乾冰所產生的霧

Since carbon dioxide is a colourless gas, the carbon dioxide produced from sublimation of "dry ice" is not visible by the naked eye.
But carbon dioxide is heavier than air, the gas thus tneds to stay around the dry ice. Because the temperature of the carbon dioxide is much lower than that of the surrounding air, it will cool down the water vapour present in the air, forming tiny water droplets. This is the "mist" that you observed.
Therefore, the second explanation in your post is correct.

二氧化碳在極低溫下就會變成乾冰,
所以乾冰是固體的二氧化碳.
乾冰在常溫下會經昇華變成二氧化碳,
人家會把乾冰放在雪榚上,把雪榚凍藏.
雪榚又不會弄濕.
乾冰也可以用作人工降雨.

2009-09-20 20:25:48 補充：
乾冰在常溫下會經昇華變成二氧化碳.
另外,它吸熱時,也會令旁邊的水蒸氣放熱變成水點.
所以兩者都對.

乾冰旁邊所冒出的霧系水蒸氣凝結來的，

因為乾冰的溫度，如果我沒有記錯是-78°C。

所以回冷卻旁邊的水蒸氣，水蒸氣凝結，邊成小水點。

就是你看到的霧。