IN RELATION TO QUANTITATIVE CHEMICAL ANALYSIS. 
327 
quently it was deemed necessary to photograph solutions containing 1 per cent., OH 
per cent., and O'Ol per cent, of the metals, and map the spectra characteristic of each. 
Other photographs could then be taken as required for comparison with solutions 
of mineral substances or for estimating the minute constituents of metallurgical 
products. 
To prevent the instrument being splashed with liquid projected outwards by the 
spark, it is necessary to use a lens in front of the slit, the focal length of which for 
similar reasons should not be much less than three inches. 
In all cases the spark should be regulated in length by passing a piece of plate 
glass between the electrodes, and adjusting them so that they just touch it. When 
these conditions are fulfilled and the spark is working properly, a spindle-shaped 
bundle of rays may be seen when a piece of card is held in the opening of the colli¬ 
mator tube, about two inches behind the slit. The rays which fall upon the centre of 
the card should not show any wavering motion, for if this be the case the electrodes 
are not quite in the proper position. 
I propose first to describe in detail the changes observable in the spectrum of 
magnesium when a series of solutions of definite strengths are examined. In the 
case of other metals detailed descriptions will not be given, but a glance at the maps 
accompanying this paper which represent both the normal spectra with wave-lengths 
and the prismatic spectrum with scale numbers for each element, will render the 
changes in the spectra evident to the eye. 
The magnesium spectrum. 
The solution examined was prepared by dissolving a weighed quantity of the metal 
in hydrochloric acid. The changes rendered visible on dilution are the following :— 
( 1 .) Solution containing 1 per cent. Mg., or 1000 parts per 100,000 of solution. 
The first and third lines of the quadruple group, wave-lengths 2801*6 and 2794*1, 
are considerably stronger than the second and fourth. Of the quintuple group the 
lines with wave-lengths 2780*2 and 2776*9 are invisible. 
( 2 .) 0*1 per cent. Mg., or 100 parts per 100,000 of solution. 
The least refrangible line, 4480, is shortened by two-thirds. The triplet with wave¬ 
lengths 3837’9, 3832*1, and 3829*2 is altered, only one-half of each line is fairly 
strong, the other half is much weakened. 
The triplet with wave lengths 3096*2, 3091*9, 3089*9 is barely visible, and that 
through only one-half the length of the lines. 
The pair with wave-lengths 2.935*9, 2928*2 exhibit a great weakening through two- 
thirds of their length. The line 2851*3 is fairly strong, one-half being weakened. The 
second and fourth lines of the quadruple group are much weakened through two- 
thirds of their length ; wave-lengths of the second 2796*9, of the fourth 2789*6. 
