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 the bulb was placed in freezing distilled water; he then marked the divisions above and below "according to the degrees of expansion or contraction of the liquor in proportion to the bulk it had when it indur'd the freezing cold."

Edmund Halley, writing in 1700, said that Hooke exhibited at Gresham College, 2nd January, 1667-8, a combination of barometer and thermometer in separate tubes, the freezing-point of water being equal to zero, and the stem being graduated from -70 to +130.

Hooke made an important step in advance when he took the freezing-point of water as a fixed point in the scale, but the statement made by Brewster that in 1684 Hooke proposed the boiling-point as a second fixed point has been examined by Poggendorff and not verified. The claim has been made that the two fiduciary points were first proposed by the Dutch mathematician Christian Huyghens, in a letter dated 2nd January, 1665, addressed to Robert Moray. (A. Momber, Schr. naturf. Gesch. Danzig, N. F. VII, 108.) Huyghens wrote: "It would be well to have a universal and determinate standard for heat and cold, securing a definite proportion between the capacity of the bulb and the tube, and then taking for the