Across
1
Unit of power
3
Infectious microscopic agent
4
Center of an atom or cell
7
Biological catalyst
8
Gas essential for breathing
9
Ball of burning gas in space
11
Form of thermal energy
Down
2
Mass per unit volume
5
Amplified beam of light
6
Nerve cell in the brain
10
Icy body with a tail near the sun
12
Chemical element in table salt