946 
EEV. T. E. EOBIKSON ON SPECTEA OE ELECTEIC LIOHT. 
to 1. Two things, however, must be remembered: these errors may be + ; and there- 
fore, in comparing a series, the limit is nearly doubled ; and further, these probabilities 
may be much modified by other circumstances. For instance, a line or band may be 
identified by some peculiarity, even if the difference be greater than the probable limit. 
In general I would fix the limit to +1'*0 from the mean. 
I at first read both verniers ; but found this consumed so much time, that I deter- 
mined the excentricity of the upper circle and allowed for it. This correction for the 
vernier A =0'-75 X sin (160°H-?i). 
The gases which I selected for experiment are — 1, air ; 2, nitrogen obtained by heat- 
ing nitrite of potassa with saturated solution of sal-ammoniac ; 3, oxygen ; 4, hydrogen : 
these last two were obtained by electrolysis of jpure oil of vitriol diluted with eight 
volumes of distilled water, in a voltameter of peculiar construction. A porous cell has 
a cover cemented on it with three tubulures ; one for admitting the dilute acid ; one 
for a strong platinum wire to which is soldered with gold a platinum sheet, exposing 
19 square inches; the third carries away the evolved gas. Round this cell a larger 
platinum is rolled, and it is immersed in a jar filled with the same dilute acid. When 
connected with the three Grove’s already mentioned, it gives 8 inches of hydrogen per 
minute. Both the gases I believe to be quite pure, except as to ozone, when thus 
obtained. 5. The carbonic oxide was got by heating sulphuric acid with ferrocyanide 
of potassium. 
As to the metals, the platinum and silver were obtained from Messrs. Johnsox and 
Matthey, the aluminium from Paris. I am indebted to the kindness of Dr. Matthiessex 
for calcium, tellurium, and gold. The palladium was given to me by Dr. Wollastox; 
tin, lead, and bismuth reduced from oxides carefully prepared ; zinc, iron, and antimony, 
deposited by electrolysis on platinum wires. Cadmium, nickel, cobalt, magnesium, 
sodium, and potassium were got from Messrs. Jacksox and Towxsox. 
The spectra at common pressure (C.P.) are in general magnificent objects. Their 
ground seems to be a continuous spectrum, of which, however, the brightness varies very 
much with different substances and at different parts. Sometimes, especially at the 
violet end, this ground is so faint that its presence might be questioned ; but I believe it 
exists even there. On this are, as it were, superposed luminous lines of every degree of 
brightness, from a splendour almost insupportable, to a faintness such that (at least 
with my optical means) the least glimmer of diffused light in the telescope totally effaces 
them. Most of these lines are (as might be expected) as broad as the image of the 
slit : a few are much broader, even to six or seven times. Such are, I think, always 
cloudy and ill defined, giving the impression that they are groups of finer lines, wliich 
the optical power is insufficient to separate. In several instances this is shown to be 
the fact by the combination of two fluid prisms of 60° (2BS.C), of which the two most 
remarkable are those which I call in the green, and x", at the beginning of the violet. 
The first was seen in the Merz prism as a broad bright band, but it is a crowd of very 
fine lines, of which the central one is much the brightest ; and some of the others are 
