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
Internet-related prefixes as a cultural reference to the rising popularity at that time of the prefix in product names. The word "virtual" is used in a similar way to the prefixes above, but it is an adjective instead of a prefix. For example, it is used in the terms "virtual reality", "virtual world", and "virtual sex". These prefixes are productive. Michael Quinion notes that most of these formations are nonce words that will never be seen again. He writes that new terms such as "e-health" are unneeded; in this case "telemedicine" already exists to describe the application of telecommunications to medicine. He