In 1772 Jean-Andr Ã Deluc studied the several substances used in thermometers at the time . At the time , new theories of heat were being developed . In 1772 Jean-Andr Ã Deluc studied the several substances then used in thermometers in the light of new theories of heat and came to the conclusion that mercurial thermometers were the best for practical use ; for example , if two equal amounts of water at x and y degrees were mixed , the temperature of the result was then the average of x and y degrees , and this relationship only held reliably when mercury was used .