M1KSSCSE OF STEAM, ETC.] 



UNDULATORY FORCES. HEAT. 



21 



downwards on its upper side. But, if either pressure be 

 removed, the other would at once become evident, and 

 its existence be perceived. In dealing with pressures 

 beyond fifteen pounds on the square inch, such are 

 reckoned by " atmospheres ;" that is, supposing air was 

 forced into a vessel until it exercised a pressure on its 

 aides equal to thirty pounds per square inch, such would 

 be called a pressure of two atmospheres ; if of forty-five 

 pounds, it would be three atmospheres; and so on. This 

 short explanation will suffice to render the succeeding 

 remarks intelligible. 



Whilst water boils at ordinary pressures at a tem- 

 perature of 212, aud at about 90 in a vacuum, it is 

 found that a much higher temperature is required to 

 produce the same result under a 

 pressure exceeding that of the 

 atmosphere. n arrangement, 

 represented in Fig. 4, is em- 

 ployed for the purpose of ascer- 

 taining the successive boiling 

 points of water under such cir- 

 cumstances. It consists of a 

 strong copper boiler, in which 

 is fixed a tall glass tube, open 

 at both ends. The bottom of 

 the boiler contains mercury, and 

 the vessel is half filled" with 

 water. On one side a stop-cock 

 is fitted, and on the other a 

 good thermometer, whose bulb 

 enters into the metal vessel. 

 On heat being applied, and the 

 steam produced being allowed 

 to escape, the thermometer will 

 not indicate a higher tempera- 

 ture than 212, and the mer- 

 cury will not rise within the tall 

 glass tube. But, on closing the 

 stop-cock, and thus preventing 

 the escape of the steam, the 

 internal pressure will gradually 

 UM>, the mercury indicating 

 this by rising In the central 

 tube. A* the mercury rises, the 

 temperature indicated by the thermometer will also rise ; 

 showing that by an increase of pressure the boiling 

 point is raised. 



If the stop-cock is opened, the (team will escape, and 

 the pressure will be simultaneously diminished. The 

 mercury in the thermometer will fall to 212, and there 

 remain, so long as free access is allowed between the 

 external air and the boiling water. 



The following table exhibits the results obtained by an 

 experiment of this kind : 



Pmrarr per Height of Merrnrj In 



Square Inch. a B iromctcr Tube. 



15 pounds 80 inches 



30 GO 



45 90 



60 120 



75 150 



. 180 ,_ 



Temperature. 



212 



230 



275 



294 



310 



323 



It will thus be observed, that the boiling point of water 

 i increased in by no mean* a regular ratio under an 

 increased pressure of vapour on its surface. Tables in- 

 dicating this result have been extended to a much higher 

 degree than that which we have presented ; but the 

 principle will r>3 understood from what we have stated. 



Liquids, on being converted into vapour, undergo 

 great expansion. Water, when changing its liquid to the 

 Taporous state, is expanded to about 1,700 times its pre- 

 vious volume. The annexed table gives the relative 

 bulk of a cubic inch, in the liquid state, to that of the 

 condition of vapour, of water, alcohol, and ether. 



Balk at W> F. 



Water . . 1 cubic inch 

 Alcohol . 1 ,, 

 Ether . . 1 



Temperature. 



212" F. 

 173 

 96 



apour 

 Hoilintr Point. 

 H>'. ii I cubic inches 

 4!.t 

 213 



In round numbers, it is estimated that one cubic inch 

 of water is converted into one cubic foot of steam, by 

 being raised to the boiling point. 



As the state of vapour depends essentially on the main- 

 tenance of a high temperature, it follows that, on the 

 abstraction of heat, such vapours regain their liquid state, 

 and, in so doing, contract from their extended to a 

 j diminished bulk. Thus steam, of a temperature indi- 

 cated by the thermometer of 212, becomes reduced to 

 -p-iyjth of its bulk on returning to the condition of a 

 liquid. The following experiment may tend to impress 

 the fact on the mind of the student :r 



Experiment 12. In an air-tight tin vessel, having an 

 open stop-cock, boil water until the vessel is tilled with 

 steam. A cylinder twelve inches high and three inches 

 in diameter, made of thin metal, answers well for this 

 purpose. When the vessel is filled with steam by the 

 ebullition of the water, close the stop-cock, and immerse 

 the arrangement in cold water. The steam will at once 

 condense and return to its diminished bulk of water. 

 The atmospheric pressure, acting on all sides, will com- 

 press the vessel, and prove that the vapour has lost its 

 bulk by the condensation and abstraction of heat, which 

 has thus been effected. 



On these principles the condensing steam-engine is 

 constructed, of which we shall have to speak in the 

 section of Applied Mechanics. A vast variety of natural 

 phenomena also depend on the laws of vaporisation and 

 evaporation. We must defer considering these in their 

 amplitude ; for the branch of science, called Meteorology, 

 embraces these effects. 



CONDUCTION OF HEAT. 



BEFORE treating on Latent Heat, we shall consider the 

 ability which bodies have of conveying the force of heat 

 by what has been termed " conduction." 



All substances are divided by philosophers into two 

 olnttifii, so far as Thcrmotios are concerned ; and these are 

 respectively termed "conductors" and "non-conductors." 

 Nevertheless, these distinctions are empiric ; because no 

 body can be called absolutely a good or bad conductor, 

 the quality existing only in comparison with others rela- 

 tively better or worse. 



Daily life affords familiar instances wherein the pro- 

 perties of conductors and non-conductors are called into 

 play. In many domestic contrivances, one part is made 

 of a good conductor, and the other portion of a non- 

 conducting substance. The body and handles of an urn, 

 a tea-kettle, teapots, <fco., are respectively made of metal 

 and wood : the latter being a bad conductor, prevents 

 tiie heat passing from the hot metal to the hand. An 

 illustration of the law of conduction may be noticed in 

 the dillerent rate at which heat passes to the hand, from 

 the bright metallic portion of a kettle, and that part 

 covered with charcoal. A kettle filled with boiling water 

 may be safely placed on the hand, if the bottom is well 

 coated with soot, <fcc. 



Another simple mode of illustrating the comparative 

 power of conducting heat, possessed by different sub- 

 stances, is that of placing in the flame of a gas-lamp a 

 copper wire and a rod of glass of equal size. It will be 

 found that the heat of the flame travels rapidly to the 

 hand by means of the metal, whilst no sensation of heat 

 is experienced through the rod of glass. Now, because 

 the metal conveys the force of heat rapidly, it is called 

 a good conductor ; whilst the glass, conveying the force 

 but slowly, has been termed a bad conductor. 



Even metals, which are generally excellent conductors 

 of heat, vary very much in their power in this respect ; 

 as the following experiment will prove : 



Experiment 13. Twist together a piece of copper and 

 p'.atina wire, and at the extreme free ends of each attach 

 a piece of phosphorus. On applying heat at the point 

 where the two metals are in contact, the heat will travel 

 along each wire until it reaches the phosphorus. In 

 doing so it will travel faster through the copper than 

 through the platina wire illustrating the fact that cop- 

 per conducts heat quicker, and is, therefore, a better 



