Radiator efficiency, restoration

Dust grains act like stones in the desert. They accumulate heat and restore it to the medium in the form of infrared radiation. They are intermediaries between light from stars and interstellar gas, for they absorb stellar photons in a most efficient manner. This is why these clouds appear so dark in photographs. In fact, they shine in the infrared. The dust strewn across the Galaxy trades the big money of stellar light for the small change of the infrared. 

To restore the efficiency of a radiator, suffering from the accumulation on noncondensables inside its condensing coils, the noncondensable gases have to be removed. The air vent shown in Fig. 13.1 serves this purpose. 

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