io On- Radiant Matter. 



may be neglected. This line is easily seen by examining 

 with a small pocket spectroscope the light reflected from a 

 good ruby. 



There is one particular degree of exhaustion more 

 favourable than any other for the development of the pro- 

 perties of Radiant Matter which are now under examina- 

 tion. Roughly speaking it may be put at the millionth of 

 an atmosphere.* At this degree of exhaustion the phos- 

 phorescence is very strong, and after that it begins to 

 diminish until the spark refuses to pass.t 



i-o millionth of an atmosphere = 0*00076 millim. 

 1315-789 millionths of an atmosphere= fo millim. 

 1,000,000- ,, ,, ,, = 760*0 millims. 



,, =i atmosphere. 



t Nearly 100 years ago Mr. Wm. Morgan communicated to the Royal 

 Society a Paper entitled " Electrical Experiments made to ascertain the Non- 

 conducting Power of a Perfect Vacuum, &c." The following extracts from 

 this Paper, which was published in the Phil. Trans, for 1785 (vol. Ixxv., p. 272), 

 will be read with interest : 



" A mercurial gage about 15 inches long, carefully and accurately boiled till 

 every particle of air was expelled from the inside, was coated with tin-foil 

 5 inches down from its sealed end, and being inverted into mercury through a 

 perforation in the brass cap which covered the mouth of the cistern ; the 

 whole was cemented together, and the air was exhausted from the inside of 

 the cistern through a valve in the brass cap, which producing a perfect vacuum 

 in the gage formed an instrument peculiarly well adapted for experiments of 

 this kind. Things being thus adjusted (a small wire having been previously 

 fixed on the inside of the cistern to form a communication between the brass 

 cap and the mercury, into which the gage was inverted) the coated end was 

 applied to the conductor of an electrical machine, and notwithstanding every 

 effort, neither the smallest ray of light, nor the slightest charge, could ever be 

 procured in this exhausted gage." 



" If the mercury in the gage be imperfectly boiled, the experiment will not 

 succeed; but the colour of the electric light, which in air rarefied by an 

 exhauster is always violet or purple, appears in this case of a beautiful green, 

 and, what is very curious, the degree of the air's rarefaction may be nearly 

 determined by this means ; for I have known instances, during the course of 

 these experiments, where a small particle of air having found its way into the 

 tube, the electric light became visible, and as usual of a green colour; but the 

 charge being often repeated, the gage has at length cracked at its sealed end, 

 and in consequence the external air, by being admitted into the inside, has 

 gradually produced a change in the electric light from green to blue, from blue 

 to indigo, and so on to violet and purple, till the medium has at length become 

 so dense as no longer to he a conductor of electricity. I think there can be 

 little doubt, from the above experiments, of the non-conducting power of a 

 perfect vacuum." 



" This seems to prove that there is a limit even in the rarefaction of air, 

 which sets bounds to its conducting power ; or, in other words, that the parti- 

 cles of air may be so far separated from each other as no longer to be able to 

 transmit the electric fluid ; that if they are brought within a certain distance 

 of each other, their conducting power begins, and continually increases till 

 their approach also arrives at its limit." 



