Royal Irish Academy, 105 



1. If two spheres be inscribed in a right cone touching the plane 

 of a conic section, the points of contact are called/oci. 



2. The radical plane of these two " focal spheres " intersects the 

 major axis in a point called the centre. 



The property from which the definition of a focus here given is de- 

 rived, although known for several years, has not been hitherto ap- 

 plied further than to show that this point is identical with the focus 

 as usually defined. 



By the help of the above definitions, and of the simplest elemen- 

 tary principles, the central and focal properties already known have 

 been deduced, generally in one or two steps, and several new 

 theorems have been likewise discovered in the development of the 

 method. 



A paper '* On Fluorine;" by G. J. Knox, Esq., and the Rev. Thomas 

 Knox, was read by Dr. Apjohn*. 



The authors having taken a summary view of all the researches on 

 fluorine up to the date of the commencement of their experiments in 

 April, 18.36, proceeded to describe the vessels of fluorspar which they 

 used in their first experiments, and exhibited those which were lat- 

 terly found best adapted for examining the gas. These vessels were 

 of fluor spar lapped with iron wire for the purpose of equalizing the 

 temperature, so as to prevent the vessels from splitting on a sudden 

 application of heat. In place of a flat cover for the vessels, fluor 

 spar receivers were used, the cavities of which were filled with ground 

 stoppers of the same material. On moving the receivers over the 

 mouth of the vessel the stoppers fall in, and their places are occupied 

 by the gaseous contents of the vessel. On the top of each of the 

 vessels is placed a flat slab of fluor spar, which answers the purpose 

 of a table, upon which the receivers of the gases can be moved. On 

 the slab are four small depressions, in which are placed the substances 

 upon which the action of the gas is to be observed, and over which 

 the receivers when filled with the gas, can be slid. In opposite sides 

 of these receivers are drilled holes, into which are fitted, air-tight^ 

 clear crystals of fluor spar, through which the colour of any gas in the 

 receiver may be distinctly observed. The vessels are supported on a 

 stand over a lamp. 



On heating pure fluoride of mercuryin these vessels with dry chlorine 

 the authors obtained a colourless gas, (as seen through the fluor,) 

 having a heavy smell not pungent or irritating, and thereby easily 

 distinguished from chlorine or hydrofluoric acid. When exposed to 

 the air it does not fume, as would be the case were the slightest trace 

 of hydrofluoric acid present. The inside of the vessel is found coated 

 with crystals of corrosive sublimate. The gas does not extinguish 

 ignited phosphorus or red-hot iron wiref , and consequently is (as 

 Sir H. Davy conjectured) a supporter of combustion. It detonates 



• SeeLond and Edinb. Phil. Mag., vol. ix. p. 107- 

 t The non-extinction of ignited iron-wire in a gas cannot give evidence 

 of its capability of supporting combustion. — Edit. 



Vhxl, Mag, S, 3. Vol. 12. No. 71. Suppl. Jan. 1838. P 



