ON RADIANT MATTER. 19 



intense red line, a little below the fixed line B in the spectrum, having 

 a wave-length of about 6,895. There is a continuous spectrum begin- 

 ning at about B, and a few fainter lines beyond it, but they are so faint 

 in comparison with this red line that they 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 favorable than 

 any other for the development of the properties of radiant matter 

 which are now under examination. Roughly speaking it may be put 

 at the millionth of an atmosphere.* At this degree of exhaustion the 

 phosphorescence is very strong, and after that it begins to diminish 

 until the spark refuses to pass, f 



I have here a tube. Fig. 5, which will serve to illustrate the de- 

 pendence of the phosphorescence of the glass on the degree of exhaus- 



* 1-0 millionth of an atmosphere = O-OOOTG millim. 



1315"789 millionths of an atmosphere = I'O millim. 

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



" " " " =1 atmosphere. 



f Nearly a hundred years ago, Mr. WilUam Morgan communicated to the Royal Society 

 a paper entitled " Electrical Experiments made to ascertain the Non-conducting Power of 

 a Perfect Vacuum," etc. The following extracts from this paper, which was published 

 in the " Philosophical Transactions " for 1 785 (vol. Ixxv., p. 272), will be read with in- 

 terest : 



A mercurial gage about fifteen inches long, carefully and accurately boiled till every par- 

 ticle of air was expelled from the inside, was coated with tin-foil five 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 pecuharly well adapted 

 for experiments of this kind. Things being thus adjusted (a small wire having been pre- 

 viously 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 color 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 color ; 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 fight 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 be 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 particles 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. 



