202 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



the stopper being then forced into place and the substance thus shut 

 up in an entirely dry atmosphere. The weighing bottle may now be 

 removed, placed in an ordinary desiccator and weighed at leisure. 

 The substance is really dry, and its weight has definite significance. 



Mention may be made also of another instrument, winch likewise 

 has greatly facilitated the recent work at Harvard, namely, the 

 nephelometer. 1 With the nephelometer, minute traces of suspended 

 precipitate may be approximately determined from the brightness of 

 the light they reflect. The construction is very simple. Two test 

 tubes, near together and slightly inclined toward one another, are 

 arranged so as to be partly shielded from a bright source of light by 

 sliding screens. The tubes are observed from above through two 

 thin prisms, winch bring then images together and produce an appear- 

 ance resembling that in the familiar half-shadow polarimeter. The 



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r 



CBOAT) 



Q^^Of 



Fig. l. 



unknown quantity of dissolved substance is precipitated as a faint 

 opalescence in one tube by means of suitable reagents, and a known 

 amount, treated in exactly the same way, is prepared in the other. 

 Each precipitate reflects the light; the tubes appear faintly luminous. 

 If the tubes show like tints to the eye when the screens are similarly 

 placed, the precipitates may be presumed to be equal in amount. In 

 case of inequality of appearance, the changed positions of the screens 

 necessary to produce equality of tint give a fairly accurate guide as 

 to the relative quantities of precipitate in the two tubes. Traces of 

 substance, which are too attenuated to be caught on any ordinary 

 filter, may thus be estimated. 



The two errors obviated by these simple devices, namely, the 

 presence of residual water and the loss of traces of precipitate, respec- 

 tively, have perhaps ruined more previous investigations than any 



i Richards, Zeitsch. anorg. Chem., 1895, vol. 8. p. 269; Richards and Wells, American Chemical Journal, 

 1904, vol. 31, p. 235; Richards, Ibid., 1906, vol. 35, p. 510. 



