Chemical and Physical Notes 55 



is rapid condensation on the sides, and after that, on the 

 thermometer, from which the water falls in a series of drops, 

 which follow each other first slowly, then rapidly, then again 

 more slowly, until, finally, a drop remains hanging from the 

 lower extremity of the thermometer, and never falls. By 

 that time the protecting film of water has established itself 

 on the sides of the tube, and effectually guards the thermo- 

 meter from external influence. If it were possible for the 

 temperature of the thermometer to fall ever so little below 

 that of the condensing temperature of the saturated steam in 

 the tube, its temperature would be immediately restored by 

 the condensation of some of the steam on the water which 

 moistens its surface. If its temperature were to rise ever so 

 little above the condensing temperature, it would be im- 

 mediately brought back again by the evaporation of some of 

 the moisture on it. Consequently, when the boiling is in full 

 operation, the thermometer is exactly at the temperature when 

 the smallest possible increase of heat will cause evaporation, 

 and the smallest possible decrease of heat will cause condensa- 

 tion. But the boiling-point of a substance is the temperature at 

 which if, as a vapour, condenses on itself as a liquid, and as 

 a liquid evaporates into itself as a vapour. Therefore, the 

 temperature of the thermometer is exactly the boiling tempe- 

 rature of the water. Further, the whole enclosure is guarded 

 by a surface of water in contact with saturated steam, so that 

 its walls are necessarily also at the boiling temperature of the 

 liquid, and it is impossible that the thermometer can be at 

 any other temperature than that of the boiling liquid and 

 condensing vapour. It follows, therefore, that for this purpose 

 we may graduate our thermometer into thousandths of a 

 degree, if we choose, and it is quite certain that the tempera- 

 ture of the thermometer will not differ by that amount from 

 that of the medium. During an operation the steam tube 

 is the ideal enclosure at constant temperature. When the 

 thermometer has taken the temperature of the steam, it 

 remains perfectly steady so long as the barometric pressure 

 remains the same. 



As evidence of the efficiency of the steam tube, the rate of 



