On the Law of Expansion of Superheated Steam. 



67 



at one uniform height. A cathetometer (g g), with vernier (A), to 

 read the lower and variable level of the mercury column, was intro 



rr~ - . ^i 



duced. To heat the globe, the oil-bath (k) was used, fitted to the 

 tube (t) by a stuffing-box (Y) ; the oil-bath is itself immersed in a 

 mercury bath (I), surrounded by a coil of jets of gas (mm). 



The globe, filled with dry and warm mercury, the air-bubbles 

 being extracted by means of an air-pump, was inverted to form a 

 Torricellian vacuum. A small glass globule of water was then in- 

 serted, the platinum wire fixed in its place, and an india-rubber cap 

 fitted to the extremity of the stem. Being transferred to its place, 

 and the india-rubber cap replaced by an open glass cistern, so that 

 the glass (d) could be elevated to its position, the jets were lighted, 

 and the temperature elevated to 300°. 



From this point the levels of the column were read off at intervals 

 of 50° until the temperature of saturation was reached. The levels 

 were taken in a series of descending temperatures, to avoid the in- 

 fluence of steam boiling out of the mercury as the temperature rose, 



F2 



