256 Proceedings of the Royal Society of Edinburgh. [Sess. 
The spiral shape was found most suitable, as a large surface is essential. 
The layer of toluol nearest the glass is the only part appreciably affected 
by the changes in temperature, and a large volume of toluol is therefore 
unnecessary provided the surface exposed to the water of the bath is 
sufficiently large. 
The tubes b and b' have an internal diameter of about 2*5 mm., and are 
both left open at the top. Through b' a platinum wire passes, which is long 
enough to be constantly in contact with the mercury in the reservoir e. 
The tube b bears a movable platinum wire and its adjusting screw. 
When the temperature of the bath rises, the expansion of the toluol 
raises the mercury in the tubes b and b' (fig. 1), makes contact in the tube b, 
and extinguishes the heating-lamps l (fig. 2). 
The first rough adjustment of temperature is made by adding or remov- 
ing mercury until the meniscus is about 6 inches from the top of the tubes 
b and b\ The final adjustment is made by raising or lowering the platinum 
wire in b by means of the screw adjustment a (fig. 1). The lower end of the 
adjustable platinum wire is twisted like the end of a cork-screw so that 
contact is made in the centre of the mercury surface. When contact is 
made at the side the delicacy of the adjustment is impaired owing to 
capillary action. 
