Across
1
Force that attracts objects together
3
Nerve cell in the brain
5
Natural satellite of Earth
8
Preserved remains of ancient life
9
Disturbance that transfers energy
10
Ball of burning gas in space
11
Gas form of a liquid
12
Biological catalyst
13
Basic unit of life
Down
2
Reddish electrical conductor
3
Center of an atom or cell
4
Unit of energy
6
Group of similar cells
7
Mass per unit volume