1878.] •'' , LKOnig. 



After grinding ODe side, the bead is pushed out, reversed and the other side 

 cut similarly. A rectangular slot in a brass plate (fig. 5) exactly 0.12 inch 

 wide serves as a gauge into which the bead must pass with friction. One 

 obtains soon such a practice that the fit will be obtained without requiring 

 a second setting. The operation only consumes 3 minutes. The rough- 

 ness of the faces causes now the bead to be only translucent, but by apply- 

 ing a thin film of Canada balsam a beautiful transparency is obtained. I 

 place the bead into a small air bath heated to 150 C° for a few minutes 

 then apply by means of a pointed tube the liquid balsam, replace in the 

 air-bath o minutes, after which the bead is ready for the chromometer, as 

 soon as it has resumed equal temperature with the air in the room. 



3. The Chromometer. — The construction of this simple instrument is 

 represented in transverse sectional view in the fig. 5. A box/, 8x1x1 

 inches, is mounted on a stand s, 18 inches high. A wedge (w) having the 

 complementary color to that of the metal which is to be determined, 

 cemented for support upon a colorless glass plate, moves by rack motion 

 in the box /. The motion is imparted by turning the knob d. In its 

 centre the box is perforated so that pure sky light or light reflected from 

 the porcelain plate c may pass through the colored glass into the tube a, 

 through the bead g, into the eye of the observer at the lens b. The 

 latter has only a small magnifying power and serves mainly to give a 

 straight line of vision parallel to the tube a. The lens is readily remov- 

 able, so as not to obstruct the insertion of the bead into a. The joint e 

 serves to incline the line of vision, if that should become necessary. The 

 sliding plate which carries the wedge is furnished with a permanent milli- 

 meter scale, which is read by the observer through the opening o in the 

 box, simultaneously with the observation of the bead. Fig. 7 gives a 

 natural size horizontal view of the sliding plate with a section through 

 the wedge. The latter is held by the projections p, p h /> tl , p Uh and can 

 be readily exchanged. 



Mode of working with the Chromometer. 



In the first place the wedge must be calibrated. A single determination 

 will suffice for this purpose, since the law pertains thai "the intensity of 

 color is directly proportional to the thickness of the wedge, and hence to the 

 percentage of metal under determination. If, therefore, the wedge is 

 accurately cut, so that its section is a perfect triangle, its length and thick- 

 ness at both ends being known, it is only necessary to dissolve a proper 

 quantity of the chemically pure metal or one of its compounds in the man- 

 ner described, to cut the bead and to determine the point of extinction on 

 the scale, and the quantities corresponding to each millimeter can be cal- 

 culated by simple proportion. Having found, for example, that a certain 

 wedge, 3 inches long, tapering to a perfect edge on one extremity and 

 being 0.1 inch thick at the other extremity, placed so that the apex is 

 exactly at zero on the scale, will extinguish the color of a normal bead 

 containing 0. mgT 08 Mn 2 3 , exactly at the division 15 on the scale, it is 



PKOC. AMEli. PHIT.OS. SOC. XVIII. 10?. K. PRINTED DEC, 12, 1878. 



