the ACL merged into the Seaboard Coast Line Railroad in 1967, none of the prefixes had the initial "A". The letters "I" and "Q" were skipped, and double-lettered prefixes (like KK, which would be AKK post-1967) were not used. A is given to the main line from Richmond, Virginia to Port of