Experimenis in the Process on Fluor Spar. 407 
Again, 20 is composed of 4 and 5: therefore divide the 
circle into 4 parts, and then by the side of 20 subdivide 
each division into 5. 
For the prime numbers take one side from the line of 
polygons, and with it mark 3 divisions as exactly as possi~- 
ble. Take these 3 divisions between the extremes, and 
divide the circle into as many divisions of 3 sides each, as 
are necessary ; then with one side between the extremes 
trisect the divisions. 
N. B. On the line of sines, the sine of 53 degrees is 
equal to the chord of three sides of a polygon of 23. sides 
nearly. 
For the purpose of very readily measuring degrees, bi- 
secting angles and lines, the sectograph (if required) may 
be made to consist of three points only in the line, besides 
the central point. 
LXVIII. Some Experiments and Observations on the Sub- 
stances produced in different chemical Processes on Fluor 
Spar. By Sir Humpury Davy, LL.D. F.RS. VP. 
R.L* 
In the Bakerian Lecture for 1808 I have given an account 
of an experiment on the combustion of potassium in sili- 
cated fluoric acid gas, in which the gas was absorbed, and 
a fawn-coloured substance formed, which effervesced with 
water, and left after its action on that fluid, a residuum which 
burnt when heated in oxygen, reproducing silicated fluoric 
acid gas; and I concluded from the phenomena, that the 
acid gas was decomposed in the process, that oxygen was 
probably separated from it by the potassium, and that the 
combustible substance was a compound of the siliceous and 
fluoric bases. 
The experiment of burning potassium in silicated fluoric 
acid gas was made likewise by MM. Gay-Lussac and The- 
nard, before I published any account of my researches on this 
phenomenon. It was indeed one of the most obvious ap- 
plications of potassium, and it occurred to many others, as 
well as to myself, that it might be made, immediately after 
I discovered that metal, 
MM. Gay-Lussac and Thenard drew the same conclu- 
sions as I did, namely, that the acid gas was probably de- 
composed during the action of potassium on silicated fluoric 
acid; but their general views differed from mine in this re- 
* From the Philosophical Transactions for 1813, part ii. 
Cc4 spect, 
