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.

Beginning with the adiabatic curve, we find that for one volume of air when compressed without cooling the curve intersects the first vertical line at a point between 0.6 and 0.7 volume, the gauge pressure being 14.7 pounds. If we assume that this air was admitted to the compressor at a temperature of zero, it will reach about 100 degrees when the gauge pressure is 14.7 pounds. We find this by following down the first line intersected by the adiabatic curve to the point where the zero heat curve intersects this same line, the reading being given in figures to the left immediately opposite. If the air had been admitted to the compressor at 60 degrees, it would register about 176 degrees at 14.7 pounds gauge pressure. If the air were 100 degrees before compression, it would go up to about 230 degrees at this pressure. Following this adiabatic curve until it intersects line No. 5, representing a pressure of five atmospheres above a vacuum (58.8 lb. gauge pressure), we see that the total increase of temperature on the zero heat curve is about 270 degrees, for the 60 degree curve it is about 370 degrees, and for the 100 degree curve it is about 435 degrees.