OF VITAL PHENOMENA 15 



The degree of turbidity of a sol or suspension is determined with 

 Richards's nephelometer. 



In order to facilitate comparison of the results of different 

 investigators, the standards used in measurement should always 

 be recorded. The practice of measuring dimensions in the metric 

 system and temperature in degrees 'centigrade (Celsius) is quite 

 general, but some confusion may arise from any assumptions as 

 to the concentration of solutions. The effect of the acceleration 

 of gravity may be neglected in weighing, because it affects equally 

 the weights used and the substance weighed. The buoyancy of 

 the air is taken into 'consideration in the finest work and especially 

 for very light substances. Certificates of the U. S. Bureau of 

 Standards give weights in vacuo, hence the volume of the weights 

 must be subtracted from the volume of the substance weighed 

 in correcting for the buoyancy of the air. 



The lengths of the two arms of the balance beam are often 

 sufficiently different to produce an appreciable error in weighing. 

 By determining the ratio of the lengths of the two arms a cor- 

 rection may be applied. Or the true weight may be found by 

 taking the geometric mean of the weights on the two pans (after 

 a second weighing on the opposite pan). 



The standard solution or m, is the gram-molecular-weight 

 of the solute in a liter of the solution, but the liter flask from 

 the Bureau of Standards should be used at the temperature for 

 which it was standardized. After the solution is standardized, 

 any change in temperature changes its volume and hence burettes 

 and pipettes must be used at the same temperature as the flask. 

 A flask standardized at 15° would hold 0.4 cc too much at 30 , 

 but if it is merely a question of volumetric analysis, this can 

 easily be corrected for as the pipettes and burettes likewise hold 

 0.04 per cent too much. If a liter flask is standardized at 15 it 

 holds about 998 g H 2 at 15°, 996.3 at 25° and 995 at 30 when 

 weighed in air. This refers to the true liter (the volume of a 

 kilogram of H 2 at 4° ) and not to the Mohr liter. A liter meas- 

 ures 1000.027 cc but in this book cc is used to indicate 0.001 liter 

 or milli-liter (ml). 



It is often desirable to use solutions under various conditions, 

 and since their concentration according to the above definition 

 may change, new definitions are substituted. Since the molecular 



