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DB. TYITD ALL'S DISCOVERIES.] UNDTJL4.TORY FORCES. HEAT. 



39 



source of 212 Fahrenheit. This is the feeblest action 

 which has been observed. At the other end of the 

 scale stands olefiant gas, which absorbs more than eighty 

 per cent, of the same rays. This gas, wliich is perfectly 

 transparent to rays of light, is opaque to the rays of 

 obscure heat. Streaming into the atmosphere, it is not 

 seen by the eye ; but if a sheaf of calorific rays be caused 

 to cross it, its presence is at once declared by a proper 

 instrument ; it intercepts such rays almost as effectually 

 as a screen of cloth drawn across their path. Coal gas 

 i acts in a similar manner. 



" When a volume of olefiant gas, say one-thirtieth of 

 that necessary to fill it, is sent into an exhausted tube, 

 through wliich calorific rays are passing, it destroys so 

 many of these rays, that a second equal volume does not 

 produce anything like the same effect. In fact, the rays 

 are not there to be destroyed by the second discharge of 

 gas. Bvit when small volleys of gas are used, the num- 

 ber of rays shot down is exactly proportional to the 

 number of shots. For example when a measure pos- 

 sessing a capacity of one-fiftieth of a cubic inch was 

 used, five such measures destroyed exactly five times as 

 many rays as a single measure ; ten such measures de- 

 stroyed ten times as much ; fifteen such measures de- 

 stroyed fifteen times as much ; but here the rays became 

 sensibly thinned in numbers, so that twenty measures 

 did not produce twenty times the effect of a single mea- 

 sure. As the quantity of gas introduced into the tube 

 augmented, the quantity of rays destroyed deviated 

 more and more from the law of proportionality ; until, 

 finally, the addition of a single measure ceased to pro- 

 duce any appreciable effect. At this point, in fact, the 

 vulnerable rays had been practically disposed of. Trans- 

 parent gases have been hitherto considered as almost 

 perfectly diathermanous ; but this opinion can no longer 

 be retained in presence of the fact, that a measure of 

 olefiant gas, one-fiftieth of an inch in volume, diffused 

 through a tube possessing a capacity of 220 cubic inches, 

 and there possessing a tension of only TT i rj of an atmos- 

 phere, exercises a measurable absorption. 



Between atmospheric air and olefiant gas, the other 

 compound gases occupy intermediate positions. In the 

 case of vapours, the most energetic is that of sulphuric 

 ether ; the least energetic is that of bisulphide of carbon. 

 Comparing small volumes at equal tensions, the absorp- 

 tive energy of sulphuric ether is ten times that of olefiant 

 gas, and ten thousand times that of oxygen, hydrogen, 

 nitrogen, or atmospheric air. This vapour, in fact, 

 which, tested by light, is as transparent as the diamond, 

 tested by calorific rays of low intensity, is almost as 

 black as pitch. Between sulphuric ether and bisulphide 

 of carbon, stand chloroform, wood-spirit, benzol, alco- 

 hol, the iodides and chlorides of ethyl, methyl, and amyl, 

 and other vapours. 



" The influencs of tho aqueous vapour in tho atmos- 

 phere may be judged of from the following fact. On a 

 lair November day, the quantity of transparent vapour 

 suspended in the air produced fifteen times the absorp- 

 tion of the true air itself. Philosophers have long specu- 

 lated on the influence exerted by a diathermanous 

 envelope, like our atmosphere on tho climates of a 

 planet. D. Saussure, M. Pouillet, and Mr. Hopkins, 

 have shown the effect of such an envelope in preserving 

 the terrestrial rays from being wasted in space. But, in 

 reality, the atmosphere itself exerts a vanishing influence 

 compared with that of the aqueous vapour it contains. 

 Here we have a constituent, whose changes must pro- 

 duce corresponding alterations of temperature at the 

 surface of the earth. Whether it is competent to ac- 

 count for tho variations of climate revealed by geologists, 

 future investigations must decide ; but it is a vera causa 

 which can never henceforth be overlooked in specula- 

 tions on the subject. 



" It is known that oxygen, obtained from the decom- 

 position of water by an electric current, contains a trace 

 of that singular substance ozone. Such oxygen exerted 

 four times the absorptive energy of tho same substance, 

 after it had been passed through the iodide of potas- 

 sium, and thus freed from its ozone. 



"The radiative power of gases was examined by 

 causing them to pass over a sphere of heated metal, and 

 to ascend in a column in front of a thermo-electric pile. 

 It was here found that the order of radiation was pre- 

 cisely that of absorption. The theory of heat now uni- 

 versally accepted is, that it is a kind of motion of the 

 atoms of bodies, and radiant heat is the propagation of 

 this motion through an ethereal fluid in which all bodies 

 are immersed. A popular exposition of this theory is 

 given by Professor Tyndall, hi his Glaciers of the Alps; 

 and his recent experiments prove, tliat every atom, or 

 molecule, which is competent to accept motion from 

 agitated ether or, in other words, to absorb heat is 

 competent, in precisely the same degree, to impart mo- 

 tion to still ether, or, in other words, to radiate heat. 

 His experiments have one peculiarity which preceding 

 ones did not possess. They deal with free atoms, and 

 thus fix upon them individually the responsibility of tho 

 effects observed. In solids and liquids the atoms form 

 joint -stock companies; considerations of cohesion aud 

 aggregation come into play, from which the molecules, in 

 a state of gas or vapour, are free. Hence the experi- 

 ments are calculated to throw light upon the physical 

 characters of the atoms themselves. 



"The lecturer pointed out, that the reciprocity of ab- 

 sorption and radiation is a necessary mechanical conse- 

 quence of the theory of an ether. But the question 

 here arises ' Why is one molecule competent to stop or 

 to generate a calorific flux so much more powerfully 

 than another? What is it, for example, which enables 

 olefiant gas to transcend so enormously in these respects 

 its constituent hydrogen?' To tliis question the experi- 

 ments prompt the following reply : The elementary gases 

 which have been examined, all exhibit extremely feeble 

 powers of absorption and radiation as compared with 

 the compound ones. Now, in tho former case, we have 

 oscillating atoms, but in the hitter case oscillating sys- 

 tems of atoms ; and if we unite the atomic theory of 

 Dalton with the perfectly independent conception of an 

 ether, it follows that the compound molecule, or group of 

 atoms, wliich furnishes paints d'apjnii to the ethereal 

 fluid, must be capable of accepting and generating 

 motion to a far greater degree than a single atom, which 

 we may figure to our minds as an oscillating sphere. 

 Thus oxygen and hydrogen, which, taken separately or 

 mixed mechanically, produce a scarcely sensible effect 

 when united chemically to form oscillating systems, as in 

 aqueous vapour, produce a powerful effect. Thus, also, 

 nitrogen and hydrogen, which, separate or mixed, pro- 

 duce but little action when combined to form ammonia, 

 produce a great action. So, also, nitrogen and oxygen, 

 which, as air, are feeble absorbers and radiators when 

 combined, as in nitrous oxide, are very powerful in both 

 capacities. Comparing small volumes at equal tensions, 

 tho action of nitrous oxide is fully 250 tunes that of air ; 

 and there is, perhaps, no fact on record which shows so 

 conclusively that air is a mixture, and not a compound, 

 as this one. Carbonic oxide is about 100 times as power- 

 ful as its constituent oxygen ; carbonic acid, 150 times 

 as powerful ; while olefiant gas, as already stated, is 

 1,000 times as powerful as its constituent hydrogen. In 

 the case of the hydro-carbon vapours, where the atomic 

 groups attain a higher degree of complexity, the action 

 is still greater than that of olefiant gas. The lecturer 

 also referred to the influence of the rate of atomic oscil- 

 lation on the amount of heat absorbed. But this subject 

 is difficult to be rendered popular, and is, therefore, 

 omitted here. The discourse concluded by applying 

 considerations similar to the foregoing to the conduction 

 of heat. It was shown how radiation, absorption, and 

 conduction were connected together, and that the phe- 

 nomena of all of them might be deduced from simple 

 mechanical principles." 



CONCLUDING REMARKS. 



IN the foregoing pages, we liavo endeavoured to present 

 to the reader, the leading facts and principles involved 

 in that branch of science which treats on heat, as one of 



