294 LECTURES TO SCIENCE TEACUEES. 



before ; but by Frankland and Ward's apparatus the baro- 

 meter tube was placed in the water-vessel together with 

 the others, so that the pressures of the gases could be read 

 off independently of changes of atmospheric pressure. At 

 the bottom there was a means of communicating the tubes 

 with one another in any way you pleased. At the bottom 

 there is a three-way cock by which communication can be 

 made between the two outside tubes, and a long barometer 

 tube which passes down below. It is a tube of iron more 

 than thirty inches long ] so that when the stop-cock was 

 turned in one position the mercury would flow out of the 

 apparatus, and the whole become vacuous. The eudiometer 

 was divided into ten equal volumes, so that it was easy to 

 reduce the gas always to certain volumes, and then measure 

 the tension or pressure in the barometer tube. In this 

 way the corrections were much diminished, and there 

 were fewer calculations to be made. We were entirely 

 independent of the variations of the atmospheric pressure, 

 and the only thing that had to be attended to was the 

 constancy of the temperature of the water in the large 

 vessel ; but this was obtained by keeping a continuous 

 stream of water flowing through the apparatus. 



Some years after Frankland and Ward's apparatus was 

 employed I used this one, which is absolutely on the same 

 principle, although the supply tube was removed and the 

 mercury was admitted by means of a reservoir which could 

 be raised or lowered by a winch ; there is at the back a 

 black screen to enable you to obtain the proper illumina- 

 tion of the surface of the mercury. You get the bottom of the 

 black line exactly on a level with the mercury and have the 

 light below. The telescope here enables you to read off the 

 height of the column, and at the same moment the volume 

 of mercury is being altered by turning the stop-cock. 



Dr. Frankland has lately improved upon his original 

 apparatus by the removal of the centre supply-tube alto- 

 gether, the mercury being admitted from a reservoir con- 

 nected to the apparatus by a flexible tube. In this 

 instrument we have the advantage of having no connec- 

 tions which are not of glass or which consist merely of 

 two pieces of steel brought together. In Kegnault and 

 Rsiset's you have metallic communication, there being steel 

 stop-cocks and communications ; and you have no idea of 



