Page’s Regulator.—­The most convenient gas regulator to use in connection with the heat-test apparatus is the one invented by Prof.  F.J.M.  Page, B.Sc.[A] (Fig. 49).  It is not affected by variations of the barometric pressure, and is simple and easy to fit up.  It consists of a thermometer with an elongated glass bulb 5/8 inch diameter and 3 inches long.  The stem of the thermometer is 5 inches long and 1/8 inch to 3/16 inch internal diameter.  One and a half inch from the top of the stem is fused in at right angles a piece of glass tube, 1 inch long, of the same diameter as the stem, so as to form a T. A piece of glass tube (A), about 7/16 inch external diameter and 1-1/2 inch long, is fitted at one end with a short, sound cork (C, Fig. 50).  Through the centre of this cork a hole is bored, so that the stem of the thermometer just fits into it.  The other end of this glass tube is closed by a tightly fitting cork, preferably of indiarubber (I), which is pierced by a fine bradawl through the centre.  Into the hole thus made is forced a piece of fine glass tube (B) 3 inches long, and small enough to fit loosely inside the stem of the thermometer.

[Footnote A:  Chemical Soc.  Jour., 1876, i. 24.]

The thermometer is filled by pouring in mercury through a small funnel until the level of the mercury (when the thermometer is at the desired temperature) is about 1-1/2 inch below the T. The piece of glass tube A, closed at its upper extremity by the cork I, through which the fine glass tube B passes into the stem of the thermometer, is now filled by means of the perforated cork at its lower extremity on the stem of the thermometer.  The gas supply tube is attached to the top of the tube A, the burner to the T, so that the gas passes in at the top, down the fine tube B, rises in the space between B and the inside wall of the stem of the thermometer, and escapes by the T. The regulator is set for any given temperature by pushing the cork C, and consequently the tubes A and B, which are firmly attached to it, up or down the stem of the thermometer, until the regulator just cuts off the gas at the desired temperature.

[Illustration:  FIG. 49.—­PAGE’S REGULATOR.]

[Illustration:  FIG. 50.—­PAGE’S GAS REGULATOR, SHOWING BYE-PASS AND CUT-OFF ARRANGEMENT.]

As soon as the temperature falls, the mercury contracts, and thus opens the end of the tube B. The gas is thus turned on, and the temperature rises until the regulator again cuts off the gas.  In order to prevent the possible extinction of the flame by the regulator, the brass tube which carries the gas to the regulator is connected with the tube which brings the gas from the regulator to the burner by a small brass tap (Fig. 2).  This tap forms an adjustable bye-pass, and thus a small flame can be kept burning, even though the regulator be completely shut off.  It is obvious that the quantity of gas supplied through the bye-pass must always be less than that required to maintain the desired temperature.  This regulator, placed in a beaker of water on a tripod, will maintain the temperature of the water during four or five hours within 0.2 deg.  C., and an air bath during six weeks within 0.5 deg.  C.