390 Dr. W. W. J. Nicol on the 



about 400 millim. long, is attached a narrow tube which 

 passes down behind the stand and leads to the pump, a small 

 WoulfFs bottle being interposed to catch any mercury which 

 may splash over. A reservoir of mercury, capable of being 

 raised and lowered, attached to the lower end, makes the " cut- 

 off" complete. 



On the back of the stand is pivoted a wooden bar about 

 800 millim. long, and capable of being fixed at any angle by 

 the thumb-screw passing through the stand. This bar carries 

 a tube about 10 millim. internal diameter, and closed at one 

 end with a side tube about 20 millim. from the other. The 

 tube is filled to a depth of about 700 millim. with mercury, 

 and a capillary passing through the cork which closes the 

 end and reaching to the bottom establishes communication 

 with the air when the pressure is sufficient to overcome that 

 of the column of mercury in the tube (the height of this 

 column being determined by the inclination of the tube). 

 Connexion with the remainder of the apparatus is made by 

 an indiarubber tube attached to the side tube and to the end 

 of the T-piece which passes through the stand. 



The pressure-regulator is used as follows : — The water- 

 pump is set in action, and the mercury-reservoir being lowered 

 both stopcocks are closed. When the pressure as indicated 

 by the gauge is about that which is required, the reservoir is 

 raised till the mercury just reaches the end of the " cut-off," 

 and a screw-clip on the tube to the pump is closed till only a 

 few bubbles of air pass the " cut- off." Then the tube at back 

 of stand is slowly inclined till air begins to bubble through 

 the mercury. The clip is slightly opened ; and if the pressure 

 be too great, the reservoir is lowered, and the inclined tube 

 placed more upright. With a little care, and with at least 

 50 millim. of mercury above the side hole of the " cut-off," 

 it is possible even with very variable water-pressure (provided 

 the pump have a Bunsen valve) to maintain the pressure in 

 the apparatus itself constant within 1 or 2 millim. ; while if 

 an air-reservoir be interposed between the regulator and a 

 separate gauge, the pressure in this last can be kept absolutely 

 constant. 



With this regulator I succeeded in maintaining a constant 

 temperature in the dilatometer-bath for any required time ; 

 while the temperature of the whole column of water, 700 

 millim. long, was so uniform, that, without stirring, the differ- 

 ence between the top and bottom did not exceed 0°*2 C. The 

 dilatometers were used in pairs : two placed side by side in a 

 small wire cage along with the thermometers being lowered 

 into the bath to within 50 millim. of the bottom. 



