ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



467 



and dry mercury in BCD. Above the mercury in C D there is a 

 barometric vacuum. When the regulator is vertical and at ordinary 

 temperature (e.g. 15°) the inferior level of the mercury in B C is above 

 the thread E. The total length varies between 45 and 75 cm., and the 

 hydrogen pressure between 0*45 and 0'75 mm. of mercury, according 

 to the size of cupboard. The regulator is intended to be suspended 

 and in circuit with the heating wire. The 

 current can only circulate when there is con- 

 tact between the platinum thread E and the 

 mercury ; the level of the mercury in B C is * 



dependent upon temperature variations of the 

 hydrogen, the inclination of the regulator to 

 the vertical and the allotment of the mercury 

 between the pocket G and the tube BCD. 

 The effect of each of these is separately con- 

 sidered by the designers. It will be understood 



m 



that by inversion of the instrument the pro- 

 portion of mercury and hydrogen in the limbs 

 can be easily adjusted and that the increase of 

 pressure, as the temperature is raised, of the 

 hydrogen in AB will depress the mercury so 

 that electric contact is at length automatically 

 severed. The authors give tables for determin- 

 ing the adjustment. 



Various Details of Construction. The walls 

 are made of wood, varnished externally ; the 

 wood is of two thicknesses, enclosing a layer of 

 wadding and lined with glass. In the larger 

 stoves there are two systems of doors, the inner 

 ones being made to slide, the outer ones hinged. 

 Certain modifications of these arrangements 

 obtain in the smaller stoves. A rheoscopic 

 lamp (fig. 73) is in the electric circuit, not for 

 illumination but for signalling the interruption 

 of current. This lamp is formed of a test-tube 

 constricted so that the lower part is bulbous. 

 Above this constriction is a block of paraffin 

 fusible at the temperature at which the circuit is 

 to work ; above the paraffin is a small quantity 

 of mercury into which pass the ends of the 

 wires. This mercury ensures contact, and if, 

 from any cause whatever, the temperature rises 

 too high, the underlying paraffin is melted by 

 the heat of the mercury, which thereupon falls into the bulb. In this 

 way the contact is broken and accident from over-heating prevented. 



Practical Results. The conditions of maintenance of uniform tem- 

 perature are fully discussed, and, as an example of the working of the 

 stove, the temperature statistics for a certain day are given. On that 

 day the mean temperature of the stove was 37 '9° C, the variations 

 being within 37 -7° and 38-2° C. The coefficient of loss of heat from 

 small stoves was found to be greater than from large ones ; or, in other 



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