578 



H Y G R O M E T R V. 



Hygrome- 

 try. 



Table of the Quantity of Moisture, in Grains, contained 

 in a Cubic Inch 'of ' Vapour, from to 100 of Fah- 

 renheit. 



height of the barometer in inches be represented by ft, Hvjron e. 

 then try. 



or - 



6 <jf> 



.0068544 F, 



30 \1+. 002086. 1] 



40. The formula, by which the above Table was cal- 

 culated, being adapted to a pressure of 30 inches of 

 mercury, if the pressure of the atmosphere be different, 

 a corresponding correction must be applied: if the 



or,g = 



. 10953 /3JV 

 ,j79.4 + <_< 



JlO^SySF, 



447.4 -I- t 



If the value of g be taken from the Table, it must be 



multiplied by . 

 30 



41. The formulae which we have deduced, for the Experi- 

 elastic force of steam, and the weight of the water con- menu of 

 tained in a given volume of vapour, when the tempe- Gay Lus- 

 rature and pressure are given, furnish also the means 5ac on . va - 

 of ascertaining, under similar circumstances, the force {^ 

 and quantity of vapour which exist, in combination or ^d, fr. 

 mixture with atmospheric air. The experiments of Gay 

 Lussac have decidedly proved, that vapours, so long as 

 they exist in the aeriform state, not only undergo the 

 same change of mechanical condition by change of tem- 

 perature and pressure, but that the same thing holds 

 true, when they are mixed with vapours of a different 

 kind, or even with permanently elastic fluids. The 

 apparatus which Gay Lussac employed for demonstra- 

 ting this important fact, is delineated in Fig. 1, Plate 

 CCCXXVI. AB represents a cylindrical tube, accu- rcrx 

 rately graduated; R and R' are two stop-cocks of iron ; Fig. 1. 

 TT' is a bent tube of glass, communicating with AB 

 at T. The whole apparatus having been well dried, 

 the tube AB was filled with dry mercury, recently 

 boiled ; a balloon, furnished with a stop-cock r, and 

 rilled with dry air, was then firmly connected with the 

 tube AB, after which the stop-cocks r and R' were 

 opened, thus making a communication between them. 

 The stop-cock R was next opened, and a quantity of 

 mercury allowed to make its escape by the recurved 

 tube a b. The gas was thus allowed to dilate itself, till 

 it was reduced to some particular state of rarefaction. 

 The stop-cocks R and R' were then shut. The inclo- 

 sed gas in the cylinder AB being rarefied, by its in- 

 crease of volume, the level of the mercury in it is 

 found to stand higher than that of the mercury in the 

 bent tube TT'; but the gas is easily reduced to the 

 pressure of the atmosphere, at the time of the experi- 

 ment, by pouring mercury into the tube TT', till H 

 and h have the same level. The liquid to be reduced 

 into vapour is introduced into AB, by removing the 

 balloon, and applying another stop cock R", the tube 

 connected with which is surmounted by a small fun- 

 nel V. The. stop-cock R" has a small cavity on one 

 side of it, capable of containing a single drop of the 

 liquid to be subjected to experiment, and by means of 

 which any given portion of it may be introduced into 

 AB. The liquid being thus brought in contact with 

 the dry gas, is gradually converted into vapour ; and 

 when the addition of it ceases to increase the volume 

 of the gas, a sufficient quantity is introduced, to exist 

 in the vaporous state, at the temperature and pressure 

 at which the experiment is made. The elasticity of 

 the gas being augmented by the vapour, the mercury 

 is elevated in the tube TT' above its former level, and 

 the gas, together with the vapour with which it is 

 mixed, thus sustains a greater pressure than that of 

 the atmosphere. It would be easy to make an allow- 

 ance for the difference of level, and determine by cal- 

 culation the volume which both ought to occupy under 

 that pressure ; but the apparatus itself furnishes the 

 means of obtaining the proper correction, by allowing 



