Intelligence and Miscellaneous Articles. 79 



produced at a fixed point; the change of state becomes possible 

 when the temperature can give to the vapour an elastic force equal 

 to the external pressure ; but this change takes place only very 

 rarely at the exact point at which its possibility commences. 



With a view to the study of ebullition, I have undertaken a great 

 number of experiments ; and among others, I have endeavoured to 

 study ebullition by arriving at this phenomenon rather by a change 

 of pressure, which the liquid undergoes, than by an increase of 

 its temperature. The apparatus resembles, with certain modifica- 

 tions, that which M. Regnault used in studying the elastic force 

 of aqueous vapour. A sheet-iron vessel communicates, by suit- 

 able tubes, (1) with an air-pump, (2) with a mercury manometer, 

 (3) with a glass retort. In this retort were placed the liquids expe- 

 rimented upon, and a thermometer with a small reservoir plunged in 

 the interior. By means of stopcocks, suitably arranged, the various 

 parts of the apparatus could be connected. An observation of the 

 manometer and of an external barometer obviously gave, at any 

 moment, the external pressure of the apparatus. 



Studied under these circumstances, the boiling of water presents 

 some characters worthy of attention. In the case of distilled water, 

 it is soon seen that after a first heating to 100°, boiling obtained 

 by diminution of pressure is only produced at the temperature 

 which the known law requires. Water remains liquid although 

 the pressure is far below the tension of aqueous vapour for the 

 temperature in question. When boiling commences, it is produced 

 with tremulous violence, and usually part of the liquid is carried into 

 the tubes with the first burst of vapour. These retardations are 

 then more pronounced the more frequently water has been raised to 

 a high temperature. They are more considerable when the water 

 has been alternately heated to 1 10° and then cooled in the apparatus 

 a certain number of times before being submitted to the test of dimi- 

 nution of pressure. The following are some examples in which are 

 noted in three successive columns, (1) the temperature of the 

 liquid when boiling commences ; (2) the pressure at this time ; 

 (3) the temperature at which normal ebullition would take place 

 for this pressure : — 



o nvm. 



71 175 64 



57 75 46 



66 108 53-5 



90'5 335 78-7 



53 37 33 



Retardations of 7°, 11°, 11°'8, 20°, &c. are thus seen; that 

 is, far more considerable than those observed for water in glass 

 vessels when ebullition is attained by reheating. 



Taking ordinary water, not distilled, and even tolerably calcareous, 

 the same facts are observed; but it is necessary that the water should 

 have been two or three times heated to boiling, then cooled in the 

 vessel, or submitted to a very prolonged ebullition before being sub- 

 mitted to diminutions of pressure. Normal ebullition is less rare 



