Mr. T. E. Thorpe on an Improved form of Filter-Pump, 249 

 To recapitulate : — 



(1) The velocity of cooling of gases — -j- is expressed by 

 Dulong^s formula 



ax 



(2) The exponents c' and dJ are the same for all gases, and 

 sensibly equal to those found by Dulong for solid bodies. 



(3) n^ is different according to the gas ; in our experiments it 

 has constantly the same ratio as in Dulong's. 



(4) The quantity of heat lost in the unit of time is 



p/^=^KW¥', (3) 



or, reducing, 



ax r 



The coefficient n' is therefore inversely as the radius of the bal- 

 loon, proportional to {l-\-ad) and to a factor K characteristic of 

 each gas. 



XXX. On an Improved form of Filter -Pump, 

 By T. E. Thorpe, F.R.S.E.^ 



[With a Plate.l 



IN the Berichte der Deutschen Ckemischen Gesellschaft (No. 7, 

 1872), Dr. Mendelejeff is reported to have described a new 

 form of filter-pump devised by M. Jogno, of Moscow, which is so 

 exceedingly simple and efficacious that it will doubtless be univer- 

 sally set up in laboratories. The disposition of the apparatus will 

 be readily understood from the annexed figure (Plate III.), which 

 represents it in the modified form about to be described. It con- 

 sists of a tube A A about 1 metre in length and from 8 to 10 mil- 

 limetres in diameter, to the side of which is affixed a side tube B 

 about 5 centims. in length. The upper end of the vertical tube A 

 is cut slantwise in the manner represented in the enlarged figure 

 (fig. 2), and is connected by means of a strong but sufficiently 

 elastic caoutchouc tube with the stopcock C in connexion with 

 the water-supply. In the original apparatus a Bunsen valve 

 was fitted into the side tube; that is, the caoutchouc tube DD 

 was stopped at the upper end with a short piece of glass rod and 

 cut along its length near the end by a smart blow from a chisel. 



* Communicated by the Author, having been read before the British 

 Association at Brighton, 18/2. 



