4 



The Philippine Journal of Science 



is taken as the depth at which the sharp image of the cross 

 disappears, giving place to a shghtly blurred one — ^that is, it 

 seems out of focus. Table II is provided for converting the 

 turbidimeter depths to parts per million of silica. 



Table II. — Conversion of turbidimeter readings in depth to parts per million 



of turbidity. 



Reading. 



Turbidity 

 (asSiOe). 



Reading. 



Turbidity 

 (asSiOz). 



Reading. 



Turbidity 

 (asSiOz). 



Reading. 



Turbidity 

 (asSiOz). 



cm. 



Parts per 



million. 



cm. 



Parts per 

 ■million. 



cm. 



Parts per 

 million. 



cm. 



Parts per 

 million. 



2.3 



1,000 



6.3 



350 



10.5 



210 



19.6 



110 



2.6 



900 



7.3 



300 



11.0 



200 



21.7 



100 



2.9 



800 



7.6 



290 



11.5 



190 



23.0 



90 



8.2 



700 



7.8 



280 



12.1 



180 



26.0 



80 



3.5 



650 



8.1 



270 



12.8 



170 



28.0 



70 



3.8 



600 



8.5 



260 



13.6 



160 



31.0 



60 



4.1 



550 



8.7 



250 



14.4 



150 



36.0 



50 



4.5 



500 



9.1 



240 



15.4 



140 



42.0 



40 



4.9 



460 



9.5 



230 



16.6 



130 



52.0 



30 



5.6 



400 



10.0 



220 



18.0 



120 



70.0 1 20 



Turbidity may be also determined with the turbidity rod, 

 which consists merely of a bright platinum wire fastened at 

 right angles to a tape. Under the proper conditions the tape 

 is lowered into the water under examination, and the end point is 

 taken as the depth at which the wire just disappears from view. 

 The tape is calibrated directly to read parts per million of silica 

 .(SiO,). 



The disadvantage of the turbidity-rod method is the required 

 nicety of adjustment of conditions, involving the use of a large 

 sample under circumstances often impossible. The turbidimeter 

 method, on the contrary, is independent of most of these con- 

 ditions. Only a small sample is required. Since the method 

 is based on the diffraction of light, the accuracy of the deter- 

 mination is almost independent of the intensity of the light and, 

 therefore, of the condition of the batteries and bulb. It fol- 

 lows directly that the original calibration as given by Leighton 

 is applicable to any well-constructed turbidimeter. No difficulty 

 was experienced in having a suitable instrument constructed 

 for our purposes. 



Sulphates are also determined with the turbidimeter, as de- 

 scribed by Leighton. To 100 cubic centimeters of the water 

 is added 1 cubic centimeter of hydrochloric acid (50 per cent 

 concentrated acid by volume) and 1 gram of powdered crystals 

 of solid barium chloride. Precipitations are conveniently made 

 in 250 cubic centimeter glass-stoppered bottles. The water is 



