The following Chemthink tutorials can now be found at Simbucket:
The Particulate Nature of Matter
Chemthink is currently not operational.
Chemthink's hosting provider has upgraded its database software and this has caused issues with the code that runs chemthink. I'm working on finding a solution but the code was written 10 years ago and may require a complete rewrite to make it work. If that is the case I will first work to make the site available without the database functionality.