10 STUDY OF ABSORPTION SPECTRA 



the Nernst glower was carefully protected from air-currents. This 

 was accomplished by inclosing the glower in a tight box of heavy 

 asbestos board. That the current must have remained invariable during 

 the two successive readings which we had to make, to obtain the per- 

 centage transmission for any given layer of solution, is shown by the 

 close agreement of the duplicate readings. As a rule, the duplicate 

 results agreed with one another to within less than 1 per cent. That 

 we could duplicate our readings so closely we regard as due to the con- 

 stancy of the zero-point of the radiomicrometer and to the steadiness 

 of the current passing through the Nernst glower. Another factor 

 which seriously interferes with the accuracy of this work is mechanical 

 vibrations. This effect becomes especially noticeable when the deflec- 

 tions of the radiomicrometer are small. By placing felt under the 

 marble slab which served as the base of support of the instrument, and 

 setting the radiomicrometer support on blocks of wood and cork, we 

 were able to eliminate all mechanical disturbances except those of a 

 very violent character. When such violent disturbances occurred, the 

 work was temporarily discontinued and the measurements repeated. 



METHOD OF PROCEDURE. 



The Nernst glower carried 0.8 ampere and 120 volts. By means of 

 an adjustable resistance we could keep the current very close to 0.8 

 ampere throughout this entire work. If any appreciable variation 

 in the current was detected between the two successive readings which 

 determined the transmission for any given layer of solution, the readings 

 were always repeated. The light from the Nernst glower, after being 

 rendered parallel by passing through a lens, was passed through a given 

 layer of solution and then focused on the slit of the Hilger spectroscope. 



By properly rotating the drum-head of the spectroscope, w r e could 

 throw the desired wave-lengths of light on the junction of the radio- 

 micrometer. The dispersion curve for the glass prism was carefully 

 worked out, some of the points in the infra-red being determined by 

 means of the water-bands. It was possible to find definite maxima of 

 absorption for the water-bands Ijii, 1.25/*, l.Sl^u, and 2.01ju. Since we 

 worked mainly with aqueous solutions, after each series of measure- 

 ments we could determine, by means of the water-bands, whether the 

 prism had shifted or not. 



Suppose it is desired to know the absolute transmission of a given 

 solution, 20 mm. in depth, for a given wave-length of light. This was 

 done by the following differential method, the correctness of w r hich has 

 already been discussed: 1 Twenty-one millimeters of the solution were 

 placed in one cell and 1 mm. was placed in a second cell, which was 

 exactly like the first one. The wave-length of light, whose transmis- 



'Carnegio Inst. Wash. Pub. 190, p. 62. 



