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, which likewise 
has greatly facilitated the recent work at Harvard, namely, the 
nephelometer.! 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, which bring their images together and produce an appear- 
ance resembling that in the familiar half-shadow polarimeter. The 
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 
1 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. 
