With regard to India teas possessing the quality of exciting the spirits, this, like every other stimulus, either by constant use loses its effect, or unnerves the system it is meant to strengthen. The nerves through which the animal spirits circulate being, like the strings of a violin or harpsichord, too frequently braced, lose, at last, their natural tensity, and thus render the human frame one system of debility.
Having thus, as briefly as possible, stated that even their ascribed virtues are either derogatory to all physical principle, or else destructive to the constitution, from their constant use, the nature of India teas is next considered, with respect to what appears to be their chief component parts, from analyzation.
Teas have been found to consist principally of narcotic salts, some astringent oil, and earth. These being found in greater quantities in bohea than in green teas, those who have very sensible and elastic nerves must be seized with a greater tremor after drinking the former than the latter. The continual and regular influx of the nervous juices is stopped by their component fibres being contracted from the roughness and restringency of such decoctions. The force of the heat, or the brain's propulsion of its nervous juice, being inferior to the resistance of the whole ramified fibres thus encreased by the sudden contraction and unequal motion, the flow of the animal spirits must be greatly impeded and disordered. In fact, the influx suffers a suspension, until the fibres, by relaxing again, admit their empty tubes to receive their appropriated liquids. Thus even green tea must, especially if taken strong and often, stop the natural circulation of humours, and produce the attendant defects of depression of spirits, deficiency of secretion, loss of appetite, decrease of strength, waste of body, and, finally, a total want of effective vigour in all the animal functions. But, as above observed, bohea tea possessing in greater quantity the pernicious ingredients, the vessels are thrown into momentary spasms and convulsive vibrations, by the relaxing power of the narcotic salts, and the contracting force of the astringent oil and earth. And here it must be noticed, that oil mixed with salt is rendered astringent: thus all vegetables, where a mixture of both prevails, are reckoned stimulating. The narcotic power of the salt is derived from its hindering the flux of the animal spirits through the nerves.