A NEW FORM OF NEPHELOMETER. 



By J. T. W. MARSHALL and H. W. BANKS, 30. 



{Read April 2^, 1915.) 



The nephelometer (Gr. vec^eA.?;, a cloud), an instrument for the 

 quantitative determination of small amounts of material in sus- 

 pension, has attracted considerable attention of late, although the 

 principles involved are by no means new. Since the time of Gay- 

 Lussac attempts have been made to estimate small quantities of 

 material by the turbidity or opalescence of their suspensions. This 

 was generally done by comparing the suspension with a graded 

 series of known suspensions prepared in the same way, and the 

 comparison was made by looking through a column of the liquid 

 and noting the turbidity, or by observing the opalescence, that is, 

 the light reflected from the minute particles when the liquid is 

 illuminated by a powerful beam of light. It is evident that matter 

 in smaller quantities or in a finer state of subdivision may be recog- 

 nized more easily by the opalescence than by the turbidity of its 

 suspension. That even excessively minute particles possess the 

 ability to diffract light has been shown by the ultramicroscope, 

 while by the Faraday-Tyndall convergent beam of light, the optical 

 in-homogeneity of solutions of crystalloids has been detected. 



T. W. Richards in the course of atomic weight determinations 

 in 1894^ devised a simple instrument to enable the opalescence of 

 very dilute suspensions of silver bromide to be more readily ob- 

 served, and in a measure, quantitatively determined. Ten years 

 later, Richards and Wells^ improved the instrument optically and 

 suggested its applicability to suspensions of substances other than 

 the silver halides. Their actual determinations, however, seem to 

 have been arrived at by a process of approximation; that is, the 

 unknown was compared in the instrument to a suspension of 

 known concentration, and from these readings a first approxima- 

 tion of its strength was calculated. A new standard of more 

 nearly the same concentration as the unknown was then prepared 



iProc. Am. Acad., XXX., 369, 1894. 



- Richards and Wells, Am. Chem. Jour., XXXL, 235, 1904. 



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