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UNDULATORY FORCES. ITEAT. 



[LATENT BEAT. 



LATENT HEAT. 



CONSIDERABLE difficulty is often found by those not far 

 advanced in scientific study, in understanding the doc- 

 trine of latent forces; and before proceeding to in\ 

 gate the theories and facts respecting latent heat, it will 

 therefore be convenient that we should define our mean- 

 ing of the term. 



Those powers or forces which are evident to, and per- 

 ceived by, the senses, are termed sensible, their presence 

 being easily recognised; but in all matter there also 

 exist forces which are not perceptible to the senses, 

 until proper means are employed to call them forth. 

 Thus, no one could predicate, by simple inspection, that 

 ! a common match contained the forces of heat and light, 

 ; although it is well known that friction easily educes 

 them. 



It is manifest, that caloric, or its proximate cause, 

 must, although unperceivod, exist in a body ; because, 

 as we can create nothing, we can only call out that which 

 really, although imperceptibly, is present. Now forces 

 thus disguised are called latent, because they lie hid, 

 until called forth by special means. We have chosen a 

 familiar illustration of the meaning attached to the term 

 latent ; but we must give this a general signification, by 

 stating that all substances contain heat, light, <fcc. , in a 

 hidden state ; and this is true even in those least likely 

 to afford them. 



Ice would, at first sight, appear to be the most un- 

 likely body from which heat could be obtained ; but 

 nothing is more easy than to produce such a result, 

 as the following experiment will prove : 



Experiment 15. Make a hole in a piece of ice, and, 

 into this, introduce a pea of the metal potassium. 

 Owing to the chemical action which will take place, both 

 heat and light will be rapidly evolved. 



From this instructive experiment, we learn that both 

 heat and light must exist in a latent state ; and, in this 

 instance, chemical action is employed to render these 

 forces sensible. Our introductory experiments on the 

 production of heat are examples of a similar kind. * 



It is owing to heat being present in a latent form, 

 that gases exist ; and hence, when gases, vapours, and 

 liquids are reduced to the solid state, they invariably 

 give out heat ; and, nice versa, when solids are converted 

 into liquids or gases, they absorb sensible heat, which 

 afterwards becomes latent. On compressing air in a 

 vessel by means of a syringe, the metal of the vessel 

 generally becomes quite hot, owing to the air forced in 

 losing a portion of its latent heat. When the air is 

 allowed to escape, it will be found that the reverse effect 

 is produced, and the vessel parts with heat, and becomes 

 cold to the touch. If a bottle containing soda-water, of 

 the temperature of the air, is suddenly uncorked, and 

 its contents are poured into a vessel containing a thermo- 

 meter, the abstraction of heat, produced by the expan- 

 sion of the carbonic acid gas, will be made evident by 

 the mercury of the thermometer at once falling. 



The amount of latent heat in various vapours diners 

 materially ; and the latent heat of steam exceeds that 

 of any other. This will be evident from the fact, 

 that it requires 5J times as much heat to convert 

 a given quantity of water into steam, as it does to heat 

 the same quantity of water from freezing to boiling 

 point. 



Again, steam, on being reduced to the liquid state, 

 parts with the heat it had previously received, which 

 passes into a latent form ; and hence, a pound weight of 

 steam would heat 65 pounds of cold water to a tempe- 

 rature of 212. The reader will here perceive another 

 proof of what has been already stated that gases and 

 vapours owe their condition to the force or agency of 

 heat. 



If we examine the effect of mixing water of two 

 different temperatures in equal quantities, we shall find 

 that the resulting temperature will bo increased just half 

 of the excess of the highest. Thus the addition of one 



-Sci- tinte, p. 15. 



pound of water, at a temperature of 100, with one 

 pound at 32, will give a mean temperature of Oli J . If, 

 however, equal weights of ice and hot water are mixed 

 together, the resulting temperature will not be a mean 

 between the two ; for it will not exceed 32" until the 

 whole ice is melted. The heat of the hot water thus 

 assumes a latent form, and is employed in liquefying 

 the ice; or, in other words, in imparting mifliciont heat 

 to enable it to assume the liquid state. As an illustra- 

 tion of this, we may state that, if a pound of ice at 32 

 is mixed with a pound of water at 1"2, the difference of 

 140 will be apparently lost in converting the ice into 

 water. 



The following table, collected from the investigations 

 of Dr. Ure, represents the latent heat, at the boiling 

 point, of 



Water, as equal to ... 1000 



Alcohol, s. gr. 0.825 . . . 457 



Sulphuric ether .... 312. 9 



The abstraction of its latent heat, and the conversion 



of water into ice, may be easily effected by means of 



rapid evaporation, by the air-pump, or in an instrument 



called the Cryophorus, or " frost bearer." 



Experiment 16. Fill a watch-glass with water, and on 

 the surface of the liquid place another watch-glass filled 

 with ether. On introducing these into the receiver of 

 an air-pump, and exhausting the air in the usual manner, 

 the rapid evaporation of the ether will abstract the 

 latent heat of the water, and will convert it into ice. 

 Generally speaking, the two glasses become solidly frozen 

 together. 



This peculiar mode of producing cold has been lately 

 employed on a large scale, for the purpose of producing 

 ice in tropical countries. Special arrangements are em- 

 ployed for recovering the ether, which may thus be 

 repeatedly used, and the economy of the process main- 

 tained. 



Experiment 17. The Cryophorus is made by blowing 

 a bulb at each end of a long glass tube. One of the 

 bulbs being filled with water, heat is applied thereto, so 

 as to expel all air, and the vessel is then hermetically 

 sealed. For the purpose of this experiment, the whole 

 of the water is retained in one bulb, and the empty one 

 is immersed in a mixture of snow or pounded ice and 

 salt. This reduces it to a temperature of Fahrenheit. 

 In a very short time the water in the other bulb will 

 be converted into ice, owing to the rapid evaporation 

 and loss of latent heat which it sustains. 



By the abstraction of latent heat, a great degree of 

 cold may be produced hence the employment of freezing 

 mixtures, for the purpose of cooling wines, etc. Whilst 

 these assume the liquid state, they abstract the latent 

 heat of bodies immersed in them. A well-known instance 

 of this is the use of salt for the purpose of removing ice 

 and snow from foot-paths. In the employment of these 

 means, the snow, <fec., melts, and a great decrease of 

 temperature results. The practice, for this reason, is 

 highly to be reprehended, because it is injurious to 

 health. Persons treading in a mixture of this kind sus- 

 tain a great loss of natural heat from the feet ; and to so 

 great an extent does this take place, that the leather 

 soles of shoes, if thus moistened, will, -for the reasons we 

 have named, become the cause of dangerous diseases, 

 such as fever, diphtheria, <fec. The following are mixtures 

 which may be employed for the purpose of producing 

 intense cold. 



Experiment 18. Mix together equal weights of sal- 

 ammoniao and nitre, both in fine powder. Place the 

 mixture in a tea-cup, and, having added cold water, .stir 

 the whole by means of a thin test-tube holding cold 

 water. In a short time the liquid in the test-tube will be 

 completely frozen, owing to its latent heat being ab- 

 stracted during the solution of the salts external to it. 



Experiment 19. Mix equal weights of snow or 

 pounded ice and chloride of calcium, in the dry state. If 

 mercury, in a test-tube, is introduced into this mixture, 

 the metal will be frozen, owing to the intense cold 

 produced. 



i:.fj:rimi'nt 20. Place a small piece of lint or cottor 



