297 



GAS. 



GAS-LIGHTING. 



298 



subtract 32 from 480, the remainder is 448 ; to which add the degrees 

 above zero indicating the temperature of the air, these are 32 and 60, 

 making 480 and 508. Then say 480 : 508 : : 100 : 105 832, the volume 

 of the air at 60. 



It is well known that air suffers diminution of volume in proportion 

 to the pressure to which it is subjected, and the same law holds good 

 with all the more incondensible gases. In chemical analyses it is often 

 requisite to make corrections for variations of barometric pressure, as 

 well as of temperature in estimating the quantity of gaseous products. 

 The following are the rules for this purpose, given by Professor 

 Faraday in his work on chemical manipulation : " A pressure of 

 30 inches of mercury, as observed by an accurate barometer, has been 

 assumed as the mean heiyht or barometric pressure, and volumes of gas 

 observed at any other pressure frequently require to be corrected to 

 what they would be at this point. For this purpose it is only necessary 

 to compare the observed height with the mean height, or 30 inches, 

 and increase or diminish the observed volume inversely in the same 

 proportion. Thus, as the mean height of the barometer is to the 

 observed height, so ia the observed volume to the volume required. 

 As an instance, suppose that 100 cubic inches of gas have been observed 

 when the barometer stood at 30'7 inches : then, as 30 inches, or mean 

 height, is to 30 P 7 inches, or observed height, so is 100, or the observed 

 volume to a fourth proportional, obtained by multiplying the second 

 and third terms, and dividing by the first: thus, 307x100 = 3070, 

 which divided by 30= 102 '333 cubic inches ; this would be the volume 

 of the gas at 30 inches of barometric pressure. Again, suppose a 

 quantity of gas amounting to 20 cubic inches standing over mercury in 

 a jar, the level of the metal within being 3 inches above that without, 

 and the barometer at 29'4 inches. Then the column of 3 inches 

 mercury within the jar, counterbalancing 3 inches of barometric 

 pressure, instead of being 29'4, the latter is effectively only 26'4, and 

 the correction will be, as 30 inches is to 26'4 inches, so is the 20 cubic 

 inches observed to 17'6 cubic inches, the volume which the gas would 

 really occupy if the mercury were level within and without the jar, 

 and the barometer were 30 inches." 



It is very commonly requisite to make corrections both for tempe- 

 rature and pressure in the same volume of gas, and it is of no con- 

 sequence which is made first. 



In chemical analyses various other considerations arise in ascertaining 

 the quantities of gaseous products ; as for example, the separation of 

 or making the requisite allowances for the moisture which they contain : 

 for these, as well as for the various modes of collecting, transferring, 

 and preserving various gases, we must refer to the very excellent work 

 just quoted. 



The solubility of gases in water is extremely various. Dr. Henry 

 thought that the volume of each gas absorbed by water is the same, 

 whatever be the pressure to which the gas is previously subjected, but 

 this hag since been proved to be not strictly correct. If the weight of 

 carbonic acid gas be doubled by subjecting it to the pressure of two 

 atmospheres, water will still absorb its own volume of it. The 

 following table exhibits the volumes of each gas absorbed by 100 

 volumes of water at 00 Fahr., and under a pressure of 30 inches of 

 mercury : 



Absorption 



It may be observed, that in general the more easily a gas is con- 

 densable by cold and pressure, the more soluble it is in water : this 

 will appear by comparing the above statements with that containing 

 the pressure at which Faraday liquefied various gases. For more 

 recent and accurate researches on the solubility of gases in water at 

 different temperatures, see Bunsen's ' Gasometric Analysis,' translated 

 by Dr. Eoscoe. 



A curious property of gages, and .possessed by them in very different 

 degrees, is that of their condensation by porous bodies, and especially 

 by charcoal. [CARBON.] 



A curious fact with respect to mixtures of gases was discovered by 

 Dr. Priestley, which he thus states : " Different kinds of air that have 

 no affinity do not, when mixed together, separate spontaneously, but 

 continue diffused through each other." This he proved to be the case 

 by several experiments ; and more especially by one, in which he found 

 that he was able to explode hydrogen and oxygen gases, which had 

 long remained together, and which he justly argues must have been 

 mixed, or he could not have fired them by an electric spark, in a vessel, 

 the wires of which were at the top. He adduces this experiment to 

 Illustrate the fact tliat the guei which constitute the atmosphere do 



not separate according to their respective gravities, though they do 

 not combine. (Priestley's ' Experiments, &c.,' vol. vi. p. 391.) 



These experiments were repeated by Dr. Dalton, and he inferred 

 from them that the particles of one gas, though repulsive to each other, 

 do not repel those of a different kind ; and that one gas acts as a 

 vacuum with respect to another. If therefore a vessel full of carbonic 

 acid be made to communicate with another of hydrogen, the particles 

 of each gas insinuate -themselves between the particles of each other 

 till they are equally diffused through both vessels. This theory ac- 

 counts not only for the mixture of gases, but for the equable diffusion 

 of vapours through gases and through each other. 



Another observation made by Dr. Priestley, and related with others 

 of a similar kind (' American Phil. Trans.,' vol. v.), appears to have 

 been entirely overlooked. He found that though a glass vessel was 

 perfectly air-tight, yet if it had been broken, and the pieces joined 

 with paint or cement, hydrogen gas contained in it would be changed 

 for the external air. Dbbereiner has since remarked the escape of 

 hydrogen gas by a fissure or crack in glass receivers. Professor Graham, 

 in an elaborate paper on this subject, has shown that gases diffuse into 

 atmospheric air and into each other, with different degrees of ease and 

 rapidity, the lighter ones escaping most readily, so much indeed, that 

 hydrogen escapes five times more quickly than carbonic acid gas, which 

 is about 22 times heavier. [DIFFUSION.] 



To Dr. Priestley also we are indebted for the important discovery 

 that gases can pass through membranes which are perfectly air-tight, 

 and by this action he explained that of the atmosphere upon the blood 

 in the lungs. In the memoir above alluded to he has also shown, that 

 when a bladder containing hydrogen is put into a vessel of oxygen, or 

 one with oxygen into a vessel of hydrogen, the bladder and the vessel 

 of gas both contain both gases, owing to the passage of the gases from 

 and into the bladder. It is also stated by Professor Graham, that if a 

 bladder, half filled with air, with its mouth tied, be passed up into a 

 large jar filled with carbonic acid gas, standing over water, the bladder, 

 in the course of twenty-four hours, becomes greatly distended by the 

 insinuation of the carbonic acid through its substance, and may even 

 burst, while a very little air escapes outwards from the bladder. This 

 however he does not consider as a case of simple diffusion ; the result 

 depends, first, upon carbonic acid being a gas easily liquefied by the 

 water in the substance of the membrane, and therefore the carbonic 

 acid penetrates the membrane as a liquid ; secondly, this liquid is in 

 the highest degree volatile, and therefore evaporates very readily from 

 the inner surface of the bladder into the air confined in it. The air in 

 the bladder comes to be expanded in the same manner as if ether 

 or any other volatile fluid was admitted into it. Professor Graham 

 further observes, that in the experiments of Dr. Mitchell and Faust 

 and others, in which gases passed through a sheet of caoutchouc, it is 

 to be supposed that the gases were always liquefied in that substance, 

 and penetrates through it in a fluid form ; and it is also to be noticed, 

 that it is generally those gases which are more easily liquefied by cold 

 or pressure that pass most readily through both caoutchouc and humid 

 membranes. 



Dr. Mitchell found that the time required for the passage of equal 

 volumes of different gases through the same membrane was 



1 minute with ammonia. 



24 miautes with hydrosulphuric acid. 



34 



54 



64 

 274 

 28 

 374 

 113 

 160 

 And a much longer 



time with . . nitrogen. 



cyanogen, 

 carbonic acid, 

 nitrous oxide, 

 arseniuretted hydrogen, 

 olefiant gas. 

 hydrogen, 

 oxygen, 

 carbonic oxide. 



In concluding we may observe that gaseous bodies are of the highest 

 importance, as connected not merely with the well-being, but even 

 with the existence of animals: two of them, oxygen and nitrogen, form 

 our atmosphere ; two of them, hydrogen and oxygen, constitute water ; 

 oxygen united with silicon and various metals forms the greater part 

 of the crust of our globe ; and chlorine is one of the elements of 

 common salt. 



GAS-LIGHTING, Chemistry of. The manufacture and consumption 

 of gas for illuminating purposes is a process involving applications of 

 chemistry at almost every step. There are, however, three distinct 

 portions of the operation, the successful carrying out of which pecu- 

 liarly require a knowledge of certain chemical principles : namely, 

 1st, the generation of gas : 2nd, its purification ; and 3rd, its combus- 

 tion for the production of light. We will, therefore, consider these 

 points seriatim. 



I. Generation of Gas. The generation of nearly all kinds of gas for 

 illuminating purposes is a process termed by chemists destructive distil- 

 lation, and consists in placing coal, or other similar substance, in close 

 vessels heated to a temperature varying from a red to a white heat. 

 In practice, the vessels used are generally retorts, constructed either of 

 cast iron or clay. The organic substances thus heated consist almost 

 entirely of the elements Carbon, hydrofen and oxygen, with small pro 



