TRANSACTIONS OF SECTION A. 425 
3. Note on Measurements of the Specific Heats of Gases. 
* By W. F. G. Swann, D.Sc. 
4. On the Series Lines in the Arc Spectrum of Mercury, and on their 
Resolution by an Echelon Grating. By Professor J. C. McLennan. 
5. Report of the Committee on Electrical Standards. 
See Reports, p. 109. 
6. Report of the Committee to aid in Establishing a Solar Observatory 
in Australia.—See Reports, p. 113. 
7. Report on the International Tables of Physical and Chemical 
Constants. 
8. Photographs of a Deposit upon the Poles of an Iron Are burning in 
Air. By Dr. W. G. Durrie.p and G. E. Cotuis. 
Soon after an arc is struck between iron poles a fine deposit appears upon 
both poles, but more abundantly upon the upper one. This increases in quantity 
as time goes on, and may extend to the whole length of the upper pole. In the 
immediate neighbourhood of the arc the deposit assumes forms which may be 
described as resembling a tree, because a large mass may be connected by one 
short trunk to the pole, or it may resemble a fern leaf, or it may be best described 
as feathery. 
With the currents used—5 to 8 ampéres—little difference was noticed when the 
polarity was reversed. In the photographs shown the negative was usually the 
top pole. 
The deposit formed in greatest abundance upon the edge of the upper pole, 
and appears to be an oxide of iron. It is magnetic, but this is possibly due to 
the admixture of small particles of iron which had been shot off from the pole. 
The growths vary from a millimetre to a centimetre in length; they increase in 
size by the condensation of metallic vapour or the vapour of an oxide of iron. 
Occasionally, when the arc has been stopped, fine filaments have been observed 
bridging the poles; in places they are as fine as one-hundredth millimetre in 
diameter. 
9. On a New Method of Determining Vapour Densities. 
By Dr. G. E. Grsson. 
All methods of vapour density determination hitherto in use, in which quartz 
or glass manometers are employed, suffer from the disadvantage that the highest 
temperature attainable is about 750° C. 
Above this temperature quartz loses its elastic properties, so that the mano- 
meter becomes useless for accurate measurements. 
It is, however, possible to overcome this difficulty whenever the vapour 
pressure of the substance under investigation is considerable below 750° C. 
If we heat the bulb—volume V—which contains the main bulk of the vapour 
in one furnace whose temperature, T, may be varied at will, and the manometer 
—volume v—in a separate furnace at a constant temperature, t—less than 
750° C.—we can calculate the vapour density A, relative to hydrogen, at the 
temperature T by the formula 
m RT vT 
4 = — - . é, 
BB pv Vi 
where R is the gas constant, m is the total weight of substance, wu is the 
