SPECTEA OE THE METALS. 
879 
which, and terminating at about 159’5, is another strong triple group of dots, — the 
spectrum terminating at 170-5, with six nearly equidistant but rather faint groups of 
dots (Plate XXXIX. fig. 9, & Plate XL. fig. 39.) 
The strongly marked character of the spectrum of silver, and particularly the renewal 
of its intensity towards the more refrangible end, rendered it very appropriate for the 
purpose of testing the diactinic quality of difierent media ; and accordingly I have used 
it more extensively than any other metal in the experiments already detailed upon this 
subject. 
[33. Thallium. — The spectrum of thallium (for a specimen of which in a pure state I 
am indebted to the kindness of Mr. Ceookes, its discoverer) is particularly interesting, 
as in its -visible portion it is remarkably simple, the single intense green line being the 
only one visible, even when heated in the intense flame of the oxy hydrogen jet. When, 
however, the sparks of the secondaiy coil are transmitted, not only do new lines make 
their appearance in the -visible spectrum, but also in the extra-violet portion, and the 
complex impression sho-wn at Plate XL. fig, 45 is developed. This character of its 
spectrum f separates thallium from the metals of the alkalies. In the less refrangible 
portion are two strong groups of lines at about 103 and 106 ; three other groups occur 
at 116, 121, and 126 respectively, the two first less intense, the third of about the same 
strength as the first pair of groups. Several feebler pairs of dots follow ; and the spectrum 
terminates with four nearly equidistant groups, commencing respectively at 136, 141, 145, 
and 151 : the first of these groups is very strongly marked, the others are fainter but of 
nearly equal intensity. — Feb. 1863.] 
34. Mercury. — Experiments were made with this metal by soldering a platinum wire 
into a small glass tube which was filled with mercury and connected by means of the 
platinum wire with one end of the secondary wire of the coil ; the other electrode con- 
sisted of a platinum -wire. The spectrum obtained exhibited few lines, excepting those 
due to the mercurial electrode. The never-failing nitrogen line, 110-5, was evident ; but 
there were numerous strong lines due to the mercury, the most distinct of which are 
those at 104, 114-5, 117-5, 119-0, 122-5, 131-0, 138, 156, 159, each of the two last 
forming a strong broad group of dots, the last group terminating the spectrum at 161-0. 
(Plate XXXIX. fig. 10.) 
35. Lead. — This spectrum is very strong for rather more than one-third of its length ; 
it then diminishes in strength, until the action ceases almost entirely at 146-5 ; at about 
161 it is renewed abruptly by a strong group of dots, which terminate the spectrum at 
162-5. The lines in this spectrum are numerous and complicated. It exhibits an 
at 109-5 and 111-5, equidistant from the nitrogen line 110-5; beyond this are three 
intense triple group of dots between 122 and 125, and two other strong groups at about 
130 and 132. (Plate XXXIX. fig. 11.) 
* In one or two instances, when using bromiodized collodion, I saw a very faint series of groups prolonged 
as far as 190 0. 
t Proceedmgs of the Eoyal Society, January 1863, vol. xii. p. 407. 
6 D 2 
