70 
SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
THE COLOR1METRIC APPARATUS AND ITS USE. 
The glasses G (fig. 12) may be of square or reetangular section, 8 to 
12 cm. high and 3 to 3i cm. inside measurement between those sides 
through which the liquid is to be observed. 1 These sides should, of 
course, be exactly parallel; the others need not be, but should be 
blackened externally. In order to further exclude the effect of side 
Fig. 12.— Apparatus for colorimetric determinations, in different aspects. G, one of two glasses of 
square or rectangular section, 8 to 12 cm. high and 3 to 3£ cm. inside measurement between those 
sides through which the liquid is to be observed. The other sides are blackened on the outside. B, 
rectangular box about 35 cm. long and 12 cm. square, stained black inside and out, one end closed 
by a ground-glass window, W, the other open, and a portion of the top removed. P, blackened parti- 
tion, with openings corresponding to the interior dimensions of the glasses when in position. S, 
blackened cardboard shutter sliding stiffly up and down between partition and glasses, so as to shut 
off all light above the lowest surface of the liquid in the glasses. 
light in this and other similar methods (chromium, for instance, p. 80), 
it is very convenient to have a simple light box (B, fig. 12) that can 
be easily held in one hand, about 35 cm. long and 12 cm. square, stained 
black inside and out and with one end closed by a piece of ground glass 
W, the other open. For a space equal to the width of the glasses the 
1 The allowable error in distance between the corresponding pairs of sides of the two glasses should 
not in any case exceed 1 per cent. Unfortunately there seems to be a disinclination or inability on the 
part of dealers in this country to furnish glasses fulfilling this requirement, and held together by a 
durable cement which shall be proof against dilute sulphuric acid. Canada balsam answers well for 
a time, but sooner or later it cracks, leaks then appear, and the sides soon drop off. It is, however, 
but a simple matter to cement them on again. 
A pair of entirely satisfactory glasses can be made from a couple of square or rectangular 3 to 4 ounce 
bottles by cutting off one pair of sides from each and grinding down till the calipers show that 
agreement is perfect. The tops are then to be sawed off and pieces of plate glass cemented on the 
sides. 
