The thermometer may be filled by the following method:--When the straight tube has been made the first dumb-bell is introduced and shaken down well towards the lower bulb, the tube is now bent to its final shape and the whole thermometer filled with alcohol as described on page 44. Now heat the thermometer to a little above the maximum temperature that it is intended to record, and pour clean mercury into the open bulb while holding the thermometer vertically. Allow to cool, and the mercury will be sucked down. The second dumb-bell is now introduced, sufficient alcohol being allowed to remain in the open bulb to about half fill it, and the alcohol in this bulb is boiled to expel air. The tube through which the bulb was filled in now sealed. _Clinical Thermometers._--The clinical thermometer is a maximum thermometer of a different type. In this case there is a constriction of the bore at a point just above the bulb. When the mercury in the bulb commences to contract, the mercury column breaks at the constriction and remains stationary in the tube, thus showing the maximum temperature to which it has risen. _Vacuum Tubes._--There are so many forms of these that it is scarcely practicable or desirable to give detailed instructions for making them; but an application of the various methods of glass-working which have already been explained should enable the student to construct most of the simpler varieties. An interesting vacuum tube is made which has no electrodes, but contains a quantity of mercury. When the tube is rocked so as to cause friction between the mercury and the glass sufficient charge is produced to cause the tube to glow. _A Sprengel Pump._--This, in its simplest form, is illustrated by _a_, Fig. 12. Such a form, although highly satisfactory in action, needs constant watching while in action, as should the mercury funnel become empty air will enter the exhausted vessel. Obviously, the fall-tube must be made not less than thirty inches long; the measurement being taken from the junction of the exhaustion branch with the fall-tube to the top of the turned-up end. [Illustration: Fig. 12] _A Macleod Pump._--One form of this is illustrated by _b_, Fig. 12. It has the advantage that the mercury reservoir may be allowed to become empty without affecting the vacuum in the vessel being exhausted. _"Spinning" Glass._--By the use of suitable appliances, it is quite possible to draw out a continuous thread of glass, which is so thin as to have almost the flexibility and apparent softness of woollen fibre; a mass of such threads constitutes the "glass wool" of commerce. The appliances necessary are:--a blowpipe capable of giving a well-formed flame of about six or eight inches in length, a wheel of from eighteen inches to three feet in diameter and having a flat rim of about three inches wide, and a device for rotating the wheel at a speed of about three hundred revolutions per minute.