474 proceedings: philosophical society 



ing rapidly with small changes in the other variables, such as tempera- 

 ture or pressure. The factors determining the intensity of the scat- 

 tered light in the simple case of particles small compared with the 

 wave length of the light were given after the treatment of Rayleigh. 



Turbidimetry is chiefly concerned with the measurement of the 

 mass of suspended matter, but, as this is small and its accurate deter- 

 mination difficult, optical measures have frequently been adopted for 

 rapid work. The most definite of these is the intensity of the light 

 scattered in a fixed direction from a layer of turbid medium of definite 

 thickness, illuminated by a parallel incident beam. The logical meas- 

 ure would be the ratio of the scattered and incident intensity. Pro- 

 visionally, however, the scattered intensity was compared with the 

 intensity of the light diffusely reflected normally from a magnesia sur- 

 face with the same parallel beam incident at 45°. The magnesia sur- 

 face was made by depositing the smoke from burning magnesium rib- 

 bon upon thin plate glass. The magnesia-glass surface was used 

 because its direction could be determined. It was accurately reproduci- 

 ble. A turbidimeter was described, by means of which these measure- 

 ments could be made. The readings obtained with doubly distilled 

 water, with turbid liquids, and with air, both pure and vitiated with 

 smoke, were given. 



Discussion. Mr. Wright inquired as to the possibility of using the 

 scattering of a beam of polarized light, as measured by the amount of 

 light emerging polarized at right angles to the plane of the entering 

 beam, as an indication of turbidity. Mr. Wells stated that such ef- 

 fects would vary with the size of the suspended particles. Mr. Mer- 

 win inquired as to the distribution of the light scattered from the mag- 

 nesia reference plate. Mr. Wells stated that it had not been meas- 

 ured, but that it probably followed Lambert's law closely. Mr. Bates 

 spoke of the difficulty of producing a perfectly clear fluid, with especial 

 reference to some attempts to remove all suspended particles from oil. 



The 788th meeting was held at the Cosmos Club, April 19, 1917. 

 President Buckingham in the chair; 85 persons present. 



The entire evening was devoted to hearing an address by Dr. Irving 

 Langmuir on The constitution of liquids. (No abstract.) A paper 

 dealing with the subject matter presented will be found in the Proc. 

 National Acad. Sci., 3: 251-257. 1917. 



Discussion. The paper was discussed by Messrs. Sparrow, Buck- 

 ingham, Swann, and Hersey, and by Messrs. T. C. Mendenhall and 

 R. A. Millikan with especial reference to the questionable but 

 persistent idea of a spherical atom. 



The 789th meeting was held at the Cosmos Club, April 28, 1917. 

 Vice President Humphreys in the chair; 39 persons present. The 

 minutes of the 787th and 788th meetings were read in abstract and 

 approved. 



