ON THE SOURCES AND EFFECTS OF HEAT. 491 



divisions, but by their effects on our senses : and we may also conclude 

 that there is some similar relation between heated and luminous bodies of 

 different kinds. 



The effects of heat, thus originating, and thus communicated, may be 

 divided into those which are temporary only, and those which are per- 

 manent. The permanent effects are principally confined to solids, but the 

 temporary effects are different with respect to substances in different states 

 of aggregation, and they also frequently comprehend a change from one 

 of these states to another. The effect of heat on an elastic fluid is the 

 simplest of all these, being merely an expansion of about one five 

 hundredth of its bulk for each degree of Fahrenheit that the temperature 

 is raised ; or an equivalent augmentation of the elasticity when the fluid 

 is confined to a certain space. This expansion is very nearly the same for 

 all gases and vapours, amounting to -^ for each degree, at the common 

 temperature of 56 of Fahrenheit, but at higher temperatures it is less 

 than 3-j-ff 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, deter- 

 mined by the degree of pressure to which 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 wotild be condensed at a temperature 36 

 higher, and with the pressure of half our atmosphere only, it might 

 be cooled without condensation 83 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 producing 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 Fahren- 

 heit. 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 

 102, 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 sufficiently 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 effect 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 



