In the old days, on a cold winter night it was common to bring a hot object to bed with you. Which would be better to keep you warm through the cold night - a 10 kg iron brick or a 10 kg jug of hot water at the same temperature?

Question

In the old days, on a cold winter night it was common to bring a hot object to bed with you. Which would be better to keep you warm through the cold night - a 10 kg iron brick or a 10 kg jug of hot water at the same temperature? 
10 kg iron brick 
10 kg jug of hot water

WHY?!!

Eman · Answer

Heat Movement
Thermal energy is always moving. No matter what substance is heated, there is always another place for the heat to move, unless all available substances are the same temperature and equilibrium is reached. This is particularly true on earth, where thermal energy is concentrated in specific places, although some substances manage to hold onto their heat longer than others. Substances of similar temperatures will exchange heat more sluggishly, while substances with a large temperature difference, such as the air and fire, will exchange heat very quickly. Some substances can hold onto heat longer than others. Water, for instance, holds heat for longer periods than metal. (eHow)

Emissivity of iron is .91-.96, while the emissivity of water is 0.98, thus holding onto it's latent heat longer than iron.