CELESTIAL SPECTROSCOPY. U7 
troscopic evidence agrees, it would be perhaps unwise at present to at- 
tempt to define too precisely the exact condition of the matter which 
forms the nucleus of the comet. In any ease the part of the light of 
the comet which is not reflected solar light can scarcely be attributed 
toa high temperature produced by the clashing of separate meteoric 
stones set up within the nucleus by the sun’s disturbing force. We 
must look rather to disruptive electric discharges, produced probably 
by processes of evaporation due to increased solar heat, which would 
be amply sufficient to set free portions of the occluded gases into the 
vacuum of space. May it be that these discharges are assisted, and 
indeed possibly increased, by the recently-discovered action of the 
ultra-violet part of the sun’s light? Lenard and Wolfe have shown 
that ultra-violet light can produce a discharge from a negatively elec- 
trified piece of metal, while Hallwachs and Righi have shown further 
that ultra-violet light can even charge positively an unelectrified piece 
of metal. Similar actions on cometary matter, unscreened as it is by an 
absorptive atmosphere, at least of any noticeable extent, may well be 
powerful when a comet approaches the sun, and help to explain an 
electrified condition of the evaporated matter which would possibly 
bring it under the sun’s repulsive action. We shall have to return to 
this point in speaking of the solar corona. 
A very great advance has been made in our knowledge of the con- 
stitution of the sun by the recent work at the Johns Hopkins University 
by means of photography and concave gratings, in comparing the solar 
spectrum, under great resolving power, directly with the spectra of the 
terrestrial elements. Prof. Rowland has shown that the lines of thirty- 
six terrestrial elements at least are certainly present in the solar spec- 
trum, while eight others are doubtful, Fifteen elements, including ni- 
trogen, as it shows itself under an electric discharge in a vacuum tube, 
have not been found in the solar spectrum. Some ten other elements, 
inclusive of oxygen, have not yet been compared with the sun’s spec- 
trum. 
Rowland remarks that of the fifteen elements named as not found in 
the sun, many are so classed because they have few strong lines, or 
none at all, in the limit of the solar spectrum as compared by him with 
the are. Boron has only two strong lines. The lines of bismuth are 
compound and too diffuse. Therefore even in the case of these fifteen 
elements there is little evidence that they are really absent from the 
sun. 
It follows that if the whole earth were heated to the temperature of 
the sun, its spectrum would resemble very closely the solar spectrum. 
Rowland has not found any lines common to several elements, and 
in the case of some accidental coincidences, more accurate investiga- 
tion reveals some slight difference of wave-length or a common im- 
purity. Further, the relative strength of the lines in the solar spec- 
trum is generally, with a few exceptions, the same as that in the elec- 
