128 Mr Millei"'s Description of a new Pyrometer. 



crease of 100° in the cylinder producing an increase of about 

 12° in the ascent of the thermometric column. From re- 

 sults with known heats, a scale was obtained, by which un- 

 known and higher heats were inferred. But as the capacity 

 of iron for heat increases rapidly, all determinations with this 

 metal must be too great in the higher ranges. 



In practice, this method was attended with some inconve- 

 niences. The sand that surrounded the bulb of the thermo- 

 meter imbibed moisture fi'om the air in the driest weather : 

 when the cylinder was applied at a red heat, the moisture 

 was converted into steam, and the thermometer quickly rose 

 from perhaps 70 or 80 degrees to the boiling point of water. 

 There was also difficulty in marking the time by a seconds 

 watch, and the ascent of the thermometer at the same in- 

 stant. A new plan was suggested, in which time does not 

 enter as an element in the investigations, and the operation 

 is thus in some measure simplified. 



The pyrometer, in its present form (Plate III.), consists of 

 a platinum* cylinder, Aveight about four ounces ; about thirty 

 ounces of mercury, contained in a vessel of sheet-iron, placed 

 in one of wood, with charcoal between ; a thermometer, gradu- 

 ated on the stem to 600°. The iron vessel has a cover of the 

 same matei'ial ; an iron tube, closed at the lower end, and of 

 rather greater diameter than the platinum heater, is secured 

 to the cover by a lip or flange, and passes through the mer- 

 cury to about one-eighth of an inch from the bottom of the 

 vessel. 



The procedure is similar to the above ; differing in this, 

 that instead of estimating the heat by its partial effect dur- 

 ing a short and given period, it is estimated by its total effect 

 on a mass of mercury. 



The platinum after receiving the temperature of the fur- 

 nace, melted metal, &c., is dropped into the tube, sand is 

 poured quickly over it, and the tube closed with a non- con- 

 ducting stopper. A lid of wood is pressed down on the outer 



-■>'= Platinum has two desirable properties not possessed by wrought iron ; it 

 sustains intense heat, without being oxidized, and its capacity for heat is not 

 a rapidly increasing one, nor irregular. 



