(5^40 tECTURE hi. 



iTientation of the elastirfty when the fluid is confined to a certain space. This 

 expansion is very nearly the same for all gases and vapours, amounting to 

 ^l^ for each degree, at the common temperature of 50° of Fahrenheit, but 

 at higher temperatures it is less than -j-^-o- of the bulk of the gas, and at lower 

 temperatures somewhat more, being nearly the same in quantity for the same 

 portion of the fluid at all temperatures. 



When an elastic fluid is contracted by cold within certain limits, determined 

 by the degree of pressure to M'hich it is exposed, as well as by the nature of the 

 fluid, its particles become subjected to the force of cohesion ; they rush still 

 nearer together, and form a liquid. Thus, when steam, under the common 

 atmospheric pressure, is cooled below the heat of boiling water, it is instantly 

 condensed, and becomes water: but with a pressure of two atmospheres, it would 

 be condensed at a temperature 36° higher, and with the pressure of half our 

 atmosphere only, it might be cooled without condensation 33° lower than the 

 common temperature of boiling water. And similar effects take place in 

 vapours of other kinds at higher or lower temperatures, a double pressure pro- 

 ducing, in all cases an equal disposition to condensation, with a depression of 

 temperature of between 20 and 40 degrees, and most commonly of about 35°, 

 of Fahrenheit. Thus, the vapour of spirit of wine is usually condensed at 

 175° of Fahrenheit; but with a double pressure it is condensed at a temperature 

 39° higher; and with the pressure of half an atmosphere, at a temperature 35° 

 lower|; and the vapour of ether, which is commonly condensed at 10£°, 

 requires a temperature 38° higher, with a double pressure, or as much lower, 

 ■with half the usual pressure. If the temperature be below the freezing point 

 of the liquid, the pressure being sufticiently lessened, the vapour may still 

 retain its elasticity, but a further reduction of temperature or increase of 

 pressure will convert it immediately into a solid. 



The expansion of liquids by the eflfect of heat is much less uniform and 

 regular than that of elastic fluids, since it varies considerably, not only in 

 different liquids, but also in the same liquid at different temperatures, being 

 in general greater as the temperature is more elevated, and sometimes almost 

 in proportion to the excess of the temperature above a certain point, at which 

 it begins. This variation appears to be the least considerable in mercury, 

 although even this fluid expands a little more rapidly as it becomes more 

 heated ; but the expansion is always very nearly one ten thousandth for each 



