The Papers of Joseph Henry

Facsimile of Henry's "Record of Experiments" entry of March 30, 1847 (p. 1), Henry Papers, Smithsonian Archives.
Facsimile of Henry's "Record of Experiments" entry of March 30, 1847 (p. 2), Henry Papers, Smithsonian Archives.

Flame Transparency of

According to the view I have taken of the cause of the phenomenon of the increased radiation of flame by the introduction of a solid body balls of clay introdued into the fire would not increase the rapidity with which water would be boiled but only the quantity of heat radiated into the room heated by coal.
The quantity of heat thrown on a dutch[A] oven would be increased so that the roasting of the meat would be facilitated while the boiling process going on over the fire would be retarded.
Count Rumform to prove that flame is transparent held a candle betwen his eye and the sun and thus eliminated the case which renderes the flame opaque namely the comparatively greater illumination of the flame than the body you attempt to view through it.[1]
A much simpler and better method is to hold the flame in the cone of diverging rays of light thrown on a screen in a dark room through a hole in the window shutter and a lens.[B]
In Flanders and in several parts of Germany and particularly in the Du[ch]ies[C] of Juliers and Bergen where coals are used as fuel the coals are always prepared before they are used by pounding them to a powder and mixing them up with an equal weight of clay and a sufficient quantity of water to form the whole into a mass which is kneaded together and formed into cakes; which are afterwards well dried and kept in a dry place for use. And it has been found by long experience that the expense attending the preparation is amply paid by the improvement of[D] the fuel. The coals thus mixed with clay not only burn longer but give much more heat than when they are burned in the crude state.
Count Rumford[2]
Make[E] experiments on the invisible heat of a lamp.
Henry Papers, Smithsonian Archives.
    [1] Benjamin Thompson, Count Rumford, "An Account of a Method of Measuring the Comparative Intensities of Light Emitted by Luminous Bodies," Phil. Trans., 1794, p. 105.
    [2] We have not been able to locate an account by Rumford corresponding exactly to Henry's discussion. However, in "Observations Relative to the Means of Increasing the Quantities of Heat Obtained in the Combustion of Fuel," Journals of the Royal Institution of Great Britain, 1802, 1:28–33, Rumford discussed the practice in the Netherlands of mixing wet clay and coal (p. 32).
    [A] Altered from duch
    [B] Altered from lense
    [C] Center of word erased.
    [D] Altered from in
    [E] Altered from making