LATE-VT H EAT DISTILLATION. ] UNDULATOKY FORCES. H EAT. 



wool on the bulb of a thermometer, and moisten it with 

 a little ether or sulphide of carbon. The evaporation, 

 wliich will thus take place very rapidly, causes the mer- 

 cury of the thermometer to fall, sustaining, as it will do, 

 a loss of latent heat. 



Without multiplying instances of this kind, we may 

 refer to an extended application of the principle in the 

 condenser of the steam-engine, and the ordinary process 

 of distillation. With respect to the mechanical details of 

 the arrangement used for the former purpose, we must 

 defer any detailed description to the section on Me- 

 chanical Philosophy, wherein the subject will be fully 

 dealt with. The steam, after exercising its force on one 

 side of the piston, is ejected into the condenser, wliich is 

 an air-tight vessel, surrounded by a cistern of cold water. 

 A jet of cold water plays into the condenser, and, imme- 

 diately on the steam entering, its latent heat is abstracted 

 by the cold water, and it sutlers condensation, suddenly 

 contracting to a bulk of -nVstn part of that which it occu- 

 pied in the state of vapour. To the researches of Dr. 

 Black and James Watt, this application of the laws of 

 Lite" t heat is chiefly due. 



In the process of distillation, heat is applied for the 

 purpose of con verting a liquid into the state of vapour, and 

 of thus separating it from others with which it had been 

 previously mixed. The vapour thus formed is again 

 condensed by passing it through a metal worm sur- 

 rounded with cold water. 



In the ordinary process of manufacturing spirits, stills 

 of great size are employed, and they are generally made 

 of copj .-. The " wash," or mixture, formed by infusing 

 malt, and fermenting it with yeast, is introduced into the 

 body of the still, and, heat being applied, the spirit, 

 which boils at a lower temperature than water, passes 

 over into the worm ; and, being there condensed, it re- 

 assumes the liquid state. The stills and general arrange- 

 ment vary in .shape, A'c. ; but a common tea-kettle, to 

 which two or three feet of metal tubing has been attached 

 at its spout, will illustrate the whole principle of the 

 process. The mechanical arrangements alone vary from 

 this simple mode of distilling, when adopted by the spirit 

 distiller. 



The latent heat of vapour seems to vary as the tem- 

 perature is increased ; but the differences hitherto ascer- 

 tained do not warrant any reliance on their accuracy. 



A very common application of the laws of latent heat 

 is that of drying and warming by means of steam con- 

 veyed through pipes. Owing to the latent heat of steam 

 being so great, it may be made a vehicle to carry heat to 

 great distance from its source ; and it must be borne in 

 mind that steam will heat six times as much space 

 (nearly) as a piece of metal of the same weight and sen- 

 sible temperature. Hence its employment for manufac- 

 turing purposes, especially in bleaching, dyeing, calico- 

 printing, paper-making, tallow-melting, <kc. It is usual 

 to cover the pipes conveying the steam with felt, until 

 they arrive at the spot where heat is required, where of 

 course they are left uncovered, so that the heat may 

 freely pass off. Steam is an economical agent for warm- 

 ing apartment*, and exceeds in this respect water, when 

 similarly applied; because, whilst the water only gives 

 up an amount of heat equal to the difference between its 

 own temperature and tliat of the air of the apartment in 

 which it is employed, steam affords also its latent heat, 

 and, therefore, more entirely and conveniently answers 

 the purpose. 



On placing the hand over a jet of steam proceeding 

 from a boiler, it will be found that the higher the pressure 

 under which the steam escapes, the lower will its sensible 

 temperature appear. At first sight this seems para- 

 al, but the difficulty at once disappears on applying 

 the laws of latent heat to its explanation. Of course, as 

 the steam rushes out of the jet, it must expand. In 

 so doing, it robs the surrounding air of heat, so as 

 to permit of its expansion ; and hence, the stronger the 

 pressure of emission, the cooler is the blast. When tried 

 with a pressure of 120 pounds on the square inch, 

 the jet of steam seems to have the same effect on the 

 hand as a jet of cool sir. A vacuum ia also formed 



VOL. L 



during this expansion, and a ball of wood may be thus 

 kept playing in a jet of steam for any length of time. 



The abstraction of latent heat, together with pressure, 

 has been applied by Dr. Faraday and others to the lique- 

 faction of gases. Of all experiments of this kind, the 

 condensation of carbonic acid gas has been the most 

 successful. The gas is manufactured on a large scale, in 

 a strong metallic vessel, and is then allowed to pass into 

 a receiver. On issuing from a jet placed in the side of 

 the receiver, the gas expands so rapidly as to produce 

 intense cold, and undergoes congelation. A very usual 

 course adopted to liquefy some gases, is to enclose the 

 materials from wliich they are procured in a strong glass 

 tube, bent at an angle at its centre. Each end being 

 hermetically sealed, the gas is received in the liquid 

 state in that end opposite to where it is being produced. 

 By these means, carbonic acid, cyanogen, <tc. , have been 

 obtained in the liquid state. 



If a jet of air issuing from an orifice under a pressure 

 of 00 to 100 pounds on the square inch, is allowed to im- 

 pinge on a good conductor, its moisture will be imme- 

 diately frozen. In some deep mines in Germany, where 

 air is pressed by a high column of water for the purposes 

 of a blast, it is no uncommon occurrence to observe snow 

 produced from the moisture of the air issuing from the 

 pipes, or from any crack in the arrangement ; and it is 

 the custom of the miners to place their caps against the 

 jet, for the purpose of showing the effect to the visitor. 



SPECIFIC HEAT. 



IT has been already stated, that if equal quantities of 

 water, at different temperatures, be added together, the 

 resulting temperature will bo a mean between the ex- 

 tremes. But that result will not occur when liquids 

 of different kinds are similarly mixed. Thus, if mer- 

 cury and water, at different temperatures and in equal 

 weights, be added to each other, the resulting tempera- 

 ture will be nearer that of the water, presuming that to 

 have been the hottest of the two liquids. 



This fact has given rise to the term Specific Heat, 

 which means that different bodies have various capacities 

 for heat some requiring more, and some less, of sensible 

 heat to raise them to an equal or standard temperature. 

 The ratio of these capacities is found by mixing liquids, 

 Arc , at different temperatures, and noticing that which 

 results from the mixture; and, in most cases, the specific 

 heat, or capacity of water for heat, is adopted as the 

 standard. 



The best illustration of the mode usually adopted, may 

 be found by taking mercury as the type of other bodies ; 

 and we shall thus be able to explain the general law which 

 governs all matter with respect to its capacity for heat. 



If one pound of mercury, at a temperature of 40, be 

 mixed with a pound of water at 100, the resulting 

 temperature of the mixture will not bo a mean between 

 the two, but will be found to be 97 i- Thus the heat 

 of the mercury will be raised about 57 J; whilst the 

 water will be reduced in temperature to the extent of 2J. 



Now, putting these figures into the ordinary form of 

 proportion, we have 



67.6 : 2.6 :: 1 : 0.04348. 



In which 57.5 represents the number of degrees by 

 which the temperature of the mercury is raised ; 2.5 the 

 loss of heat sustained by the water ; and 0. 04348 is the 

 specific heat of mercury compared to water, as the 

 standard, or 1.00000. The figures here obtained are 

 somewhat too h-gh ; but the illustration will give an 

 insight into the means usually employed for the purpose 

 of obtaining the specific heat of bodies. The plan ia 

 varied with solids ; but the principle is still the same. 



The following table gives the specific heat of some 

 bodies, water being the standard : 



Water f . 1.000 



Alcohol O.C60 



Iron 0.114 



Copper 0.095 



Zinc 0.095 



Platina 0.032 



