APPARATUS 9 



Hshed constancy of temperature. The change of volume could be 

 sharply read in a connecting capillary tube. 



In order to determine the changes of volume suffered by this bulb 

 and tube under various pressures, we used mercury as a standard sub- 

 stance, depending upon the very satisfactory results of Amagat l for 

 our knowledge of its absolute compressibility. The results are care- 

 fully stated below in such a way that if any error is discovered in the 

 compressibility of this standard substance, the more accurate value may 

 be easily applied. It will be seen that when the absolute compressi- 

 bility of a single substance is once accurately known, the reference of 

 all other substances to it becomes a very simple matter. It is to be 

 hoped, therefore, that many investigators will determine by many 

 methods the absolute compressibility of mercury, a substance which 

 for several reasons forms an exceedingly convenient standard. We 

 propose ourselves to do this in the future ; but for the purpose of this 

 paper the value of Amagat is quite certain enough. 



The apparatus thus constituted demands a stout envelope around 

 each end of the bulb tube, with a free space of capillary tube between. 

 The lower of these two envelopes consisted of the compressing barrel 

 of the admirable Cailletet machine made by the Societe Genevoise for 

 liquefying small amounts of gas. In our arrangement the free space 

 of capillary tube was visible through an oval opening or window in 

 the brass support which carried the upper envelope. This upper 

 envelope in turn was closed by a heavy steel screw cap with an 

 attached tube for transmitting the pressure. The capillary was 

 cemented into each envelope by means of marine glue, and was cali- 

 brated with suitable care. 



Because the use of this apparatus was subsequently abandoned, 

 further description of it is unnecessary. We are indebted to Mr. 

 Frederic Bonnet, Jr., for assistance in this part of the work. 



While the device overcomes the difficulties which it was expected to 

 overcome-, three other causes of trouble still remained. These are, 

 the mutual solubility of the liquids at the point of contact in the capil- 

 lary tube, the adhesion of the compressed liquid to the wall of this 

 tube from which it has retreated, and the frequent fracture of the free 

 portion of the tube at pressures of even less than four hundred atmo- 

 spheres. 



In our first experiments with bromine, we used water saturated 

 with this substance as the containing or compressing liquid ; but we 



1 Loc. cit. The value found by Amagat is 0.000003 92 if the unit of pressure is 

 the atmosphere, or 0.000003 8 if the unit is the kilogram per sq. cm. 





[Ml IBRARY 



