184-4.] of Bodies generally existing as Gases. 103 



The pressure of the vapour of this substance at the tempera- 

 ture 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 

 Fahr., with two different specimens obtained at different 

 times, and it will illustrate this point. 



Fahr. Atmospheres. Atmospheres. 



-100 .... 4-60 .... 9-30 



- 90 . .-; V . 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 



27-18 



10 . . * '.' ,i. ' : * * u . 31-70 



20 . . * . * v . 4 . . 36-80 



30 . . . - 42-50 



I have not yet resolved this irregularity, but believe there 

 are two or more substances, physically, and perhaps occa- 

 sionally chemically different, in olefiant gas ; and varying in 

 proportion with the circumstances of heat, proportions of in- 

 gredients, &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 redistilled, 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 re- 

 sembling those that run through ice. Its solidifying tempera- 

 ture is nearly 60 Fahr., and then its vapour has not the 

 pressure of one atmosphere ; at a point a little higher it be- 



