ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



811 



the highest point desired the wire above the mercury is pushed down so as 

 just to toTich the surface of the latter. It is obvious that if the bath be 

 filled with water below the temperature desired the mercury will not rise 

 and touch the wire B, thus making connection with the other wire, until 



Fig. 221. 



Fig. 222. 



^^ 





the bath reaches that temperature, and that as soon as the temperature falls 

 below this point the mercury will fall with it and away from the wire B ; 

 also that by raising or lowering this wire the connection can be made 

 to take place at any desired higher or lower temperature. This regulating 

 thermometer will be found to be sufficiently delicate to keep the tem- 

 perature to within two degrees. It can be made by simply blowing a bulb 

 on a glass tube and filling the bulb and tube with mercury alone. 



Fitting over the top of the lamp-chimney is a chimney D, 11 in. long 

 and 2f in. square, having at its top a hot-air chamber E, into which the 

 water-bath fits. This chamber has holes round the sides near the bottom 

 for the escape of air. The chimney D has at one side a branch chimney 

 F, 12 in. long, opening into it at an angle of 45°. In the opening 

 between the chimneys is hung a valve G, turning on a hinge H, and moved 

 by a lever I on the outside. This valve should be very light, and must 

 turn easily on the hinge which is made by hanging the valve fastened on 

 a wire through holes on the sides of the chimney ; to this wire is attached 

 on the outside the lever which is to be weighted on the end with a small 

 bullet, so as nearly to balance the valve which must just fall of its own 

 weight. At K is a shelf 2^ in. wide extending into the chimney D. This 

 shelf leaves an opening in the chimney 1/2 in. wide which is sufficient for 



