18 



UNDULATORY FORCES. HEAT. 



[LIQUEFACTION, ETC. 



interesting feature in Physical and Natural Science, the 

 results being more definite and constant than those 

 found in the expansion of solids and liquids. 



The simplest mode of illustrating the fact, that a gas, 

 such as air, expands by an addition of sensible heat, is 

 by plunging the stem of a retort or thermometer tube, full 

 of air, beneath the surface of water, and applying heat to 

 the bulb. On the air expanding, a portion wiU escape 

 in bubbles through the water ; and on the source of heat 

 being removed, the water will rise in the neck, to take 

 the place of the air which has been expelled by heat. 



Air thermometers are constructed on this principle ; 

 and a very simple one may bo made by fixing a glass 

 tube, by means of a cork, into the neck of a glass flask. 

 If the flask is partly filled with a coloured liquid, and 

 inverted so that the end of the tube shall rest in a cistern 

 of water, the air remaining in the flask will expand on a 

 slight increase of its temperature, and the liquid in the 

 tube will be driven downwards, recovering its position on 

 the application of heat ceasing. Differential thermome- 

 ters are of this kind, but have two bulbs, into one of 

 which the liquid passes on the expanded air pressing it 

 downwards. 



Nearly all gases expand equally on receiving an equal 

 increase of temperature ; at least, in practice, their 

 differences may be neglected for all ordinary purposes. 

 After careful experiments, it has been ascertained that 

 they increase in the proportion of j-.V^rd part for every 

 degree of temperature rising from the freezing point of 

 water, or 32 Fahrenheit. 



In other words, if a volume of air measure 493 parts 

 at 32, it would become expanded to 986 parts on attain- 

 ing a temperature of 525. Any intermediate result may 

 ] be obtained for other temperatures by adding T hrd part 

 | for each degree of increase. The following formula ex- 

 presses this rule, where x represents the volume sought, 

 o the normal volume, and t the number of degrees of 

 increment of temperature; 32 Fahrenheit being the 

 standard : 



* = *+< 



To illustrate this we will suppose that it is desired to 

 ascertain the expansion which air will undergo by being 

 heated from 32 to 212. Supposing a is taken' as 100 

 parts, t as 180, being the difference between 32 and 

 212, we shall have 



TOO 

 x = 100 -f x 180 = 136.5 



or, one hundred parts of air, on being heated from freez- 

 ing to boiling point, expand to one hundred and thirty- 

 six parts and a-half. 



In applying this rule to other gases, an allowance has 

 to be made for very slight variations in the ratios of 

 expansion. We have, in our example, supposed that 100 

 parts of air expands to a volume of 1865, on being heated 

 from 32 to 212 Fahrenheit. In the following table, 

 the ratio of the different expansion is stated for a similar 

 change of temperature in various gases, unity being the 

 standard, and on the authority of Regnault : 



Air 1.36700 



Hydrogen .... 1.36613 

 Carbonic acid . . . 1.37099 



By multiplying any number of volumes by the above 

 factors, the actual expansion which they will undergo by 

 being heated from 32 to 212 is at once obtained. 



Having thus explained the laws which govern the 

 expansion of aeriform or gaseous bodies, we may pass on 

 to consider some instances in which their application 

 becomes a matter of general interest. 



The various changes of the wind depend on the unequal 

 expansion of the atmosphere by the agency of heat. 

 Expanded air being lighter, of course tends to rise, and, 

 consequently, air of a more dense and cooler nature 

 rushes in on all sides to take its place. We thus have 

 either the gentle breeze, the hurricane, or the whirlwind. 

 To the engineer, the expansion of gases and vapours is of 

 the utmost importance ; and, within the last few years, 

 heated air has been extensively employed as a source of 



motive power by Ericson and others ; of which we shall 

 speak under the head of Mechanics. In daily life we 

 have to study and apply the laws of expansion in 

 attempting the ventilation of public and private buildings 

 a subject to which we shall draw special attention here- 

 after. It would be impossible to enumerate all the 

 instances wherein these laws exercise an influence. We, 

 however, cannot fail to remark, that an astonishing range 

 of application exists in the effects of expansion by heat ; 

 for, under the same cause, the fierce storm in one place 

 may be spreading desolation, whilst the gentle current of 

 air is supporting existence, in its quiet obedience thereto 

 in other situations. We thus observe how one cause may 

 be productive of a multiplicity of effects ; and, perhaps, 

 one of the greatest charms of experimental science, is the 

 frequent disclosure of such intcri-sting instances of design 

 and adaptation of means to an end, which the attentive 

 student is continually witnessing. 



THE EFFECTS OF HEAT. LIQUEFACTION AND 

 FUSION. 



WE proceed to consider the effects of increasing the tem- 

 perature of bodies by the addition of sensible heat ; and 

 we shall find that unequal results are produced on equal 

 increments of heat being made to various substances. 

 We have already noticed, that whilst some bodies are 

 generally found in a solid state, others take that of the 

 liquid, or gaseous, at ordinary temperatures; and the 

 most cursory observation will show, that an addition or 

 abstraction of heat would be necessary to render all bodies 

 of a uniform condition. 



The terms "liquefaction" and " fusion, " although they 

 appear at first sight almost synonymous, have, neverthe- 

 less, very different significations. We liquefy, or render 

 liquid, ice ; but, generally speaking, the term "fusion" is 

 applied to the process of melting solids, such as lead or 

 rock-crystal ; and although ice is as much a solid as glass, 

 still there is considerable convenience in maintaining this 

 distinction of terms. 



On increasing the sensible temperature of a substance, 

 the effects vary considerably. On heating ice, the solid 

 is immediately converted into a liquid; and the most 

 accurate observations fail to trace any notable effect 

 intervening between these two conditions. If, however, 

 zinc is submitted to the same process, we find that its 

 solid state is considerably modified ; and, whilst brittle 

 at first, it becomes softer and more malleable before it 

 enters into a state of fusion. The metal tin becomes 

 pulverisable, or, rather, easily divisible at high tempera- 

 tures ; and masses of iron or platina may be welded or 

 united together at a heat below their point of fusion. 



It will be thus observed, that a variety of phenomena 

 may occur between the solid state and its conversion into 

 that of a liquid. The separate causes which induce this 

 chain of variation are entirely hidden from us ; but wo 

 presume that the series of effects results from a peculiar 

 arrangement of the molecules or particles of which a body 

 is composed, and is thus owing to the relative position 

 which they may assume whilst undergoing the process of 

 heating. 



In the conversion of the solid into the liquid state, bodies 

 of different kinds absorb unequal amounts of caloric ; and, 

 consequently, we find that their melting points vary ac- 

 cordingly ; or, which is the same thing, the liquid state 

 becomes converted to the solid at different temperatures. 



We shall have to consider the cause of these variations 

 more fully when we treat on Specific and Latent Heat ; 

 but the following table will give an idea of the great 

 difference which exists amongst solid bodies when changing 

 their condition to that of the liquid state : 

 The Melting or Fusing point, according to the scale of 



Fahrenheit, of 



Mercury, is . . . . 40 

 Water (ice) . . . . + 32 



Tallow 98 



Phosphorus .... 110 

 Stearine (variable) . . . 120 

 Sulphur 226 



