Papailias, George. "Sight Glass". www.papailias.com. Archived from the original on 2017-10-29. Retrieved 2017-12-21.
udel.edu
www1.udel.edu
University of Delaware, Department of Chemistry and Biochemistry. "Glass Physical Properties". www1.udel.edu. Archived from the original on 2017-12-05. Retrieved 2017-12-21.
web.archive.org
Papailias, George. "Sight Glass". www.papailias.com. Archived from the original on 2017-10-29. Retrieved 2017-12-21.
University of Delaware, Department of Chemistry and Biochemistry. "Glass Physical Properties". www1.udel.edu. Archived from the original on 2017-12-05. Retrieved 2017-12-21.