316 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



ordinary gas Thermo-regulator. The object (fig, 80) is placed upon the 

 •warm stage a, which consists simply of a brass box resting upon the stage 

 of the Microscope, and with a tubular aperture in the centre to admit light 

 to the object. The box is connected by indiarubber tubes with a hollow 

 metal jacket /, and the whole system thus constituted is completely filled 

 with water previously boiled to the exclusion of air. The water is warmed 

 at ^ by a small gas-flame and rising through the tube c communicates its 



Fig. 80. 



heat to the box a, the temperature of which is measured by a small ther- 

 mometer h inserted through an obliquely placed tube quite into the central 

 opening and immediately under the preparation. The cooled water from 

 the stage passes down the tube c', and so to the flame again, and in this way 

 a constant circulation is kept up. 



The bulbed tube d filled with mercury serves to regulate the flow of gas 

 so as to keep the temperature constant at any desired point. This is efiected 

 by turning the steel screw e when this point, whatever it may be, is reached, 

 so as to raise the mercury in the glass tube, and almost block up the 

 lower end of a small steel or glass tube which is fixed into the upper end 

 of the tube d. The gas used for heating passes through the small tube and 

 then above the mercury and between the two tubes to be conducted by the 

 side-piece to the burner below. If now the temperature rises higher in the 

 reservoir / surrounding the mercury the latter will expand and rising in the 

 tube will cut off more of the gas, and thus reduce the flame, on which the 

 mercury will again contract and the flame increase in consequence, and so 

 on. It is found that an equilibrium soon becomes established, and the 

 temperature of the water and stage remains almost absolutely constant. To 

 raise or lower the temperature all that is required is to screw e out or in. 

 The smaller tube enclosed in d is pierced with a minute aperture to allow a 

 constant passage of gas, so as to prevent the flame from being extinguished 

 in the event of the mercury completely blocking up the lower end of the 

 tube.* 



* To provide against the danger resulting from accidental extinction of the gas, 

 Prof. Koch devised a self-acting apparatus, which, simultaneously with the extinction 

 of the flame of the burner, shuts oft" the supply of gas. Cf. Crookshank's ' Practical 

 Bacteriology,' 1886, p. 36 (I fig.). 



