633 



HEAT. 



HEAT. 



634 



symptoms ; and in men in whom a naturally nervous temperament has 

 been rendered more irritable by the too free use of stimuli or by 

 depressing passions. In these cases strong pulsations of the heart are 

 experienced, increased by exertion to such an extent as to produce 

 palpitations, a sense of faintness, and shortness of breath amounting to 

 suffocation. Sometimes a loud bellows sound is heard at the heart, 

 and in patients labouring under anaemia and chlorosis this is frequently 

 accompanied by a noise like snoring, heard chiefly in the carotid and 

 crural arteries. In all these cases great attention should be paid to the 

 general health : in the cases of anjemia and hysteria, iron medicines 

 and the shower-bath will be of the greatest service ; and, as far as 

 possible, sources of anxiety should be removed. The morbid sounds 

 will all disappear as the mind and body of the patient are restored to 

 their healthy condition, but it must be borne in mind that the heart 

 is essentially a muscular organ, and by severe exercise it may be 

 increased in bulk like other muscles; consequently if these nervous 

 states which give rise to so much action of the heart be not removed, 

 they may in process of time lay the foundation for more permanent 

 and serious disease. 



Malformation of the Heart. Cases of permanent malformation of 

 the heart are few. The only one demanding attention here is the 

 permanent foramen ovale. [CYANOSIS.] 



HEAT. This great natural agent, which is universally diffused, 

 becomes sensible to us in the first instance by our bodily sensations, 

 but we find afterwards its effects in the various changes which it is 

 capable of producing on all substances. Expansion, fusion, evapora- 

 tion, thermo-electric currents, and various physiological phenomena, 

 are effects of heat, or at least accompany its absorption. 



Every existing substance may be regarded as a source of heat. In 

 this respect the sun is the most important natural source which our 

 system possesses, its heat when condensed in the focus of a lens being 

 exceedingly intense. When concentrated by a number of powerful 

 lenses on one scale of a balance of extreme sensibility, no derangement 

 of equilibrium ensues ; as far therefore as experiment can inform us it 

 is imponderable, and the increase or diminution of heat in any body is 

 therefore unaccompanied by any alteration of weight. 



Heat may be produced artificially by any means which propagate 

 agitations internally in bodies : hence friction, percussion, sudden con- 

 denw< "I combination, including combuition and repiratioa, 



and electrical dUcharyet, are all proper to produce or rather to develope 

 heat. Pouillet has also shown that the act of moistening any dry sub- 

 stance produces a rise in temperature of from 4 to 2 in dry mineral 

 substances in powder, and from 2 to 10 or 11 in certain animal and 

 vegetable substances, such as cotton, hair, wool, paper, &c. 



As to the nature of heat, whether it should be regarded as a sub- 

 stance or an accident, has been discussed from the time of Bacon to 

 the present day. Those who regard it as having a material existence, 

 suppose that a subtle fluid, called caloric, capable of permeating the 

 densest substances, is universally diffused ; that its parts are mutually 

 repulsive, but are attracted by the material particles of bodies, and 

 hence they account for the expansions and contractions of bodies, 

 while the effects of radiant heat are explained on principles analogous 

 to those on which the undulatory theory of light is founded. 



Those who regard heat as only accidental to matter, rest their opinion 

 on the fact, that the artificial production of heat is accompanied by 

 vibratory motions in the material molecules of the heated substances. 

 The measure of the quantity of heat produced mechanically would on 

 this hypothesis have a direct connection with the sum of the ft* rien 

 of the system of vibrating particles. Hence the communication of 

 heat by contact would be the same as the propagation of vibratory 

 motions from the system of particles composing the heated body to 

 that of the touching body. This hyixrthesis is liable to a great objec- 

 tion ; for heat is propagated through a vacuum, and if even we suppose 

 all space filled with some fluid, in order to explain solar radiation, the 

 hypothesis loses its simplicity, and differs from the former principally 

 by ite greater vagueness. It has been suggested by Brewster, that the 

 solar rays are nothing more than heated light, but this opinion is open 

 to several objections founded on the different laws by which heat and 

 light are transmitted and modified. 



It is of greater advantage to study the properties of heat, and make 

 those properties the ground for its measurement and the calculation of 

 iU effects, than to speculate on the nature of an agent intimately con- 

 nected, not only with light and electricity, but with the absolute 

 nature of the material molecules. 



Heat radiates from all bodies in straight lines and in all directions, 

 and in the law of its emanation it resembles light, its intensity dimi- 

 nishing in the same proportion as the sine of the angle of emanation. 

 If we conceive two balls which are heated unequally to be enclosed by 

 a concave surface which by any means is preserved at a uniform 

 tfinjierature, the radiation of heat from the warmer ball, directed on 

 the colder, being more copious than the radiation from the latter on the 

 fin HUT, the temperature of the hotter will sink, and of the cooler rie, 

 in proportion t the difference of radiations, and this will continue 

 until the temperatures become permanent, in which case the radiations 

 are necessarily equal. In the same manner, when the different parts 

 nf .1 room in which there are one or several sources of heat have 

 acquired a permanent temperature, that temperature for each part 

 will then be nuch that the heat which it emits by radiation must be 



equal to the quantity absorbed, and which it has received by radiation 

 from all other parts of the room. 



One consequence of the free radiation of heat in open space is, that 

 its intensity must vary inversely as the square of the distance from the 

 origin. Hence every portion of space has a determinate temperature 

 due to the amount of radiation, not only from the sun and fixed stars, 

 but also of the non-luminous bodies of each system. It seems however 

 difficult to conceive with Poisson, that in addition to such temperature 

 from heat emitted, it can have any temperature peculiar to its locality ; 

 in fact, that vacuum can possibly have any proper heat. 



But though the laws of the free emanation of heat and light are 

 similar, those of their transmission through substances are very 

 different. When a metallic body is but a little heated in a fire, we 

 have heat unaccompanied by sensible light ; and in the lunar rays the 

 light, though originally transmitted from the sun, arrives at our 

 planet without any sensible heat, even when collected in the focus of a 

 burning-glass. Again, a plate of glass placed before a common fire will 

 intercept the heat until it becomes itself sufficiently heated to radiate. 

 When however the source of heat is more intense, a small portion will 

 be directly transmitted ; while for the solar rays we find the heat is 

 transmitted as well as the light. It is still more remarkable that 

 when the solar rays are decomposed by transmission through a glass 

 prism, the differently coloured rays of the spectrum have each a dif- 

 ferent intensity of heat, the least refrangible possessing the greatest 

 portion ; the greatest heat is found at the place occupied by the ex- 

 treme red rays, or even a little beyond them. 



Not only may radiant heat be collected in a focus by refraction 

 through a lens, but also by reflexion from a polished concave mirror. 

 If we employ a pair of mirrors, and if a heated body be placed in the 

 focus of one, and a thermometer in the other, the reflected heat falling 

 on the bulb of the instrument will cause the mercury to rise ; and con- 

 versely, a colder body will make the column of mercury descend, for 

 the excess of radiation proceeds in this case from the thermometer. 

 Recent experiments on heat show that the analogy of the laws of heat 

 and light extends even to polarisation. 



The experiments of Leslie have shown how greatly the quantity of 

 radiant heat may be affected by the state of surfaces from which it 

 finally emanates. The method adopted by Leslie for examining the 

 powers of radiation possessed by different substances was extremely 

 simple and ingenious. Having employed the system of two specula 

 above mentioned, he placed a tin canister filled with hot water in the 

 focus of one, and a differential thermometer [THERMOMETER] in the 

 other focus. The four sides of the tin canister were covered with the 

 substances of which he proposed to seek tha radiating powers ; when 

 three of the sides were respectively covered with lamp-black, paper, and 

 crown-glass, and then turned so as to radiate directly on the speculum, 

 the heat reflected by the other speculum raised the thermometer ac- 

 cordingly to 100, 98, and 90 ; but when the fourth side, which was 

 uncovered, was similarly directed, the thermometer fell 12. Thus it 

 appears that polished metallic substances are bad radiators, which may 

 be attributed to the internal reflexion of the heat from their surfaces, 

 for the sources of radiation must evidently be at some small deptli 

 below the geometrical surfaces. A similar apparatus served to measure 

 the absorptive power of different substances, by covering the bulb of 

 the thermometer as uniformly as possible with an envelope of the sub- 

 stance to be examined ; this power was thus found to be nearly in pro- 

 portion to that of radiation. The maximum effect of the reflected 

 heat was not however in the strict geometric focus, but, in consequence 

 of aberration, it was found to be a little beyond it. At a later period 

 Ritchie contrived a species of differential thermometer from which, by 

 direct experiment, he was enabled to prove that the power of radiation 

 from any surface in exactly equal to that of absorption at the same 

 surface. A hollow air-tight cylinder of tin, having one half of ita 

 exterior surface bright and the other covered with lamp-black, was 

 fixed in a vertical position at each extremity of a bent glass tube con- 

 taining a coloured spirit ; and midway between these was placed a 

 vessel similar to the others, and like them coated on half its surface. 

 When the instrument was to be used, the middle vessel was filled with 

 boiling water; then the bright side of one of the first-mentioned 

 cylinders being turned towards the coated side of that which con- 

 tained the water, while the coated side of the other was turned towards 

 the bright side of the latter, the rarefaction of the air within the 

 cylinders at the extremities of the tube, in consequence of the heat 

 absorbed by them, was proved to be equal by the coloured spirit 

 remaining stationary in the tube. (' Journal of the Royal Inst.,' 

 December, 1831.) It is evident, here, that if the quantity of heat 

 which radiated from the bright side of the middle cylinder were repre- 

 sented by th of that which issued from the coated side ; the bright side 



1 

 of the cylinder at one extremity of the tube absorbed -th of the 



latter quantity while the coated side of the cylinder at the other 

 extremity absorbed all the former, that is, an equal quantity. 



The power of radiating heat is certainly the most universal mode of 

 its propagation between different particles of matter as well as through 

 considerable spaces. However, it is usual, for greater simplicity, to 

 designate this propagation through solid bodies as propagation of heat 

 by contact, Poisson has shown in his memoirs on that subject, that the 



