530 



ILL USTRA TI VE EXPERIMENTS 



(d) Dead-space experivient (p. 87). Fi]i a glass tube,' of about 4 in. diameter 

 and 6 in. in length, and open at both ends, with a 5 per cent, gelatin sol to 

 which sufficient silver nitrate has been added to give a Normal solution. 

 Immerse the tube, when the gelatin has set, in a 10-15 per cent, solution of 

 sodium chloride. Ring formation, starting from each end, stops before the 

 middle is reached. 



(e) Rings formed between gases in air. Put a cork into a 1 oz. bottle. 

 Through the cork pass a glass tube about a metre long and between 1 and 

 2 cm. in diameter. Plug the upper end of the tube loosely with glass wool. 

 Put a few cubic centimetres of HCl into the bottle and moisten the wool with 

 concentrated ammonia. Leave overnight. 



31. Determination of the Relative Viscosity of a Liquid. 

 When a liquid flows through a narrow tube the velocity of flow depends 

 mainly (a) on the force producing the flow and (/3) on the resistance to flow 

 produced by the viscosity or internal friction of the liquid. In Chap. XXV. 

 we considered the shearing of the difTerent layers of the blood stream. The 

 liquid, we saw, coiild be regarded as made up of a number of concentric 

 tubes sliding past one another. When the liquid is 

 -^TTT-T' moving through the narrow tube there will be, under 

 constant conditions, a constant difference in velocity 

 between the different tubular layers. The force per unit 

 area necessary to maintain this condition is proportional 

 to the difference of velocity, v, of two adjacent layers, 

 and inversely proportional to their distance apart, x. 



V 



Briefly, Force = r] X -, where rj is the coefficient of vis- 

 cosity, which is the force per unit area when v = x. If, 

 now, the quantity of fluid and the pressure be kept constant 

 and the time observed which the fluid takes to travel a 

 certain distance, the viscosity of two liquids with densities 

 s' and s" with times of flow t' and t" will be as 



V Iv =^ s t js t . 

 Neglect of the difference of density introduces an error 

 of less than 1 per cent, and materiallj' simplifies the 

 o])eration, i.e. 



rj'j-q" = t'jt". 



Apparatus (Denning- Watson Viscosimeter). The instru- 

 ment is a modification of the Ostwald-Poiseuille viscosimeter. It consists of 

 a U-tnbe with a long and a short arm (Fig. 108). The long arm (6 cm. in 

 length) is blown out at its free end into a cup-shaped receiver with a thin 

 edge. 



On the short arm (2 cm. in length) there is a small elliptical bulb, the capacity 

 of which is defined by the two lines ni' and m". 



Method of use. The receiving cup at the end of the long arm is filled (and 

 kept filled) with the fluid to be tested, which passes down the capillary tube. 

 A stop-watch is started when the fluid reaches the point in' and stopped when 

 it reaches ;;/". The time taken is compared with the time reading of water 

 which is recorded on the back of the tube used. Denning and Watson urge 

 attention to the following points. (1) The tubes should be scrupulously 

 clean and jirrferlly dig. (2) The viscosimeter and the fluid must be at the same 

 fixed temperature. (3) The receiver of the instrument must be kept filled 

 with the fluid, for pressure-height must be kept constant, i.e. compared with 



m 





'-V 



m 



I'lG. 1(18. — ViscosiiiK tor 

 (HairMcii). 



