60 Prof. J. P. Cooke on a new form of Eudiometer. 



ference of pressure as possible, in order to ensure that all the 

 water is vaporized, and also in order to avoid the somewhat un- 

 certain correction arising from the difference of temperature of 

 the two limbs of the apparatus. This correction, which in most 

 cases is insignificant, becomes important when steam is used; 

 and it is then best to measure directly the difference of level be- 

 tween the mercury in the open mouth of the iron cistern (fig. 9) 

 and that in the tube. This we do by the millimetre divisions 

 on the eudiometer-tube with the aid of a very simple contrivance. 

 A split ring of blackened sheet brass carrying a bent steel wire 

 is slipped down over the outer tube until the end of the wire 

 just touches the surface of the mercury. The vertical height of 

 this simple measure being known, we have only to add to it the 

 height of the mercury-column above the upper edge of the ring 

 in order to know the exact difference of level. Of course care 

 must be taken when the measurement is made that the mercury- 

 column is vertical ; but it is unnecessary to dwell on such obvious 

 precautions, which are essentially the same with this apparatus 

 as those so fully detailed by Bunsen in his work on ' Gasometry/ 

 It is obvious, moreover, that the number of these corrections is 

 considerably diminished in using this apparatus as compared 

 with the ordinary mercury pneumatic trough; and besides the con- 

 venience and comfort of working in a warm room, it has the 

 additional recommendation that it requires a very small amount 

 of mercury. 



The above apparatus, as represented in fig. 9, may also be used 

 with great advantage in Gay-Lussac's process of determining the 

 density of vapours. The method of using it is obvious to any 

 one who is familiar with the process. After a weighed amount 

 of the liquid under examination, enclosed in a thin glass bulb, 

 has been passed up into the eudiometer-tube, it is readily converted 

 into vapour by passing steam, or the vapour of some liquid of a 

 still higher boiling-point*, through the outer tube. All the 

 measurements required in the determination are now easily made. 

 The temperature is measured by a thermometer placed in the 

 annular space between the two tubes ; and the difference of level 

 of the mercury in the two limbs of the instrument may be most 

 accurately determined by means of a cathetometer, aided by the 

 simple contrivance just described. 



* Hydrocarbons of high but still constant boiling-points, obtained by 

 the methods of Mr. C. M. Warren, might be used with proper precautions 

 in such determinations. 



