Foods, their composition and analysis : a manual for the use of analytical chemists and others : with an introductory essay on the history of adulteration / by Alexander Wynter Blyth.

Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Foods, their composition and analysis : a manual for the use of analytical chemists and others : with an introductory essay on the history of adulteration / by Alexander Wynter Blyth. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    123/844 page 85
    § 566.] QUANTITATIVE SPECTROSCOPY. 80 glass body, a solid bit of glass, which occupies 10 mm. ; hence, it is obvious that when the trough is charged with a liquid, the light will pass through 1 mm. in the lower half and 11 mm. in the upper half of the trough. In other words, light passing through the upper part will be weakened in the proportion to the lower as 10 is to 1 ; it will pass tlirough 1 mm. of the lower half and 11 mm. of the upper half. This trough is so adjusted before the divided slit of the spectroscope that the light passing tlu'ough the one half slit is wholly derived from the upper half of the trough, and that passing through the other half slit is wholly derived from the lower half. Before the trough is placed in position both slits are opened an equal width, and the light adjusted until an exactly equal illumination is obtained. On now adjusting the trough the illumination will, of course, be unequal. A complete I'evolution of the micrometer screw is equal to 100 divisions, and supposing that to make the light equal, the one micrometer screw has to be turned down to 30, then, as the intensity of the light falling through the 1 mm. is to be con- sidered as equal to 1, the unabsorbed light is in the proportion of 30 to 100, or as 0*3 to 1. The light passing through the 1 mm. being equal to 1, and the light passing through the solution to 0'3, then the extinction coefficient is equal to the reciprocal or arith- metical complement of the logarithm of 0*3, or e = - log 0'3. If a table of logarithms be consulted, the logarithm of 0-3 will be found to be 1 47712; if the last figure in the decimal portion be subtracted from 10, and the remainder from 9, and the index or characteristic be diminished by 1, this gives the arithmetical complement—that is, 0-52288, which is the extinction coefficient. The extinction coefficient being known, the absorption pro- portion must be obtained, and this is known if the concentration is known, for If c = the concentration expressed as grammes per cubic centimetre, e = the coefficient extinction. A = the absorption proportion. Then, A=^. e If the absorption proportion is known, the concentration unknown, all that the experimenter has to do is, for one or more definite regions of the spectrum, to work out the extinc- tion coefficient, multiply this by the absorption proportion for that particular region of the spectrum, and the result equals the grms. per c.c.—that is,
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