VISCOSITY. 



171 



VISCOSITY. 



The viscometer used was a modified form of the one described by Ostwald 

 and Luther. 1 The form described by them was found to be unsatisfactory 

 for liquids having a much greater viscosity than water. 

 The most serious objections are : 



(1) The small capillary, above and below the bulb, 

 renders it impossible to make an accurate observation of 

 the meniscus of the descending column of liquid as it 

 passes the mark. 



(2) In case a liquid more viscous than water is used, 

 the small capillary invariably fails to drain perfectly, and 

 the liquid bridges over the capillary just above the bulb, 

 thus forming a sort of valve which renders the results of 

 the measurement entirely inaccurate. 



To overcome these difficulties the following modified 

 form was made : A large capillary was placed above and 

 below the bulb, and the small capillary was joined to it 

 as shown in fig. 78. The upper bulb of the viscometer 

 had a capacity of about 3 c. c., and the lower bulb a 

 capacity of about 5 c. c. The liquid was raised from the 

 lower to the upper bulb by means of an aspirating bottle, 

 the air being dried over soda-lime. The time of flow 

 through the capillary tube was determined by means of a 

 stop-watch reading to fifths of a second. The stop-watch 

 used was found to be accurate to one-fifth of a second. 

 This was established by a number of comparative tests 

 with other standard watches. 



The specific gravity of the solution is needed in the 

 calculation of its viscosity. For specific-gravity measure- 

 ments a slightly modified form of the pycnometer em- 

 ployed by Jones and Bingham l was used. This form 

 of pycnometer is shown in fig. 79. The modification 

 consists in the elongation of the large bulb, which brings 

 a larger surface of the pycnometer, per unit volume, in 

 contact with the bath. The small bulb was made oval in shape so as to 

 secure a better draining of the bulb, and the whole instrument was of 

 much lighter construction and of somewhat greater capacity than that em- 

 ployed by Jones and Bingham. 



The zero-bath consisted of a large glass cylinder filled with crushed ice 

 moistened with water. It was found necessary to have the viscometer 



Fia. 78. 



1 Amer. Chem. Journ., 16, 479 (1894). 



1 Ibid., 34, 495 (1905). 



