20 



UNDULATORY FORCES. HEAT. 



[EVAPORATION, KTC. 



of silver platina and gold, soldered together. Its upper 

 end is fixed to a stand ; and to the lower one an index is 

 attached, which, by traversing over a graduated plate, 

 indicates a change of temperature, owing to the unequal 

 expansion of the metal forming the spiral This is one 

 of the most elegant adaptations of a scientific law which 

 has yet been made ; and for ascertaining minute changes 

 between low rates of temperature, it is a valuable instru- 

 ment in the hands of the philosopher. 



It is scarcely necessary to point out instances in which 

 liquefaction and fusion operate to our advantage in daily 

 life. The processes of metallurgy, such as smelting and 

 refining, casting, moulding, and other such operations, 

 are sufficiently familiar. We may remark, however, that 

 the difference of temperature at which bodies cease to be 

 solids is a matter of great importance, and leaves an ex- 

 tensive choice of material for various purposes.extending 

 from the refracting crucible used by the chemist, to the 

 more easily melted, and therefore more tractable, sub- 

 stances employed for economic processes. 



THE EFFECTS OF HEAT. EVAPORATION, 

 VAPORISATION, ETC. 



CONTINUING to restrict our remarks to the effect of sen- 

 tibU heat, we now examine the processes of Vaporisation 

 and Evaporation. At first sight, these terms seem conver- 

 tible ; but in scientific phraseology they hold a distinct 

 meaning. Vaporisation is that process in which anything 

 is converted into a state of vapour, by the addition of a 

 considerable amount of heat; whilst the term "evapora- 

 tion" refers to a process continually and imperceptibly 

 going on at or about the ordinary temperature of the 

 atmosphere. Thus steam is produced by vaporisation, 

 and clouds. <fec., by evaporation. 



It will be here necessary to point out the distinction 

 which is maintained between the terms "vapour" and 

 "gas. " By vapour we understand an unstable condition in 

 which a body exists, so long as a comparatively high tem- 

 peratxire is maintained. Thus the steam of water, alcohol, 

 ether, etc., fall under this category. The air, coal gas, 

 <fec. , are, however, under all ordinary circumstances, in a 

 constant state ; and hence are ranked under the term 

 "gas." It is true that some gases, as carbonic acid gas, or 

 fixed air, may, by great pressure and intense cold, be 

 reduced to the solid state ; but these results being extra- 

 ordinary, are not considered to militate against the jus- 

 tice of the rule which we have laid down. We may add, 

 that vapours can easily be converted into liquids ; whilst 

 gases present great resistance to that process ; hence the 

 distinction of the terms. The consideration of a perma- 

 nent gaseous state, we shall take up under the head of 

 Latent Heat. 



The process of evaporation is continually going on at 

 the ordinary temperatures. By placing an inverted 

 glass over grass during the heat of a summer day, it will 

 be found that moisture soon forms at the top of the 

 vessel. The insensible perspiration which continually 

 exudes from the skin of animals, the gradual loss of fluid 

 from an uncovered vessel, and other similar instances, 

 are due to this constant process. 



Whenever evaporation takes place, cold is at once pro- 

 duced ; and hence, if a porous vessel, containing a liquid, 

 is placed in any position where a draught of air can act 

 on it, the result is that its sensible temperature is much 

 diminished. The cause of these phenomena will bo more 

 fully dealt with presently. 



On applying heat to liquids, and some solids, they may 

 be made to assume a vaporous form; but, as in the 

 conversion of a solid to the liquid state, different liquids 

 require unequal increments of heat to effect tliis purpose. 

 Ether, for instance, is converted into vapour at a tem- 

 perature of 90 Fahrenheit ; whilst mercury requires 

 that of 660 Fahrenheit : both of these, however, may 

 evaporate at lower temperatures. The temperature of 

 ebullition, or rapid conversion into vapour, is termed the 

 boiling point. 



We here refer to vaporisation as taking place under 

 ordinary circumstances ; but the boiling point of liquids 



varies when the pressure of the air is raised or lowered. 

 We sliall take these different cases in the order of com- 

 mon pressures, the vacuum of the air-pump, and extraor- 

 dinary pressure, as in the high-pressure steam-lxiili-r. 



The boiling point of liquids can easily be ascertained 

 by immersing a mercurial thermometer in them ; but a 

 metal vessel must be employed to hold them, because 

 those of glass or porcelain have the effect of raising the 

 apparent boiling point, perhaps through the inferior con- 

 ducting power of their material, and from other causes. 



Annexed is a skeleton table, which affords the boiling 

 point of some liquids ; and it is supposed that the trm- 

 perature has been taken whilst the barometer stands at 

 30 inches. 



Boiling point, on Fahrenheit's scale, of 

 Sulphuric ether . . . . % to 98 

 Pure alcohol (spirits of wine) . . . 17-1 



Water 212 



Nitric acid 242 



Sulphuric acid .... 550 



Sulphur 500 



Mercury 000 



If liquids are heated under a lower pressure than that 

 of the atmosphere, they will boil at a much lower tem- 

 perature, as the following simple experiment will illus- 

 trate : 



Experiment 11. Boil some water in a glass flask, and, 

 when in full ebullition, fit a cork air-tight into the neck 

 of the vessel. Remove the latter from the source of 

 heat, and dip it into cold water. Because a vacuum is thus 

 formed, and the pressure of the air removed, the water 

 will recommence to boil. 



If ether, spirits of wine, or water heated to 98, is 

 placed under the receiver of an air-pump, and the air is 

 exhausted, each may be made to boil at ordinary tem- 

 peratures, because the pressure of the atmosphere, of 

 course, cannot affect them in a vacuum. This circum- 

 stance has been taken advantage of by sugar-refiners 

 and pharmaceutists, who are in the habit of boiling their 

 liquids in vacuo at a low temperature, and thereby pre- 

 vent the formation of certain products, which might 

 prove injurious to their processes : economy, also, is found 

 to accrue thereby. 



The property of liquids boiling at lower temperatures 

 when the superincumbent pressure of the air lias been 

 removed, has been used as a means of ascertaining the 

 heights of mountains. As we ascend in the atmosphere, 

 the pressure decreases; the mercury in the barometer 

 descends ; and, therefore, water will boil at a lower tem- 

 perature. An approximate ratio of the decrease of the 

 boiling point, may be taken as one degree of Fahrenheit's 

 scale to an elevation of 520 feet above the level of the 

 sea ; hence, on the top of a mountain 10,000 feet high, 

 water would boil at a temperature of 192 Fahrenheit. 

 Heights found in this manner are, after all, only rough 

 calculations, inasmuch as that barometric changes may 

 not be synchronous between the base and summit of 

 the mountain. 



The following table gives the three elements to which 

 we have alluded namely, the boiling point of water, the 

 barometric height, and the corresponding elevation, in 

 round numbers, above the surface of the earth. The 

 barometer is presumed to stand at 30 inches at the level 

 of the sea : 



Boilini; Point 



of \Vnter. 



212 



210 

 20&5 

 200.5 

 205 nearly 



Elevation of 

 Observer, 

 surface 

 1050 feet 

 1850 

 2900 

 3700 



Hi-ightof 



l'..unineter. 



30 

 29 

 28 

 27 

 20 



Having occasionally employed the term " atmospheric 

 pressure," it is necessary that we should define its mean- 

 ing before proceeding further. At the level of the sea 

 the air presses on every surface with a force of nearly 

 fifteen pounds on a square inch. It ia true that this 

 pressure is not felt, because it acts on each and every 

 side ; and thus, on holding out the hand, wo iiutain as 

 much pressure upwards on its under surface, as we do 



