Scientific American Supplement, No. 520, December 19, 1885 | Page 8

and three minutes, depending on the sensitiveness of the plates.
The instrument is then removed to the dark room, and the plates
developed by immersing them all at once in a solution consisting of
four parts potassium oxalate and one part ferrous sulphate. After ten
minutes they are removed, fixed, and dried. Their readings are then
noted, and compared with those obtained with the silver chloride. The
chloride experiment is again performed as soon as the plates have been
removed, and the first result confirmed. With some plates it is
necessary to make two or three trials before the right exposure can be
found; but if the image disappears anywhere between the second and
eighth divisions, a satisfactory result may be obtained.
The plates were also tested using gaslight instead of daylight. In this
case an Argand burner was employed burning five cubic feet of gas per
hour. A diaphragm 1 cm. in diameter was placed close to the glass
chimney, and the chloride was placed at 10 cm. distance, and exposed
to the light coming from the brightest part of the flame, for ten hours.
This produced an impression as far as the third division of the scale.
The plates were exposed in the sensitometer as usual, except that it was
found convenient in several cases to use a larger stop, measuring 0.316
cm. in diameter.
The following table gives the absolute sensitiveness of several of the
best known kinds of American and foreign plates, when developed with
oxalate, in terms of pure silver chloride taken as a standard. As the

numbers would be very large, however, if the chloride were taken as a
unit, it was thought better to give them in even hundred thousands.
SENSITIVENESS OF PLATES.
Plates. Daylight. Gaslight. Carbutt transparency 0.7 .. Allen and Rowell
1.3 150 Richardson standard 1.3 10 Marshall and Blair 2.7 140 Blair
instantaneous 3.0 140 Carbutt special 4.0 20 Monroe 4.0 25 Wratten
and Wainwright 4.0 10 Eastman special 5.3 30 Richardson
instantaneous 5.3 20 Walker Reid and Inglis 11.0 600 Edwards 11.0 20
Monckhoven 16.0 120 Beebe 16.0 20 Cramer 16.0 120
It will be noted that the plates most sensitive to gaslight are by no
means necessarily the most sensitive to daylight; in several instances,
in fact, the reverse seems to be true.
It should be said that the above figures cannot be considered final until
each plate has been tested separately with its own developer, as this
would undoubtedly have some influence on the final result.
Meanwhile, two or three interesting investigations naturally suggest
themselves; to determine, for instance, the relative actinism of blue sky,
haze, and clouds; also, the relative exposures proper to give at different
hours of the day, at different seasons of the year, and in different
countries. A somewhat prolonged research would indicate what effect
the presence of sunspots had on solar radiation--whether it was
increased or diminished.
* * * * *

NATURAL GAS FUEL AND ITS APPLICATION TO
MANUFACTURING PURPOSES.
[Footnote: Read before the Iron and Steel Institute of London, May 8,
1885.]
By Mr. ANDREW CARNEGIE, New York.
In these days of depression in manufacturing, the world over, it is
specially cheering to be able to dwell upon something of a pleasant
character. Listen, therefore, while I tell you about the natural gas fuel
which we have recently discovered in the Pittsburg district. That
Pittsburg should have been still further favored in the matter of fuel
seems rather unfair, for she has long been noted for the cheapest fuel in
the world. The actual cost of coal, to such as mine their own, has been
between 4s. and 5s. per ton; while slack, which has always been very

largely used for making gas in Siemens furnaces and under boilers, has
ranged from 2s. to 2s. 6d. per ton. Some mills situated near the mines
or upon the rivers for many years received slack coal at a cost not
exceeding 1s. 6d. per ton. It is this cheap fuel which natural gas has
come to supplant. It is now many years since the pumping engines at
oil wells were first run by gas, obtained in small quantities from many
of the holes which failed to yield oil. In several cases immense gas
wells were found near the oil district; but some years elapsed before
there occurred to any one the idea of piping it to the nearest
manufacturing establishments, which were those about Pittsburg.
Several years ago the product of several gas wells in the Butler region
was piped to two mills at Sharpsburg, five miles from the city of
Pittsburg, and there used as fuel, but not with such triumphant success
as to attract much attention to the experiment. Failures of supply, faults
in the tubing, and imperfect appliances for use