Viscosity of Solutions. 279 



right limb having been just previously removed, the extra 

 quantity of liquid clue to expansion is withdrawn by a suitable 

 instrument, the meniscus being made to coincide with the 

 upper mark on the right limb when the meniscus in the left 

 limb coincides with the lower mark on that side. Connexion 

 is again restored between the right limb and the right tap. 



The left tap is now for a few seconds turned to pressure, 

 and then the right tap is turned also. 



The first reading of the pressure is now taken, and the left 

 tap turned to air. The upper mark in R ( = right side) is 

 observed through a telescope, and, when the meniscus crosses 

 it, the watch is started. Immediately the first reading of the 

 temperature is made, through another telescope. The first 

 telescope is moved down on its graduated vertical scale to 

 view the lower mark in R. 



The stop-watch is stopped at the instant of coincidence, the 

 second reading of the temperature is taken, and the left tap 

 turned to pressure. The second reading of the pressure is 

 now taken. A second set of observations is taken with the 

 liquid flowing in the opposite direction. When this set is 

 complete, the stirrer is stopped, unless the whole set is to be 

 repeated, and the experiment at this particular temperature 

 is at an end. 



The value for the viscosity is determined for each set of 

 readings ; and if the temperature has remained unaltered, the 

 mean of the two values is taken as the first absolute value 

 at that temperature. A second absolute value is obtained 

 from the third and fourth sets of observations, that is the 

 second complete set : here a complete set represents two 

 ordinary sets — one from right to left, and the other from left 

 to right. 



The temperature,, however, usually alters slightly during an 

 experiment at a particular temperature; and we may obtain 

 four different readings of the mean temperature and four 

 different values of the viscosity. We have then to reduce these 

 four values to what they would be at the mean of the four 

 temperatures, for which purpose the value of drjjdt is obtained 

 from a curve drawn through the points representing the first 

 values of viscosity and temperature in each set, and by means 

 of it the four values are reduced to values at the mean tem- 

 perature. Then, as before, the mean of the first and second 

 values gives the first mean value, and the mean of the third 

 and fourth the second mean value. The mean of these values 

 is taken as the final mean value. 



U2 



