M 



UNDULATORY FORCES. HEAT. [IUDUTIOX UEW ON PLANTS. 



The student will find considerable difficulty La recon- 

 ciling the laws of latent and specific heat with those ol 

 the undulatory theory (which we entered into at the 

 commencement of this section), more especially if ho 

 observe such phrases as "the quantities of heat which a 

 body contains ;" which are often employed in treatises on 

 this subject It is tnie that these are only used to 

 popularise our ideas ; but still there will be a tendency 

 to take a material view of the nature of heat whilst ex- 

 perimenting with the force ; and this will be, to some 

 extent, continued when the effects of specific and latent 

 heat are kept alone in view. Theories, however, are 

 valuable only so far as they generalise numerous facts , 

 and the xmdulatory theory is insisted upon by philo- 

 sophers, because it collects under one broad view a 

 chain of allied causes. In its application to the branch 

 of Thermotics, with which we are now dealing, many 

 difficulties arise, which cannot be cleared up until our 

 researches have been so far extended aa to give a greater 

 insight into the nature of heat, <fec. , than we at present 

 possess. When we refer to radiant and polarised heat, 

 the value of the uudulatory theory will be at once per- 

 ceived, because we shall be able to range so many pheno- 

 mena under one comprehensive idea. 



Whatever theory is employed to connect the various 

 facts of Thennotics, it is evident that the actual amount 

 of caloric in any body cannot be measured ; and that the 

 specific heat of any substance is merely a term signify- 

 ing its relative power of absorbing the force as compared 

 with that possessed by other bodies. There is, however, 

 a veiy singular relation existing between the atomic 

 weight, or combining proportion, and the specific heat of 

 many substances. 



In our prefatory remarks, we stated that all matter is 

 composed of atoms ; and by examining the chemical 

 relationship of these atoms, we find that they have a 

 tendency to combine in certain proportions only, and 

 every elementary body has accordingly had assigned to 

 it a number which expresses the relative value of its 

 power of combination. As an instance of this, the 

 analysis of water shows that eight parts, by weight, of 

 oxygen combine with one part of hydrogen, and, bv their 

 union, nine parts of water are produced. These" num- 

 bers are called the equivalents, atomic weights, or com- 

 bining proportion of these gases ; and it is with these 

 relative weights that the specific heat of a body seeuis to 

 have some relation in certain instances. If, for instance, 

 the number 3. 1 is divided by the number expressing the 

 specific heat, say of iron, zinc, or copper, we find that 

 the quotient very nearly corresponds with tire atomic 

 weights or equivalents of those metals when expressed in 

 a series of which that of hydrogen is unity. This will be 

 observed by the annexed table. 



Specific heat of 



Iron 



Copper 

 Zinc 



Calculated 

 Equivalent. 



Equivalent obtained 



by Chemical 



' Analysis. 



28.0 

 31.7 

 32.0 



0.114 27.2 



0.095 32.6 



0.095 32.6 



These relationships are, however, by no means so evi- 

 dent ,in other bodies, although, generally speaking, the 

 product of the specific heat and the atomic weight is 

 cither nearly a constant sum or one which, is a multiple 

 thereof. In this, as hi many other connections of dif- 

 ferent phenomena, wo meet with difficulties which pro- 

 vent our affirming the existence of a law of which we 

 have as yet but a glimpse. 



Wo have hitherto referred solely to the specific heat 

 of solids and liquids; and, with respect to gases, the 

 information which lias been obtained is by no means 

 satisfactory. This is chiefly owing to the difficulty 

 which exists in carrying out the investigations in an 

 accurate manner, in any case, with aeriform bodies. 

 This is generally done by passing the gas under ex- 

 amination, at a certain temperature, through water, also 

 of a known temperature; and, in proportion as the heat 

 of the liquid is raised by the same bulk of each gas, their 

 relative capacity for heat is ascertained, and the specific 

 heat arrived at. 



RADIATION, ABSORPTION, AND REFLECTION 

 OF HEAT. 



HAVING directed attention to the leading facts which the 

 study of sensible, latent, and specific heat affords, we 

 proceed to consider those phenomena which present 

 themselves when rays of heat are emitted from a central 

 source. 



The intensity of all radiant forces is diminished in a 

 geometrical ratio as the centre of force is departed from ; 

 and it is found that this intensity is inversely as the 

 square of the distance. The student will, perhaps, more 

 readily understand this by placing before a candl.' !l;un ; 

 a square piece of cardboard, of any size. If this gives a 

 shadow of one foot square at a distance of a foot from 

 the source of light, then that shadow will cover an area 

 of four square foot at double the distance, nine square 

 at three times the distance, and so forth. The intensity 

 or amount of light will diminish in the same ratio as the 

 space over which it spreads is increased. The law holds 

 good with respect to gravitation,* light, electricity, mag- 

 netism, and other central forces. So far as heat is con- 

 cerned, we shall at present confine our attention to its 

 radiation through air ; because, like light, it sutlers both 

 refraction and polarisation when it is passed through 

 certain media subjects on which we shall subsequently 

 dwell. 



Bodies vary much in their power of radiating heat ; and 

 those having a dark and rough surface hold the highest 

 place in tho scale. Pointed surfaces especially fall 

 under this class, whilst polished ones scarcely radiate heat 

 at all. 



Absorption is a property by which bodies are enabled 

 to take in heat from any source, and thus have their 

 temperature rapidly raised. Generally speaking, good 

 radiators are also good absorbers, because each effect is 

 due to a similar state of the body under examination. 

 Of this class are lamp-black, cloth, and bodies of a dark 

 colour ; and the truth of this will be at once made evident 

 by the following experiments : 



Experiment 21. Inclose a thermometer in a metal 

 cylinder which has a polished external surface, and 

 introduce ono into a vessel which has been blackened by 

 means of lamp-black and glue. Fill these with water, 

 and expose them to the action of the rays of the sun. It 

 will be found that, owing to the blackened surface, the 

 rays of heat will be rapidly absorbed, and the temperature 

 of that vessel will quickly rise. In the one having a 

 polished surface, very little change of temperature will 

 be produced, because the rays of heat are reflected, and 

 therefore not absorbed. 



Experiment 22. If the two vessels are exposed in the 

 open air on a clear night, it will be found that tho 

 temperature of the blackened vessel will speedily fall, 

 owing to the loss of heat caused by radiation, wliilst that 

 of the polished surface will scarcely be affected. 



Hence we find that good radiators are good absorbers, 

 and that bodies liaving a polished surface reflect heat 

 freely, but neither radiate nor absorb it to any extent. 

 The best radiator, such as lamp-black, will emit eight 

 times as many rays in the same period as a polished sur- 

 face, sucli as a silver plate would do under the same 

 circumstances. 



A great variety of interesting phenomena in nature 

 depend on these laws of radiation, &c. Dew is formed 

 owing to the radiating power of tho earth's surface ; and 

 if a thermometer is plarod at night on a grass-plot, when 

 tho sky is quite free from clouds, tho temperature indi- 

 cated will be much below that shown by a thermometer 

 raised a few foet from tho ground. The radiation of 

 lieat thus cools down the temperature of the surrounding 

 lir, and the moisture, being condensed, falls down and 

 rests on tho surface of plants, grass, <tc. 



The temperature at sea is always higher at night-timo 

 ;han on land, because the level surface of tho water is a 

 >ad radiator. The peculiar effect of the sea and laud 

 Breezes is owingto the changes resulting from the radiating 



See Introductory Chapter; p. 3, ante. 



