MAINTAINING TEMPERATURE. 



35 



c communicates its beat to the box , tbe temperature 

 of which is measured by a small thermometer 6, in- 

 serted through an obliquely placed tube, quite into 

 the central hole and immediately under the prepara- 



Fig. 9. 



Apparatus for maintaining a constant temperature under the microscope. 



tion. The cooled water from the stage descends down 

 the tube c', to pass again round to the flame, and in 

 this way the water constantly circulates. The bulbed 

 tube d, tilled with mercury, serves to regulate the flow 

 of gas, so as to keep the temperature constant at any 

 desired point. This is effected by turning the steel 

 screw <?, when this point, whatever it may be, is 

 reached, so as to raise the mercury in the glass tube, 

 and thus almost block up the lower end of a small 

 steel or o-lass tube, which is fixed into the upper end 

 of the tube d. The gas passes through the small tube, 

 and then above the mercury and between the two 

 tubes to be conducted by the side piece h to the 

 burner below, and it will be understood that if the 

 temperature now rose higher in the reservoir/, which 

 surrounds the mercury, this on being warmed will 



