54 



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 

 purposeof a table, upon which the receivers of the gases 



