178 MARSHALL-BANKS— NEW NEPHELOMETER. [April 23, 



in blood, by McKim Harriot for acetone^ and by S. S. Graves in 

 ammonia determinations. 



A number of instruments' and methods have been devised for 

 determining the amount of substance in suspension by the turbidity 

 of its solution and these find considerable use in industrial chem- 

 istry. While the theory underlying this method is undoubtedly 

 simpler than the nephelometric theory, it may easily be seen from 

 the following considerations that the turbidimeter cannot equal the 

 nephelometer in delicacy or sensitivity. Let us suppose that a 

 standard as used in the turbidimeter a^bsorbs about 10 per cent, of 

 the light, then an unknown of twice the concentration will absorb 

 about twice that quantity. However, it is not the amount of light 

 absorbed, but the amount transmitted that is observed in this instru- 

 ment; consequently the quantities measured would be in the ratio 

 of about 9 : 8. The reflected lights measured in the nephelometer 

 on the other hand would be nearly in the ratio of i : 2. Clouds 

 which may be measured with considerable accuracy in the nephel- 

 ometer show very slight absorption when observed by transmitted 

 light in the turbidimeter. 



Our reason for devising a new nephelometer may be made more 

 apparent by a brief review of some of the considerations involved 

 in the use of such instruments. The following are the chief fac- 

 tors involved in the amount of light reflected by an opalescent 

 solution. First, the amount of substance in suspension. Second, 

 its physical state, i. e., the number and size of the particles, and 

 their albedo which depends upon their own refractive index and 

 that of the medium in which they are suspended. The amount of 

 light observed is again modified by the fact that the light from any 

 particle is reduced by a:n amount dependent upon the absorbing 

 power of that part of the liquid above the particle. Thus we re- 

 ceive less light from the bottom layers of the suspension than from 

 those nearer the top. This complex relation between reflection 

 and absorption demands less consideration when the lengths of the 

 illuminated columns are kept equal than when they are varied. As 

 far as we are aware, in the nephelometers hitherto described the 

 light from the two tubes has been equalized by changing the lengths 

 of the illuminated columns of suspension. Although in purely 



