WBIGHIKQ AIR.] 



METEOROLOGY. 



1129 



the degree of atmospheric pressure operating at the time 

 of the experiment, is, at least, of equal consequence 

 with the degree of temperature. 



We are now in a position to trace the theoretical steps 

 necessary to be followed in obtaining the weight of a 

 given volume of any gas. 



Necessarily, as in the previous case, a vessel of capa- 

 city is required ; but, inasmuch as a gas does not admit 

 of being poured into the vessel like water, some practical 

 expedient for accomplishing this must be devised. We 

 had better omit all consideration of this for the present, 

 and assume that the vessel (which will be a globe or 

 flask having a neck with stopcock attached) is already 

 filled with the gas to be weighed, at a definite tempera- 

 ture and definite pressure. This accomplished, the 

 operator has only to weigh his flask full of gas, deduct 

 the weight of the flask from the total weight of flask and 

 gas, and the result is gained. Practically, however, 

 many points have to be considered. 



The Gas must be Pure. Whether gas, liquid, or solid, 

 any body, the weight of which we desire to know, must 

 be pure ; but this precaution applies in the highest 

 degree to gases. 



The Gas mint be either Dry, or its Amount of Moisture 

 mutt b Definite. The property which gases have of 

 taking up vapours, especially aqueous vapour, is well 

 known ; and it will readily be seen, that to the extent 

 of the presence of such vapour, will the weight of a 

 given bulk of gas, and vapour mixed, fluctuate. One of 

 two processes has now to be followed : either the gas 

 must be artificially dried by exposure to one of the hy- 

 groscopic bodies used by chemists for that purpose ; or, 

 i must be saturated with moisture to the fullest capa- 

 city at gome given temperature. 



The further steps of the calculation are based upon a 

 consideration of the ratio between the specific gravity of 

 uteam or vapour, and the specific gravity of dry gas ; and, 

 lastly, the amount of vapour which a gas absorbs at a 

 definite temperature. According to Gay Lussac, the 

 ratio between the specific gravity of aqueous vapour and 

 air, under imilnr conditions of temperature and pres- 

 sure, is 



<M520 _ vapour 



1 = atmospheric air ' 



The amount of aqueous vapour which a unit volume 

 of gnu can absorb at given temperatures, has been ascer- 

 tained and tabulated. 



Applying this knowledge to practice, let us assume 

 that 100 cubic inches of moist air, at 60" Fall, and 30 

 inches barometer, weigh 31 grains, it is required to know 

 how much 100 cubic inches of dry air would weigh. 



We begin by turning to a table indicating the quan- 

 tity of vapour present in a gas saturated with vapour at 

 any given temperature. Dal ton's table gives this quan- 

 tity for 60 Fah. as 0-524. We next perform the follow- 

 ing calculation : 



30 : 0-524 : : 100 : 1-747 = the volume of vapour in 100 

 cubic inches of moist air at 00 Fah. 



And as 100 cubic incites of aqueous vapour weigh 19 

 grains, 1747 cubic inches weigh 332 of a grain. 



Weight of 100 cubic inches of moist air . 31 

 Deduct 0-332 



30-068 



Therefore the weight of 100- 1747 = 98 '253 cubic 

 inches of dry air = 30 668 grains, and 98-253 : 30 "COS : : 

 100 : 31-213 grains. 



\Vli''uce it follows, according to the foregoing calcu- 

 lation, that the weight of 100 cubic inches of dry air, at 

 ^iO inches barometer and 00 Fah., is 31'213 grains. 



.Such, then, are the practical operations by which the 

 weight of a known volume of gas is determined. We 

 have chosen atmospheric air as the subject of illustra- 

 tion, but the processes are identical whatever the gas 

 may be. 



Although the result of the calculation just effected 



VOL i. 



gives 31-213 grains as the weight of 100 cubic inches of 

 atmospheric air at 30 inches barometer and 60 Fah. . and 

 although the number may be accepted for all purposes of 

 meteorologic calculation, nevertheless it must not be 

 viewed in an implicit sense. In point of fact, the exact 

 weight of atmospheric air is not yet made out. Pro- 

 bably the determinations of MM. Dumas and Boussin- 

 gault are most reliable. According to their experiments, 

 1 litre or 61-02791 cubic inches of air, at centigrade 

 and 076 metres barometer, weigh 20 '065 grains ; whence 

 it follows that 100 cubic inches, under the same condi- 

 tions, must weigh 31-093 grains at 60 Fah. 



Barometric Pressure at the time of Exueriment must be 

 an Element of the Calculation. A consideration of the 

 laws of pressure, as influencing the volume of gases and 

 vapours, will have made the student aware that due 

 allowance requires to be made for variations referable to 

 this cause. Now we are acquainted with the amount of 

 expansion and contraction dependent on variations of 

 pressure. This information is conveyed by a study of 

 i the law of Marriotte, which proves that a rule of propor- 

 tion will furnish the information required. Thus, for 

 example, suppose we have 100 measures of any gas at a 

 pressure of 29 inches of the mercurial barometric column, 

 and it is required to ascertain what volume the gas will 

 fill at 30 inches of the same this being the normal 

 pressure to which all calculations as to the volume of 

 gases are reduced then we say 



As 30 : 29 :: 100 : 96 66. 



In other words, the 100 volumes of gas, under those 

 conditions, would contract into 90 '00. 



Temperature must be an Element of the Calculation. 

 When it is considered to what extent gases and vapour 

 suffer expansion and contraction by variations of tempe- 

 rature, the necessity of this calculation will be obvious. 

 We have already explained the ratio of thermal expan- 

 sion, to which gases and vapours are subjected by varia- 

 tions of temperature. Practically, we have seen at page 

 ] 127, that this ratio may be considered identical for all 

 of this class of bodies, and to be equal to r J r th part of 

 their bulk at 32 Fah. and 30 inches barometer for every 

 degree of temperature between 32 Fah. and 212 Fah. 

 Let us now apply this information to practice. 



If the temperature of the gas be above 32 Fah. , mul- 

 tiply its total volume by 491, and divide the product by 

 491 plui the number of degrees that the temperature of 

 the gas exceeds 32 Fah. The numeral result of this 

 operation gives us the correct volume the gas in question 

 would occupy at 32 Fah. 



For example, we have 100 cubic inches of gas at 50 

 Fah. ; it is required to know what volume this gas would 

 occupy if raised to 60 3 Fah. : thus 



100 X 491 

 491+ 18 



And 



= 96-46 = the volume at 32 Fah. 





96 46 + ^- - = 101-96 = the volume at CO. 



Recapitulation artd Deductions. It appears, then, that 

 the operation of weighing a gas demands in all cases that 

 due allowance should be made for variations of heat and 

 of pressure ; and if the gas be charged with vapour, due 

 allowance has to be made for moisture also. 



THE THERMOMETER. In the course of our preceding 

 investigations relative to the atmosphere, we have seen 

 that temperature is an important element. Not only 

 are the chemical functions of the atmosphere intimately 

 related with temperature, but, without allowing for the 

 expansion produced by increments of heat, the me- 

 teorologic observer would be unable to comprehend some 

 of the most ordinary physical conditions of the atmo- 

 sphere. We have already seen that the general effect of 

 heat, as regards alteration of dimensions, is expansion. 

 If the amount of this expansion be determined for any 

 particular range between fixed points, and the linear ex- 

 tension or space thus intercepted be divided into smaller 



7 B 



