462 Dr. Carl Barus on the Absorption of 



described (I. c), a clear thread of water being enclosed in a 

 stout capillary tube between terminal threads of mercury, the 

 upper of which was sealed in place, while the lower trans- 

 mitted the pressure of a force-pump. The motion of each 

 meniscus was observed in the lapse of time through a clear 

 boiling tube (vapour-bath, 185° to 210°) with the catheto- 

 meter. 



The progress of the experiments may be described, as a 

 whole, as follows : — During the first stages of heating the 

 clear thread of water expands, inasmuch as the constant 

 temperature in question is being approached. After this an 

 initially rapid contraction of the thread is manifest, which 

 must have begun much before the period of constant tem- 

 perature w T as reached, so that the full thread-length for 210° 

 is never quite attained. During the early and most marked 

 period* of contraction (and some time before) the tube 

 appears white and opaque, and the observer can only with 

 difficulty follow the rise of the lower mercury meniscus. The 

 top meniscus remains in place. Compressibility is a rapidly 

 increasing quantity. During the later stages f of heating 

 the tube becomes transparent again, the mercury-threads 

 stand out brilliantly, and the whitish opaque matter gradually 

 vanishes in the axis of the tube. Contraction becomes less 

 marked and finally ceases ; and with it the accentuated com- 

 pressibility of the aqueous silicate, now so thickly viscid as to 

 retain cavities, also disappears. During this second stage, 

 threads of mercury invariably break off if there is change of 

 pressure. Nevertheless, measurement by means of these 

 indices is not impossible, and in the telescope the observer 

 notices a slow advance of the viscous mass, moving as a whole 

 continually towards the upper end of the tube and carrying 

 the little mercury globules along like debris in a common 

 current. 



To measure compressibility at this stage is to face a 

 dilemma : on increasing pressure from below, there is marked 

 increase in the upward motion of the viscous current. It is 

 difficult to state when this accentuated motion ceases. On 

 removing pressure the mercury does not retreat proportion- 

 ately, if at all. However, when pressure is reduced too far, 

 the mercury may retreat several centimetres, quite out of the 

 field of view, as a whole, leaving well rounded or ovoid 

 cavities behind. Thus it is impossible to make measurements 



* Undissolved glass coagolum. 

 t Dissolving- coaffulum 



