354 Chemical Evidence on a late Trial. 



a common suction-pump it is necessary there should be a com- 

 munication with the atmosphere. Thirdly, it is designed to 

 carry off the aqueous vapour from the fresh oil, which has a 

 very bad smell, and such vapours would injure the sugars, if they 

 got abroad in the sugar-house. 



D. Is a cast-iron pump with a spring metallic piston commu- 

 nicating with the oil-vessel A, by means of its suction-pipe E. 

 It is set in motion in the usual manner, by some mechanical 

 power. 



F, is a copper vessel, the bottom of which is covered in the 

 inside by a coil of pipe, communicating at one of its ends with 

 the pump at G, and at the other end with the oil-vessel through 

 the pipe H. Through this coil of pipe the heated oil circulates, 

 and being surrounded on all sides by the liquid in the pan F, 

 it gives out about 100° of heat in its passage, and returns to 

 the oil-vessel to obtain a fresh increase of temperature. This 

 pan is surrounded by brick or wood work, to prevent cooling* 

 Of course it has no fire under it. 



Fig, 2. — Is a ground plan of the same apparatus in which the 

 coil of pipe in the evaporating vessel F may be seen. 



A, is the oil-vessel in which are inserted the thermometer B, 

 and the vent-pipe C. 



D, is the pump. 



E, G, the pipes forming the communication between the oil- 

 vessel and evaporating pan; which, after circulating in the 

 form of a coil, passes out at the centre of the bottom, and re- 

 turns to the oil-vessel by the pipe H. 



Art. XII. On the Vapour of Mercury at common Tempera- 

 tures , ^y M. Faraday, Chemical Assistant at the Royal 

 Institution, 



It has long been admitted, that in the upper part of the 

 barometer and thermometer an atmosphere of mercury exists, 

 even at common temperatures, but having a very small degree 

 of tension. The following experiment renders it easy to shew 

 this atmosphere even when the air has not, as in the instru- 



