Experiments made by M. Regnault and others on the influence of heat on pressures and volumes of gases have enabled us to fix the absolute zero of temperature as -461 degrees Fahrenheit. This point, 461 degrees below zero, is the theoretical point at which a volume of air is reduced to nothing. The volume of air at different temperatures is in proportion to the absolute temperature, and on this basis Box gives us the following table:

TABLE l.—OF THE VOLUME AND WEIGHT OF DRY AIR AT DIFFERENT TEMPERATURES UNDER A CONSTANT ATMOSPHERIC PRESSURE OF 29.92 INCHES OF MERCURY IN THE BAROMETER (ONE ATMOSPHERE), THE VOLUME AT 32° FAHRENHEIT BEING 1.

Temperature Volume in Weight of a
in degrees. cubic feet. cubic foot in lb.
32 1.000 0.0807
42 1.020 0.0791
52 1.041 0.0776
62 1.061 0.0761
72 1.082 0.0747
82 1.102 0.0733
92 1.122 0.0720
102 1.143 0.0707
112 1.163 0.0694
122 1.184 0.0682
132 1.204 0.0671
142 1.224 0.0660
152 1.245 0.0649
162 1.265 0.0638
172 1.285 0.0628
182 1.306 0.0618
192 1.326 0.0609
202 1.347 0.0600
212 1.367 0.0591
230 1.404 0.0575
250 1.444 0.0559
275 1.495 0.0540
300 1.546 0.0522
325 1.597 0.0506
350 1.648 0.0490
375 1.689 0.0477
400 1.750 0.0461
450 1.852 0.0436
500 1.954 0.0413
550 2.056 0.0384
600 2.15[2] 0.0376
650 2.260 0.0357
700 2.362 0.0338
750 2.464 0.0328
800 2.566 0.0315
850 2.668 0.0303
900 2.770 0.0292
950 2.872 0.0281
1,000 2.974 0.0268
1,100 3.177 0.0254
1,200 3.381 0.0239
1,300 3.585 0.0225
1,400 3.789 0.0213
1,500 3.993 0.0202
1,600 4.197 0.0192
1,700 4.401 0.0183
1,800 4.605 0.0175
1,900 4.809 0.0168
2,000 5.012 0.0161
2,100 5.216 0.0155
2,200 5.420 0.0149
2,300 5.624 0.0142
2,400 5.828 0.0138
2,500 6.032 0.0133
2,600 6.236 0.0130
2,700 6.440 0.0125
2,800 6.644 0.0121
2,900 6.847 0.0118
3,000 7.051 0.0114
3,100 7.255 0.0111
3,200 7.459 0.0108

The effect of this heat of compression in increasing the volume, and the heat produced at different stages of compression, are shown by the following table:

TABLE 2.—HEAT PRODUCED BY COMPRESSION OF AIR.

--------+-----------------------+----------+------------+-------------
| Pressure. | | |
Atmo- +-----------+-----------+ Volume |Temperature | Total
spheres.|Pounds per |Pounds per | in Cubic | of the Air | Increase of
|Square Inch|Square Inch| Feet. | throughout | Temperature.
| above a |above the | |the Process.| Degrees.
| Vacuum. |Atmosphere | | Degrees. |
| |(Gauge | | |
| |Pressure). | | |
--------+-----------+-----------+----------+------------+-------------
1.00 | 14.70 | 0.00 | 1.0000 | 60.0 | 00.0
1.10 | 16.17 | 1.47 | 0.9346 | 74.6 | 14.6
1.25 | 18.37 | 3.67 | 0.8536 | 94.8 | 34.8
1.50 | 22.05 | 7.35 | 0.7501 | 124.9 | 64.9
1.75 | 25.81 | 11.11 | 0.6724 | 151.6 | 91.6
2.00 | 29.40 | 14.70 | 0.6117 | 175.8 | 115.8
2.50 | 36.70 | 22.00 | 0.5221 | 218.3 | 158.3
3.00 | 44.10 | 29.40 | 0.4588 | 255.1 | 195.1
3.50 | 51.40 | 36.70 | 0.4113 | 287.8 | 227.8
4.00 | 58.80 | 44.10 | 0.3741 | 317.4 | 257.4
5.00 | 73.50 | 58.80 | 0.3194 | 369.4 | 309.4
6.00 | 88.20 | 73.50 | 0.2806 | 414.5 | 354.5
7.00 | 102.90 | 88.20 | 0.2516 | 454.5 | 394.5
8.00 | 117.60 | 102.90 | 0.2288 | 490.6 | 430.6
9.00 | 132.30 | 117.60 | 0.2105 | 523.7 | 463.4
10.00 | 147.00 | 132.30 | 0.1953 | 554.0 | 494.0
15.00 | 220.50 | 205.80 | 0.1465 | 681.0 | 621.0
20.00 | 294.00 | 279.30 | 0.1195 | 781.0 | 721.0
25.00 | 367.50 | 352.80 | 0.1020 | 864.0 | 804.0
--------+-----------+-----------+----------+------------+-------------

A cubic foot of free air at a pressure of one atmosphere (equal to 14.7 pounds above a vacuum) at a temperature of 60 degrees, when compressed to twenty-five atmospheres, will register 367.5 pounds above a vacuum (352.8 pounds gauge pressure), will occupy a volume of 0.1020 cubic foot, will have a temperature of 864 degrees, and the total increase of temperature is 804 degrees.

The thermal results of air compression and expansion are shown by the accompanying diagram.

The horizontal and vertical lines are the measures of volumes, pressures and temperatures. The figures at the top indicate pressures in atmospheres above a vacuum, the corresponding figures at the bottom denote pressures by the gauge. At the right are volumes from one to one-tenth. At the left are degrees of temperatures from zero to 1,000 Fahrenheit. The two curves which begin at the upper left hand corner and extend to the lower right are the lines of compression or expansion.

The upper one being the Adiabatic curve, or that which represents the pressure at any point on the stroke with the heat developed by compression remaining in the air; the lower is the Isothermal, or the pressure curve uninfluenced by heat. The three curves which begin at the lower left hand corner and rise to the right are heat curves and represent the increase of temperature corresponding with different pressures and volumes, assuming in one case that the temperature of the air before admission to the compressor is zero, in another sixty degrees, and in another one hundred degrees.