260 Modern Microscopy 



shelf on which to dry the balsam at the edges of finished 

 slides and for other purposes. Secondly, being supported 

 about an inch above the top of the oven, it protects from 

 dust, objects from which the clearing agent is being 

 evaporated in the infiltrating process. 



Upon the shelf below the oven is seen the gas-pressure 

 regulator. The use of this apparatus is to maintain an 

 equal pressure of gas in the pipes that serve the oven. 

 This is of great importance in places where the gas-pressure 

 is subject to great variation. Upon the top of the oven, to 

 the right of the slide rack (Fig. 72), will be seen the long 

 cylindrical thermometer, the mercury bulb of which passes 

 into the interior of the oven, not into the water. 



Gas Regulators. — To the left of the slide rack will be 

 seen the gas-regulator in situ with its connections. This 

 particular regulator is Page's — a very simple and efficient 

 instrument if care be taken in its adjustment. Eeichert's 

 regulator is in some respects preferable, on account of the 

 easier control which it affords — by means of a screw in the 

 mercury — over the amount of gas allowed to pass. 



The regulating action of these instruments is as follows : 

 The mercury bulb, being immersed in the water contained 

 between the walls of the oven, responds readily to every 

 variation of temperature. The gas is admitted into the 

 interior of the regulator by a small hole at the end of the 

 glass tube. If the mercury is in close proximity to the 

 small hole, it will be obvious that any rise of temperature 

 will expand the mercury, so that it will approximate more 

 closely to the aperture, and thus cut off or reduce the 

 amount of gas passing through to the burner. It is con- 

 venient to get up the heat of the oven by means of a 

 Bunsen burner, as the pinhole burner acts slowly. When 

 the temperature has reached the point required, as indicated 

 by the thermometer passing into the oven, the regulator is 

 adjusted so that any rise of temperature reduces the gas- 

 supply. In Eeichert's instrument the adjustment is brought 

 about by turning a screw, so that the mercury may rise or 



