a4 
them from new definitions, of which the following may be 
considered the principal : 
1. If two spheres be inscribed in a right cone touching 
the plane of a conic section, the points of contact are called 
foci. 
2. The radical plane of these two “ focal spheres” inter- 
sects the major axis in a point called the centre. 
The property from which the definition of a focus here 
given is derived, although known for several years, has not 
been hitherto applied 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 
elementary 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 re- 
searches on fluorine up to the date of the commencement of 
their experiments in April, 1836, proceeded to describe the 
vessels of fluor spar which they used in their first experi- 
ments, and exhibited those which were latterly 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 
