190 
DR. W. J. RUSSELL ON THE FORMATION OF 
the cross disappears, and there is uniform deposit over the whole of the plate. On 
still further increasing the heat below the plate the reverse action sets in, and the 
amount of deposit decreases. These changes will he described in detail later on. 
Some expeiiments made with a Bunsen lamp show how these figures are affected hv 
I'adiant heat, and the singular efiects which it produces. The flame of an ordinarv 
Bunsen burner was })laced on a level witli the plate and allowed to burn while the 
de])Osit was being formed. When the flame was at a distance of 12 inches from the 
centre of the plate, the cross was distorted, as shown in fig. 11, the heat having 
ti-avelled not only tlie 12 inches to the plate, but also passed through the glass of the 
receiver containing the fumes. In the next experiment the lamp was removed to a 
distance of L(! inches, then less distortion took place. At a distance of 21 inches the 
eflect produced was still visible (fig. 12), and even with the lamp at 26 inches the 
I'ij'. J1. Fig. li’. 
two rays nearest to it ai-e slightly thickened and distorted (fig. 13), but at 30 inches 
no eflect was produced. Another experiment of a little more definite character was 
tried. A small copper cylinder, 95 millims. in diameter and 100 millims. high, was 
filled with boiling water and placed at a distance of 12 inches from the centre of the 
jflate outside the tume vessel; tlie cross was afl’ected as before, the nearest rays were 
shortened and bulged out. A small candle burning at a distance of 8 inches from the 
plate is also sufficient to distort the figure which is being produced ipjon it. 
It has already been shown that by increasing the heat lielow a plate the amount 
of dejiosit is increased; Init if this heating be carried on to still higher temperatures, 
the phenomena are reversed, and less and less de])osit occurs. If the copper cylinder 
used in the former experiment be heated to 200°, and be placed below the centre of 
the plate, no dejiosit forms immediately above it. The same effect is more readilv 
produced, and at a lower temperature, if the plate is in absolute contact with the 
warmed cop|te]' cylinder, and it may be mentioned here, that the only way of 
