OA8 AND GASES 



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heat by a kind of diffusion and redistribution of 

 energy nt In-lit -motion. In hydrogen a heated wire 

 in verv rapidly cooled ; in u heavier gas, lens ru|iidly 

 so. Tin- conductivity of air, when the heat conducted 

 is reckoned in units such that each will raise a 

 cuhir cm. of the substance (air) itself through 

 one decree Centigrade, is ()"25o ; under similar con- 

 .In ions that of iron is 0'183, and that of cornier is 

 ' <>77 ; so that the rate of propagation of thermal 

 Meets in air is intermediate between that in iron 

 iiinl that in copper. This apparently hig_h rate 

 is due to the small density of air and to its low 

 .-pccific heat; and when we turn to the actual 

 propagation of heat-energy as distinguished from 

 tliat of temperature, we find the conductivity of 

 air, in this sense, to be only about one 20,000th 

 that of copper. 



Gases have as a rule small specific heat : air 

 has at constant pressure a specific heat = 0*2375, 

 at constant volume, 0*1684 ; that is, to raise a 

 pound of air 1, allowing it to expand, takes 

 i i "_': 1 7."i as much heat as it would take to raise a 

 pound of water, whereas if it be not allowed to 

 expand and thereby absorb energy, it will take 

 only 0*1684 times as much-. The specific heat of 

 j;a<fs is stated in tables with reference to 'air = 

 i'"j''75' as a starting-point; an equal volume of 

 hydrogen has a specific heat at constant pressure 

 = 0*2359, and, roughly, equal volumes of^ all the 

 ordinary gases have equal thermal capacities ; but 

 ml i nary vapours have, volume for volume, much 



eater thermal capacities than ordinary gases. 



ydrogen has a specific heat, weight for weight, 

 :'<)I!K) times (at constant pressure) as great as 

 water ; and it is the solitary exception to the state- 

 iin-iii that water has of all substances the highest 

 specific heat. In general the specific heat of a gas 

 at constant pressure is about 1*4 times its specific 

 heat at constant volume ; in the latter case no heat 

 is absorbed in doing the work of expansion against 

 resistance. The specific heat of gases rises slightly 

 with increasing temperature (Mallard and Le 

 Chatelier), and this becomes at furnace heats very 

 well marked : at 2000 C. the specific heats of car- 

 bonic acid and water- vapour are double, and those of 

 nitrogen, oxygen, and carbonic oxide about one and 

 a half times as great as what they are at 200 C. 



Different <jases have different actions upon radiant 

 heat and light ; they characteristically absorb 

 special portions of the heat and light spectrum, 

 and thus produce absorption bands : the dark lines 

 A and B seen in the solar spectrum are traced by 

 Egoroff and Khamantoff to the absorptive action of 

 oxygen. In some gases the absorption is carried 

 so far that the gas appears coloured e.g. chlorine, 

 which is yellowish-green : iodine vapour in coin- 



ever, gases are poor absorbents and correspondingly 

 poor radiators : there is comparatively little radia- 

 tion from a Bunsen flame. At the same time the 

 radiation from an incandescent gas tends to be 

 very precise in its frequencies ; it tends to produce 

 line-spectra as distinguished from the continuous 

 spectrum produced by the mutually jolting particles 

 or an incandescent solid. Each gas has its own 

 index of refraction also ; oxygen has, for example, 

 aa compared with vacuum, a mean index at atmo- 

 spheric pressure of 1 O00272. In vapours the dis- 

 persion is great ; and iodine vapour strangely re- 

 fracts red most and violet least. 



In Electricity (q.v.) the different gases have 

 different properties which sometimes present curi- 

 ous anomalies ; air at ordinary pressures is an 

 insulator ; warm air at rest is an insulator, but 

 above a Bunsen burner it is a conductor ; at 

 lew pressures it conducts and glows while con- 



ducting ; at extremely low pi-ensures it is again 

 an insulator. Different gases set up different 

 potential-differences lietween themselves and metals 

 with which they may be in contact, a* in gas- 

 l':ii*'-i i---. and they have different specific induc- 

 tive capacities. Oxygen i- magnetic in the same 

 sense an iron ; hydrogen and nitrogen are dianiag- 

 netic, and tend to lay themselves acrotw the poles 

 of a magnet. See also MATTER. 



ANALYSIS OF GASES. The gas is collected in 

 small glass vessels, the contents of which, consist- 

 ing of mercury, water, or air, are displaced by the 

 gas to be analysed. For the best methods of col- 

 lecting gases from mineral springs and waters, 

 from volcanic lakes, geysers, or boiling springs, 

 from openings in rocks, clefts of glaciers, furnaces, 

 fissures in volcanic craters, &c., reference may be 

 made to Bunsen 's Gasometry, translated by ItoHCoe. 

 Air is only used when a considerable current of the 

 gas to be analysed can be procured, which may 

 sweep out the last traces of air from the collecting 

 vessel. Water often affects the composition of 

 mixed gases which it is attempted to collect over 

 it ; for to various extents it absorl*, among 

 others, hydrochloric, hydriodic, hydrobromic, and 

 sulphurous acid gases, chlorine, sulphuretted 

 hydrogen, ammonia, fluoride and chloride of 

 boron, methyl- and ethyl-amine, methyl chloride 

 and methyl ether, cyanogen, and chlorine cyanide; 

 and it decomposes silicon fluoride with precipita- 

 tion of gelatinous silicic acid. Mercury is generally 

 employed because it is inert to most gases ; but it 

 is attacked by chlorine, which it absorbs. 



There are two leading principles made use of in 

 the analysis of gases. First, a given volume is sub- 

 jected to a chemical reaction, which results in the 

 condensation of one of the constituents of the gase- 

 ous mixture or compound ; then by simple observa 

 tion, or from the known laws of gaseous volume, 

 it is determined how great a volume of the original 

 gas has disappeared through l>eing amenable to the 

 reaction employed, and, accordingly, how great a 

 proportibn of the constituent in question was 

 originally present. In the case of air, for example, 

 a measured volume may be exposed to the absorp- 

 tive action of a strong alkaline solution of pyro- 

 gallol ; the solution becomes dark ; the oxygen is 

 absorbed ; the original volume of air is diminished ; 

 the loss of volume is ascertained, and represents 

 the quantity of oxygen originally present in the 

 measured volume or air. Or again, if the mixture 

 of gases be a somewhat more complicated one, as, 

 for example, a mixture of carbonic acid and oxide, 

 olefiant gas, and oxygen, the various absorbent 

 reagents appropriate to each constituent may be 

 successively introduced, and the successive shrink- 

 ages noted by remeasurement at the original tem- 

 perature and pressure. A few drops of a solution 

 of caustic potash will in this way take up the 

 carbonic acid ; pyrogallol will take up the oxygen ; 

 anhydrous sulphuric acid dissolved in oil of vitriol, 

 and introduced on a coke-pellet, will slowly take 

 up the olefiant gas, and the sulphurous acid and 

 anhydrous sulphuric acid vapour, which contamin- 

 ate the gas after this reaction, may be removed by 

 caustic potash ; and carlranic oxide may be absorbed 

 by means of a solution of cuprous chloride (pre- 

 pared by leaving copper turnings with a saturated 

 solution of cupnc chloride in a stoppered bottle for 

 some days), which will take it up in about ten 

 minutes. The principal absorption reagents are ( 1 ) 

 caustic potash solution, which alisorbs sulphuretted 

 hydrogen, hydrochloric, carbonic, sulphurous, and 

 other acid gases, chloride and fluoride of boron, 

 and chloride of cyanogen, and decomposes siliciu- 

 retted hydrogen with evolution of 4 volumes of 

 hydrogen ; (2) dry caustic potash, which acts like 

 the solution, but more slowly, and also absorbs 



