Final answer:
First-day cover envelopes sent by stamp collectors to themselves are not empty; they contain the new stamp and sometimes a stiffener.
The USPS issued a special stamp for the 2017 eclipse with thermochromic ink, which changes when touched.
Step-by-step explanation:
The statement in your question seems to have a typo, but if you are asking about whether a first-day cover envelope is empty when stamp collectors mail it to themselves, it is generally not.
Stamp collectors often use first-day covers to get the postmark of the first day of issue on the envelope. These envelopes typically include the new stamp, and people sometimes include a card or piece of paper as a stiffener, but the envelope is not meant for sending messages or letters.
The United States Postal Service issued a special commemorative stamp for the August 2017 eclipse which featured thermochromic ink. This type of ink changes properties with temperature change.
In this case, the ink would change appearance when you touch it, revealing underlying images, which made the stamp quite unique and collectible.