662 
Proceedings of the Royal Society 
3 t On a Method of Determining the Explosive Power of 
Gaseous Combinations. By James Dewar, Esq. 
( Abstract .) 
The author describes an apparatus by means of which the 
explosive power of gaseous combinations can easily be deter¬ 
mined, and from this, by Bunsen process, the temperature may 
readily he calculated. The essential feature of the apparatus is 
the registration of the “ compression volume ” of a given initial 
volume of air, on which the gaseous explosive mixture has been 
allowed to act. As the duration of the pressure is all hut instan¬ 
taneous, the well-known formula 
p , = (Yx\ 1-4 
p . V v J 
may be employed to ascertain the final pressure, more especially as 
the sudden rebound prevents any great loss of heat. In order to test 
the apparatus many experiments were made with mixtures of hydro¬ 
gen and oxygen, and the mean result arrived at was a condensa¬ 
tion to one-fifth the original volume of air (the initial volume 
being measured at 30 in. bar), when pure electrolytic gas was 
employed. This is equivalent to a pressure of 9-5 atmospheres, 
and therefore agrees with Bunsen’s previous determination. The 
author hopes to he able to execute a series of determinations 
under varying conditions of temperature and pressure. 
4. Note on Sprengel’s Mercurial Air-Pump. By James 
Dewar, Esq. 
The ordinary Sprengel, requiring careful manipulation, and 
being apt to get out of order, has not yet become an essential 
piece of lecture apparatus as it ought to be. The author exhibited 
to the Society two modifications adapted to lecture illustration. In 
both instruments the mercury receptacle is made of iron, and instead 
of the india-rubber joint of the original, a well-ground iron stop¬ 
cock is substituted, the portion of iron tube before the stopcock 
terminating in a Y-shaped piece bored out of the solid. In the 
one form the drop-tube is of glass, attached by means of marine 
