My first puzzle: "#2"

[bbbbbbbbba]

A big box was filled with a gas, while a small box enclosed a vacuum. I connected the boxes with a pipe, and I saw the gas quickly flows from the big box to the small one. I wanted to force more gas into the small box, and I did do it. At first nothing seemed to be wrong, but then my eye told me the gas in the small box is decreasing in quantity! I weighted the small box, and it did weigh more and more. I did the experiment again and again, and I got the same result again and again. By the time I realized what's happening, the gas was almost used up. Can you do better than me?

[Jack_Ian]

I'm puzzled.
Not sure what you did. Not sure what constitutes "better".

[Thok]

I'm assuming the gas transformed into liquid form. Looking at wiki's comments about condensation of water, that suggests that the proper solution would be to heat the small box (which should prevent condensation.)

[bonanova]

first puzzle #2 = second puzzle :p
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[bbbbbbbbba]

My answer, which may only be one of the possible answers: (SPOILER)
The gas is nitrogen dioxide(NO ₂ ). It is brown. When you try to force it into a small box, pressure increases, and the gas turns into N ₂ O ₄ , which is colorless.