iNDi SUM VI. in<;ii:M: mickoscoi'Y 



index. A l;iifi;(' uiiioiiiit of red iiidicjilcs a iiifi; ai-ca may not always repres(?iit affurate 



slifrlilly IiI^Ikt index than the li(|nid and inloiniat ion as to 1 ho amount of toxic dust 



orange and \'cll(i\v a still higher index. If hrcat lied hy t lie op(!rator. For more accurate 



solid and liiiuid arc \'ci'y fai' apart in index, results, the sample can l)e colle(;ted in water 



dispersion coloi's will not l)e observed, tlic or alcohol in a ^ li-cenl)Ui'}i;-Smith or Midget 



sample appearin}:; white. To deteiinine as inipingei- at t he breathing area of the opera- 



to whether the while appearance indicates tor. 



an index difference far above <)V fai' below, After collection, the iinpin}j;ei- tube is 



the dark-held stop must be removed and the placed in a water bath to evaporate off the 



I^eeke line metliod of index detei'ininat ion water or alcohol leaving the dry dust in the 



(Muployed. impiiig(!r tube. A small amount of index 



This same procedure of dark-field illuini- Tuiuid such as styrene, used in the identifica- 

 nation can be used for the determination of t ion of (juailz, is placed in the impinger tube 

 oilier toxic dusts sucii as isoti'opic opal (I'ig. and thoioughly mixed with the dust sample. 

 2). This foi'm of silicon dioxide having only A rubber policeman is of value for l)o1h mix- 

 one i'efracti\-e index shows no change in ing and transferring the sampk; to a micro- 

 color as the cap analyzer is rotated since it scope slide. The preparation is covered with 

 has the same optical properties in all direc- a cover glass and inspection made using the 

 tions. Posit i\'e identification of opaline silica dark-field dispersion staining microscope to 

 is more dilliciilt I lian ot hei- forms of silicon detei'mine the approximate percentage of 

 dioxide since it has a variable refractive in- toxic dust such as (juartz present in the 

 (lex. That ind(\\ can vaiy from l.ll to l.f(i preparation. More accurate (}uantitative re- 

 depending on the amount of water present suits can be made by comparison with a 

 ranging fi'om 1 to '20 per cent. Another prob- standard sample containing a known amount 

 lem with the lower index dusts such as ()j)al, of toxic dust. Two microscopes can be used 

 cristobaIit(> and tridymite is to find identify- in this determination. The stage of one mi- 

 ing index li(|ui(ls that have a much greater eroscope contains the unknown sample and 

 disp(M'sion than the sample such as to give the stage of the second microscope the known 

 biilliaiit dispersion colors in the case of small sample. The eyepieces of both microscopes 

 particle size. In general, the lowei- index are connected by means of a comparison eye- 

 liquids have low dispersion, in sonu> cases piece (Fig. 4) which permits examination of 

 iiaving a dispiM'siou only slightly exceeding both samples in a circular field of view di- 

 that of the sam))le examined. This probUnn vided vertically, one half being the unknown 

 can be largely solved by the use of ethyl sample and the other half the known, 

 cinnamate. Like styrene, it has a high dis- Other methods of (luantitative micro- 

 persion foi' its index of 1.557. When mixed scopic analysis of mixtures are described by 

 with ethyl phosphate, a series of licjuids Chamot and Mason (2). Ross and Sehl (3) 

 ranging in index fi'om l.flO to 1.557 can be describe a method for determination of the 

 prepared having higlu^r dispersions than the percentage of cjuartz in a mixture using the 

 index licjuids commonly used for the Becke usual Becke line methods of identification, 

 line method of index determination. F.mploying this procedure, particles are 



As indicated in the fii'st part of this articl(% counted and assigned weights dependent on 



determination of the approximate amount of their size as compared to the size of scjuares 



the toxic dust can in some instances be made of a Whipple disk placed in the eyepiece of 



by use of the microscope. This analysis is the microscope. This procedure should be 



sometimes made on ledge or rafter samples more easily and accurately done by use of 



which altiiough collected close to the work- dispersion staining. By proper selection of 



404 



