OP ARTS AND SCIENCES. 



23 



In the plethysmograph here presented, the bal- 

 ance of Von Basch is replaced by a steel spring, 

 the elongation of which, under a given weight of 

 fluid, is equal to the rise of fluid in the receptacle. 

 The accompanying figure shows the construction of 

 the instrument. The steel spring A has the follow- 

 ing dimensions : — 



Length (unstretched) 95 mm. 



Dium. of coil 12 " 



Diam. of wire 0.55 " 



Number of coils 165 



To the lower end of this spring is suspended by 

 two threads a large-sized test-tube, B, into which 

 enters through the bent tube C the fluid coming 

 from the organ, the changing volume of which is 

 to be measured. The length of the steel spring is 

 so chosen, that the entrance of any given quantity 

 of fluid into the test-tube B causes an elongation 

 of the spring equal to the rise of the fluid in the 

 test tube. The vertical portion of the tube C is of 

 the same length as the test-tube, and since its posi- 

 tion is such that its lower end is always just below 

 the level of the fluid in that receptacle, the fluid 

 will not only be driven into, but will be withdrawn 

 from, the test-tube in precise conformity with the 

 varying volume of the organ to be measured. A 

 delicate metallic pointer, D, attached to the lower 

 end of the spring A, is brought in contact with 

 smoked paper, covering the surface of a revolving 

 cylinder, and thus records the varying position of 

 the test-tube B, and consequently the changing 

 volume of the organ to be measured. 



It is evident that the elongation of the spring 

 under the weight of a given quantity of fluid will 

 be equal to the rise of the fluid in the test-tube 

 only so long as its specific gravity remains constant. 

 If, therefore, the instrument is to be used with 

 fluids of different specific gravities, a special adjust- 

 ment of the apparatus will be necessary. This is 

 most readily effected by shortening the spring A in 

 proportion to the increase of specific gravity of the 

 fluid used. In order to do this, it is only necessary 



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