216 PHYSIOLOGICAL PHYSICS, [Chap.xx. 



tlie resistance being directly as the length. The flow 

 of different liquids through tubes of the same length 

 and diameter, and under the same pressure, varied 

 greatly. For example, it took water 535-2 seconds 

 to pass through the same tube that ether passed 

 through in 160'5 seconds; alcohol took 1184-5 

 seconds; serum of blood, 1029; serum with alcohol 

 took longer time, 1223' 4; and with ammonia less, 

 9^1 '6. Salts like iodide of potassium and nitrate of 

 potassium increased the speed, chloride of sodium and 

 sulphate of soda diminished it. 



The movement of liquids through elastic 

 tubes is not always the same as that described for 

 rigid tubes, because a new force, elasticity, is intro- 

 duced into the question. It is proper to observe, 

 however, that this new force need not always come 

 into play. Thus, suppose a constant flow of fluid 

 through an elastic tube, under the influence of a 

 constant pressure. The pressure may not be sufficient 

 to distend the tube beyond the normal, and in 

 that case the fluid will obey the same laws as if it 

 flowed through a rigid tube. The pressure may even 

 be sufficient to distend the tube, and even to distend it 

 to the uttermost, without any variation being produced 

 in the flow of the fluid. For the pressure, however 

 great it may be, is at the same time constant, and the 

 only influence it exerts through the elasticity is to 

 make the tube wider or narrower according as the 

 pressure is greater or less. The elasticity comes into 

 play only when the constancy (the equilibrium) is 

 disturbed. Thus, suppose an elastic tube, distended 

 already to some extent by a certain pressure, to come 

 under the influence of increased pressure, acting only 

 for a short time, by the introduction of an added 

 quantity of fluid, it dilates further in response to 

 the demand, but as soon as the additional pressure 

 passes away it is restored to its former calibre by the 



