The pressure of the vapour of this substance at the temperature of the carbonic acid bath in air (-103° Fahr.) appeared singularly uncertain, being on different occasions, and with different specimens, 3.7, 8.7, 5 and 6 atmospheres. The Table below shows the tension of vapour for certain degrees below 0° Fahr., with two different specimens obtained at different times, and it will illustrate this point.
| Fahr.° | Atmospheres. | Atmospheres. |
| -100 | 4.60 | 9.30 |
| -90 | 5.68 | 10.26 |
| -80 | 6.92 | 11.33 |
| -70 | 8.32 | 12.52 |
| -60 | 9.88 | 13.86 |
| -50 | 11.72 | 15.36 |
| -40 | 13.94 | 17.05 |
| -30 | 16.56 | 18.98 |
| -20 | 19.58 | 21.23 |
| -10 | 23.89 | |
| 0 | 27.18 | |
| 10 | 31.70 | |
| 20 | 36°80 | |
| 30 | 42.50 |
I have not yet resolved this irregularity, but believe there are two or more substances, physically, and perhaps occasionally chemically different, in olefiant gas; and varying in proportion with the circumstances of heat, proportions of ingredients, &c. attending the preparation.
The fluid affected the resin of the gauge graduation, and probably also the resin of the cap cement, though slowly.
Hydriodic Acid.—This substance was prepared from the iodide of phosphorus by heating it with a very little water. It is easily condensable by the temperature of a carbonic acid bath: it was re distilled, and thus obtained perfectly pure. The acid may be obtained either in the solid or liquid, or (of course) in the gaseous state. As a solid it is perfectly clear, transparent and colourless; having fissures or cracks in it resembling those that run through ice. Its solidifying temperature is nearly -60° Fahr., and then its vapour has not the pressure of one atmosphere; at a point a little higher it be-
