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is the weightof unit volume of the gas pided by the weight of an equalvolume of pure dry air, the conditions of temperature andpressure being the same.The weight per cubic foot of a gas, ordinarily designated byS, is, under standard conditions, called the specific weight. Withthe weight of air at atmosphere pressure and varying temperatureconditions known, the weight of any gas at the same temperaturemay be computed from the relations of density and specific weightas indicated by5,=5, (air) x 6. (, )the subscripts t simply indicating that the air and the gas, theweight of which is required, are at the same temperature.The specific volume of a gas, usually designated by the symbolV, or the cubic feet per pound, will obviously be the reciprocal ofits specific weight, or(.)While it is perhaps easier and more convenient to computeweight and volumetric data of gases from their relative densitiesand a table of weights and volumes of air, such values may becomputed from the characteristic equation of a perfect gas, viz :111where P=absolute pressure in pounds per square feet,V=volume per pound in cubic feet,T =absolute temperature,R=a constant varying with tbe gas and derived from therelations existing between the pressure, volume andtemperature of the gas in question.This pressure.volume.temperature relation for any gas, asindicated by the constant R, represents the expressionR=PoVo lJ•a)T,where the subscripts 0 represent a set of standard conditions.Since the volume (and hence the specific weight) of a gas is afunction of both temperature and pressure. it is necessary, inorder that there may be a suitable basis for comparison, thatall volumes be reduced to some such standard set of conditions.These conditions, as ordinarily accepted, are a pressure of14.6963 p:>unds per square inch (2116.27 pounds per square foot)and a temperature of 32 degrees Fahrenheit.Table 4 gives the weights and volumes of air at atmosphericp.essu.e and different temperatu.es.TABLE 4VOLUME AND WEIGHT OF AIRAT AT.'OSpnItRIC PJU•.$SUR.1t.~ ~ ~ . ., -< ~" ... i i.§.,. o"'~~ i,8.;•• ~.,; .. !,8-a • • .,.'• •.!~ • • .. ar:-. -< .. ":"!~ l~ -~~ ,.gl.!ll -'. !iii: .!II~.... ~ '.. ~ t~ ~~s h ' ." "'1• ~ .. ;5 -.< - .. .,'& • ~ao., '. ~6t3 ','~. >ou.. ~ > u~~ 'u• Q Q Q3'12•390 .0S0710.'" 15.615 .06404 ,34° :0.151 .049625'0 12.84) .077863 "0 15.867 .063024 3'" 20.655 .04841455 12.1)69 .017 107 .80 16.1 '9 .062039 J80 :u.159 •°47261'"13•095 .076365"'"16.37 1 .061084 '00 21.66J .046 16:6, 1J.121 '°15631 '00 16.623 .o6ol,S8 425 %2•:93 .044851'0 13•341 .°14923 '"0 16.875 •°59:1.59 "0 22.923 .04362415 1].473 .01'4223'"16.925 •°59084'"23•554 .04245680 13•599 .o7l535 no 11•121 .058,388 '00 %4.184 .04135°8, 13•7:15 .0'72860 '3° 11•379 •°5154' 5:15 :4•8'4 .04°300go 13.851 .01:'197 "0 11.6l 1 .°56, 18 55° 250444 .°393°:'9'13"177 .07 1546 "0 17.883 .°55919 '15 26.011 .Ol8J5Z.00 14. 1°3 .010'}07 ,'" 18.'35 .°55 142 600 26•104 '°31448"0 14•355 .06966' "0 18.]87 .°54386 6'0 27•964 .035160"0 14•601 .068460 >So 18.639 '°53651 '00 :9.224 .034: 19'30 14--859 •067299 'go 18.&}1 .°52935 15° 30.484 .o328o.t_'0 15. 111 .066177 300 19.143 .05:.:138 800 31.744 .03'50%." 15-363 .065092 3'° 19•647 .0508g8 8'0 33.004 '°3°299With the values of Po and To thus fixed (see absolutetemperature, below) the value. of the constant R for any gas asgiven in formula (J-a) may be expressed asR- 2116.27 V - V (,]6)-459.64+32 0-4.3045 0 .thus offering a means of detennining the value of R directlyfrom the specific volume of the gas. Since the specific volumeof a gas is the reciprocal of the weight per cubic foot, and forany two gases the weights per cubic foot vary directly as theirmolecular weights, where the value of R for any gas is known,the value for any other gas may thus be determined from therelations of the molecular weights of the two gases, viz:N,-Molecular Weight=28 R=55.r3a,-Molecular Weight=32 R=x55.13 : x :: 32: 28R (a,)~48.24From the value of R as given in formula (J-IJ) it is possibleto express the characteristic equation of a perfect gas in what isperhaps a more convenient form for general use, asPV PoVoT = To ~)From the characteristic equation (J), of a perfect gas, it isobvious that the volume of a gas will vary inversely as the abso-lute pressure and directly as the absolute temperature. Incombustion work the variation in the pressure of the gasesencountered is sman. The temperature range covered. however,is large. and because of the effect of temperature change onvolume, it is perhaps well to define here Ifabsolute temperature."Experiment shows that if the temperature of a perfect gas at32 degrees Fahren.heit is increased one degree. the pressure beingkept constant, the gas expands ~::ii part of its volume. If such arate of expansion per one degree increase in temperature held goodat all temperatures, and experiment shows that such is the caseabove 32 degrees, if its pressure is kept constant, the gas woulddouble in volume with an increase in temperature above 32 degreesof 491.64 degrees Fahrenheit. Under a reduction of tempera-ture of 491.64 degrees below 32 degrees (corresponding to an ultimate temperature of 491,64-32=459.64 degrees Fahrenheitbelow zero) the gas would disappear. Vv"bile undoubtedly somechange in the law would occur before the lower temperature couldbe reached, there is no reason why the