Do things weigh heavier or lighter when hot or cold?

This question about gravitation is really extremely interesting, because it so happens that this is one of the very questions on which a great many remarkable experiments have been made. There is no doubt about the answer to it, but we must understand what that answer really is. It is that the power of gravitation is not in slightest degree affected by temperature. In other words, one and the same thing – if nothing is taken from it or added to it – weighs just the same, no matter how much it is heated or how much it is cooled. But we must not be confused. When a thing is heated it expands, and occupies more space; and on the other hand, it becomes a little lighter in proportion to the space it occupies. Thus hot water will float on top of cold water; hot air will rise in cold air and so on.

This, however, is not a question of absolute weight, but of the relation between the weight, which is not changed, and the volume of the thing.

More reading:
Weight (Wikipedia)

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