98 



HEAT OR CALORIC. 



be exhibited several times, but each time in a diminishing degree, 

 until it ceases. 



8. CULINARY PARADOX. 



Ebullition by Cold* Dr. Hare, 8 to 14. 



" A matrass, half full of water, being 

 heated until all the contained air is ex- 

 pelled by steam ; the orifice is closed 

 so as to be perfectly air tight. The 

 matrass is then supported upon its neck, 

 in an inverted position, by means of a 

 circular block of wood. A partial con- 

 densation of the steam soon follows, 

 from the refrigeration of that portion 

 of the glass which is not in contact 

 with the water. The pressure of the 

 steam upon the liquid of course be- 

 comes less, and its boiling point is ne- 

 cessarily lowered. Hence it begins 

 again to present all the phenomena of 

 ebullition ; and will continue boiling, 

 sometimes for nearly an hour." 



" By the application of ice, or of n 

 sponge soaked in cold water, the ebullition is accelerated ; because 

 the aqueous vapor, which opposes it, is in that case more rapidly 

 condensed : but as the caloric is at the same time more rapidly ab- 

 stracted from the water, by the increased evolution of vapor, to re- 

 place that which is condensed, the boiling will cease the sooner." 



* This fact is pleasingly exhibited, by providing two cylindrical glass vessels, of 

 one quart or two in capacity, (the quart or three-pint tumblers, sold in the shops, 

 answer very well) ; into one of them pour cold, and into the other hot water ; then 

 immerse alternately in each, a flask which contains water that was, just before, 

 while boiling, cut off, by a good cork, from the atmosphere ; in the cold water it 

 will boil vehemently, and in the hot it will cease boiling. 



A retort if treated in a similar manner, is a still better instrument, because it pre- 

 sents in the ball, a large surface for warming or cooling; and a little cold or hot 

 water poured on cautiously, while the retort is hanging in a ring, produces a very 

 striking effect. If the retort be very thin, and especially if large, there is danger of 

 its being crushed by the pressure of the atmosphere. I have repeatedly met with 

 this accident, with both retorts and flasks $ but it is not dangerous, as the fragment 

 do not fly about. 



