160 On Steam and Brines 



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 ioo'i8 C., and it was 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 indi- 

 cated by the thermometer was not lower than i8o'i79 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 contamination, 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 per- 

 fectly 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. 



