240 Mr. C. Tomlinson on the Action of Solid Nuclei [Jan. 21, 



IV. " On the Action of Solid Nuclei in liberating Vapour from 

 Boiling Liquids." By Charles Tomlinson, F.R.S. Received 

 November o, 1868. 



History. — During many years after the invention of the barometer and 

 the consequent discovery of atmospheric pressure, the boiling-point of a 

 liquid was denned to be the temperature at which its evaporating tendency 

 is .equal to the common pressure of the atmosphere, or the lowest tempera- 

 ture at which its vapour can have the elasticity of common air. 



About the middle of the last century it was noticed by several distin- 

 guished Fellows of this Society that the boiling-point of water under a 

 constant pressure varies within certain limits, according to the depth to 

 which the thermometer is plunged into the boiling liquid. 



In the Report on Thermometers published in the Transactions for 1 7 / 7, 

 it was stated that the steam from boiling water fairly represents the atmo- 

 spheric pressure ; and it was recommended that, in determining the boiling- 

 point, the water be boiled in a metal vessel constructed so as to allow the 

 bulb, and nearly the whole of that part of the stem that contained mercury, 

 to be surrounded by the steam. 



In the fine experiments undertaken by Dalton, Watt, Robison, Southern, 

 and others for determining the pressures of saturated steam at different 

 temperatures above and below the standard boiling-point, it was noticed 

 that if a minute portion of soda or of some salt soluble in water, and not 

 capable of rising in vapour with it, be allowed to ascend to the top of the 

 mercury, the column rises, thereby indicating a diminished pressure of 

 steam. The adhesion of the soda to the water tends to restrain the water 

 from evaporating, and thus the steam-emitting tension of a solution of soda 

 is measurably less than that of pure water at the same temperature. 



In 1785 Achard* showed by a number of experiments that the boiling- 

 point of water, under a constant pressure, is much more inconstant in 

 metallic than in glass vessels. He also noticed that if, while water was 

 boiling steadily in a glass vessel, a drachm of iron-filings or of some other 

 insoluble solid were added to the water, the boiling-point was lowered 1° 

 Reaumur or even more, and that there were considerable differences in the 

 amount of depression, according as the same substance was in powder or in 

 lump. 



In 1803 Be Lucf stated, in very precise terms, that boiling is produced 

 by the bubbles of air which the heat disengages from the liquid. If the 

 water be completely purged of air it cannot boil, because steam can only 

 form on free surfaces, such as the air- bubbles present. Deprived of air, 

 water can boil only on the upper or free surface. Water in a tube from 

 which the air ha d been carefully expelled was raised to 234|-° F. without 

 boiling. 



* jSouveaux Memoires de l'Acadeniie Eoyale de Berlin for 1784-85. 



t Introduction a la Physique terrestre par les Fluides expansibles. Paris. 1803. 



