The **mass** of body 2? Mass is not influenced by temperature at all.

To be precise increasing the temperature of an object does increase its mass...

Energy is equivalent to mass .... so if we have a 1 kilo mass (at sea level) and take it up mount everest we have increased it's mass ...mgh gives a number in Joules , convert this to mass (by E=mc2) to find the increase in kilograms...

If we throw our mass through the air at velocity v , we have increased it's mass 1/2mv2 , converted to kilos ...

And if we heat it , we have added energy , convert to mass by E=mc2

In a heated object the molecules are moving more quickly , increased speed increases mass of molecules ....

In practice the increase is so small it's negligible...

To answer the question from Euler ... the only way mass will change is by movement of moisture in the wood ... even "dry " wood has 5% moisture , by heating the middle plate to 120C you will drive the moisture out to the cooler plates , this process transfers heat and mass.