474 Dr. Carl Barus on the Absorption of 



nitrate, has already been mentioned. The water-glass was not 

 coloured blue, but the salt rejected as a scum in the axis of 

 the tube. Hence the solution appears to be colloidal. In 

 other respects the preceding phenomena were all reproduced, 

 all incisions of the mercury column indicating a current 

 upward in the direction of pressure. Compressibilities were 

 obtained as high as ft = 400 x 10 -6 . 



8. If the uncertain result for the opaque tube 3 be excepted, 

 the above comparisons of compressibility and volume-con- 

 traction as a rule contain data similar to those which corre- 

 sponded to tubes 1 and 2, viz. — 



No 1. 2. 3. 4. 5. 



S/3^...11xl0" 6 llxlO- 6 18xl0" 6 12xl0- 6 13xl0- 6 , 



•L'o 



where 8ft is the mean change of compressibility corresponding 

 to the change of length SL, and where L is the length of the 

 thread when cold (used throughout for want of an available 

 datum at the high temperatures). In view of the difficulty 

 of measuring ft, and of the later stage of reaction for which 

 Nos. 3, 4, 5 apply, the results may be looked upon as of the 

 same order and as a corroboration of Nos. 1 and 2. 



Since the values ft are apparent compressibilities, the 

 association of values of ft and volume-contraction may well 

 be looked upon with suspicion : for as the capillary tube 

 during the progress of the reaction increases continually in 

 bore, or in other words is growing less thick- walled, it must 

 at a given pressure yield more and more, and at the same 

 time show continually increasing apparent compressibilities 

 of the contents. A plausible explanation of the correlative 

 variation of ft and v/V is thus suggested. That it does not 

 hold may be shown as follows: — 



To treat the elastic problem first, Tait's equation* for the 

 volume increment of hollow cylinders per unit of volume is 

 available for computation. The true compressibility ft thus 

 appears as 



u p \L v J ■ a^—atf \k a 2 n / 



where p is the internal pressure, a and a x the internal and 

 external radius of the tubes, k the bulk-modulus, and n the 

 rigidity of the glass. Thus it suffices to find the correction 

 8ft for the original and the corroded tube in order to estimate 

 the value of the elastic discrepancy in question. The folio w- 



* ' Challenger Reports/ 1882, Appendix, p. 29. 



