I have proposed an experiment to measure the relative temperature of a closed system at different heights under the assumption that a relative decrease in density occurs as the relative size of an object increases as it rises into more dilated space. I am concerned about the prospects for successfully conducting such an experiment. If someone were to employ mercury as a thermometer, and mercury expands, then the expansion of mercury would be liable to return a false negative as the result of that experiment. So then, is there some mans of conducting such a tricky experiment without having the results skewed by the inappropriate choice of a thermometer, or an inappropriate experimental method?