us no more than the relative coldness of the air, but leave us in the dark as to the positive degree thereof; whence we cannot communicate the idea of any such degree to another person. For not only the several differences of this quality have no names assigned them, but our sense of feeling cannot therein be depended upon; and thermometers are such very variable things that it seems morally impossible from them to settle such a measure of coldness as we have of time, distance, weight, etc." (1665).
Boyle endeavored to overcome this difficulty; believing that the melting-point of ice varied with geographical latitude, he proposed using the oil of aniseed for getting a fixed point, placing it around the bulb of an alcohol thermometer, allowing the oil to freeze and marking the height of the spirit of wine in the bulb "when the oil begins to curdle."
This scheme for getting a fixed point has been wholly misunderstood by some historians who state that Boyle filled his thermometers with aniseed oil!While Boyle tried to secure one fixed point he overlooked the advantages of having two; he strove to compute the absolute expansion