SCIENTIFIC SUMMARY. 
433 
“Nature,” calling attention to a curious passage in the “Expose des Appli- 
cations de l’Electricite,” by M. Le Comte du Moncel. In this work, dated 
1857, the author refers to a friend’s prediction of the possibility of transmit- 
ting speech by electricity, but regards it of course as simply a wild dream, 
scarcely worth mentioning. “ Imagine,” says this far-seeing friend, “ that 
one speaks near a mobile plate, flexible enough not to lose any of the vibra- 
tions produced by the voice; that this plate establishes and interrupts 
successively the communication with a battery. You would be able to have 
at a distance another plate which would execute at the same time the same 
vibrations.” Here then is a clear foreshadowing of the principle not merely 
of the tone telegraph, but of its highest development — the talking-telegraph. 
After all, there is nothing new under the sun — even in telephones. 
A New Photometric Unit . — In comparing one source of illumination with 
another, it is of first importance to possess some standard unit by which the 
comparison may be made with scientific accuracy. To test the illuminating 
power of coal-gas, use is generally made of a photometric standard, which 
has been defined by Act of Parliament as a sperm-candle of six to the 
pound, consuming 120 grains of sperm per hour. For example : u 14-candle 
gas ” is gas which gives a light equal to fourteen of these standard candles. 
But it is impossible to get candles uniform in composition and in structure 
of the wick, while the amount of light which a given candle emits varies with 
the temperature and pressure of the surrounding atmosphere. Hence a good 
deal of uncertainty hangs over all photometric determinations. Mr. Vernon 
Harcourt, one of the Metropolitan gas-referees, has recently devised a new 
unit of light for photometric purposes, which he believes will satisfy all the 
conditions required for a trustworthy standard flame. The combustible 
used ought to be of definite chemical composition, burning under simple and 
definable conditions, and affected as little as possible by atmospheric changes. 
Mr. Harcourt’s standard is a mixture of air with that portion of American 
petroleum which distils at a temperature not above 50° C. This liquid 
consists almost exclusively of pentane, the fifth member of the 
paraffin series. To form the standard mixture 600 volumes of air are mixed 
with one volume of the liquid pentane ; the liquid rapidly volatilizes, and 
an uniform mixture of air and vapour is soon obtained. This combustible 
mixture is to be burnt from a ^-inch orifice, at the rate of half a cubic foot 
per hour, under a temperature of 60° F. and a pressure of 30 inches of 
mercury. No change will be needed in the nomenclature of photometry, 
by the introduction of Mr. Harcourt’s standard-flame ; for the new unit is 
equal to the old ordinary sperm-candle unit, but much more constant and 
trustworthy . — Chemical News , Sept. 7, 1877. 
Rain-bands in the Solar Spectrum . — By means of a miniature spectroscope, 
which can be carried in the waistcoat pocket, it seems possible to make 
rapid observations on the proportion of moisture in the atmosphere, and 
consequently to form a fair opinion as to the chance of coming rain. Pro- 
fessor Piazzi Smyth has described some interesting results of this kind, 
obtained during a recent journey from Edinburgh to Lisbon. The spectrum 
of daylight is examined through a spectroscope having a very fine and clean 
slit ; and as it is necessary to look through a great thickness of atmospheric 
-air, the instrument is directed to the sky nearest to the horizon. If the 
