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Tetrahydrocycline is apparently a highly volatile chemical compound.
According to Gaius Baltar, it is one of the compounds used in the precursor test of his original (i.e. ficticious) Cylon detector.
He brings it forward as a means of posturing, preventing Commander Adama and others in the know from realizing that he has no true means of detecting humanoid Cylons in the Fleet after Galactica's port water tanks are blown open to space, likely by a Cylon saboteur (TRS: "Water").
- Baltar's medical experience (by way of his panic) may have meant hydroxytetracycline, which is far from a volatile chemical compound on the real-world Earth. The compound is an broad-spectrum antibiotic used to treat a variety of infections, from acne and anthrax to the Plague. Another possibility is Cyclohexene, assuming "cycline" refers to benzene.