STEAM AND BBIKES. 535 



when the thermometer had taken the temperature of the steam the top of the mercury 

 was exactly even with the centre of the line on the scale marking 100'18° C. It 

 occupied this position when steam was being generated at its highest rate of 8 "4 

 grammes per minute. The flame of the lamp was reduced by degrees until it reached 

 its lowest point, when steam was being generated at the rate of 2 "4 grammes per 

 minute. It was then issuing continuously, that is, there was no regurgitation, but it 

 partook more of the nature of an exhalation than of a stream ; yet the mercury 

 remained exactly on the centre of the line of 100"18 o , and it w r as only when the lamp 

 had been reduced to its very lowest that it could be said to have fallen to the lower 

 edge of the line. The temperature then indicated by the thermometer was not lower 

 than 180"l79° C. When the reading of such a thermometer remains unaltered while 

 the supply of steam is varied in the proportion of nearly four to one, the efficiency of 

 the steam tube may be said to be perfect. 



A further condition affecting the usefulness of these tubes is that they and the 

 thermometer shall be perfectly clean. The thermometer is the most liable to contamina- 

 tion, and I generally found it convenient to wash it with soap and water before every 

 experiment. The steam which condenses on the thermometer and on the inside of the 

 tube should do so in a film and not as a dew, and it does so if the surfaces are perfectly 

 clean. The inside of the tube is more difficult to deal with than the outside of the 

 thermometer. On the other hand, when once cleaned it remains clean much longer. 

 Soap is used here also, and the best way of using it is to smear the inside of the 

 upper part of the tube with soap, preferably soft soap. Steam is raised, and so soon 

 as it reaches the soap it condenses and forms a uniform film of solution which drains 

 down back into the boiler and by continuing to boil the steam condensing washes 

 the inside walls quite clean from everything, and for a considerable time afterwards 

 there is no trouble about the steam condensing in the tube as dew. If the boiler has 

 been charged for this operation with distilled water, there is the disadvantage that it 

 immediately primes, and the steam, instead of washing down the sides of the tube, 

 continues to blow soap bubbles at its upper end, without washing the tube. I 

 always use ordinary tap water, which supplies equally pure steam, with distilled 

 water, but it has this advantage, that when the soap solution drains back into 

 it, being in comparatively small quantity, it is immediately precipitated by the 

 earthy ingredients of the water, which continues to supply pure steam without 

 priming. 



Attention to these small matters is all-important, not only in order to secure 

 accuracy but also comfort in experimenting. 



Thermometers. — Two thermometers were used for these experiments, both made 

 especially for me by Mr Hicks, of Hatton Gardens. One of them, A, was intended for 

 use at ordinary levels; the scale was in Fahrenheit's degrees, and ranged from 210° F. 

 to 240° F. Each degree was divided into tenths, and had a length of 7 '2 mm. The 

 length of a Centigrade decree in this thermometer would thus have been 13 mm., and 



