^30 Dr Hamilton's Account of a Map f 
sible during the transmission, unless the electrical machine was 
very active, in which case there was a pale phosphorescent light 
above, and a spark in the mercury below, and a brilliant light 
in the common vacuum. A Leyden jar, weekly charged, was 
ample to transmit its electricity, by explosion, through the cooled 
Toricellian vacuum, but the electricity was slowly dissipated 
through it. When the jar, however, was strongly charged, 
the spark passed through nearly as much space as in common 
air ; and, with a light visible in the shade. Sir Humphry found 
also, that the mercurial vacuum was a much worse conductor 
than highly rarified air, at all temperatures below 200°; and 
when the tube which contained it was included in the exhausted 
receiver, its temperature being about 50°, the spark passed 
through a distance six times greater in the Boylean than in the 
mercurial vacuum. 
From these very interesting results. Sir Humphry concludes, 
that light, and probably the heat generated in electrical dis- 
diarges, depends principally on some properties or substances 
bdonging to the ponderable matter through which it passes; 
and also that space, containing no appreciable quantity of this 
matter, is capable of exhibiting electrical phenomena 
Art. VITI. — An Account of a Map of the Vicinity of Pauk- 
gan^ or Pagan. By Francis Hamilton, M. D., F. R. S. 
& F. A. S. Lond. & Edin., &c. Communicated by the 
Author. 
TChe accompanying Map, Plate IV., was constructed by the 
town-clerk (Mro Za-re) of Faukgan, during a short stop I 
made there, on the return of the embassy from the court of 
• In the course of his experiments, Sir Humphiy Davy found, that recently dis- 
tilled mercury, which has been afterwards boiled and cooled in the atmosphere, 
emits air when heated in vacuo, and that the lower stratum in a barometer tube 
had imbibed air. Hence he considers it very probable, that air exists in mercury, 
in the same invisible state as in water, that is, distributed through its pores ; and, 
consequently, the mercury in barometer and thermometer tubes, should be long, 
boiled, and should even be reboiled after a certain lapse of time. Sir Humphry 
also thinks that melted tin contains air. 
