920 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHE^nCAL EESEAECHES. 
IV. THE PHOTO-CHEMICAL ACTION OE THE SUN COMPAEED VHTH THAT OF A 
TEEEESTEIAL SOUECE OF LIGHT. 
We have made a few experiments for the purpose of comparing the light evolved 
from the sun’s body with that liberated from a teiTestrial source. The most suitable 
light for this object, appeared to be the intense illumination produced by the combustion 
of magnesium wire in the air. In the following experiments we have employed a piece 
of magnesium wire, the radius of which, measured under a microscope, was r=0T485mm. ; 
this wire was pressed out of a steel press by the method employed by one of iis for the 
preparation of potassium and sodium wire, and applied by Dr. Matthiessex to the metals 
of the alkaline earths. When the end of such a wire is raised in temperature, it takes 
fire and burns on regularly throughout its entire length, leading a coherent thread of 
magnesia. 
In order to compare the light thus evolved with that of the sun, it was necessary to 
know the length of wire which remained incandescent during the combustion. This 
cannot be measured directly, because a surface of a few square millimetres appears 
during the ignition like a ball of fire as large as a nut. If the enormous intensity of the 
light be weakened by transmission through dark coloured glasses, the edges of the Vtire 
can be more distinctly seen ; but, owing to the rapidity with which the combustion 
proceeds, even in this way no exact measurement is possible. The length of the 
incandescent piece of wire may, however, be obtained, with sufficient accuracy, in the 
following way : — -Between 30 and 40 millimetres of wire of the above thickness were held 
in front of a photometric stearinized diaphragm illuminated by a gas-flame burning in 
the tin box described in our former papers, and then burnt at such a distance from the 
diaphragm that the ring just disappeared. If, now, all the arrangements remainmg the 
same, shorter pieces of wire of exactly known length are burnt, the ring always disap- 
pears until the length of the wire taken is less than the part which during the com- 
bustion is kept incandescent. From this time forward the ring appears plainly, and is 
seen to be black on a white ground. Two experiments made in this way showed that 
pieces of wire above 10 millims. in length rendered the ring invisible, whilst with pieces 
shorter than 10 millims. the ring was seen to be dark on light ground. The length of 
wire kept continually incandescent during the combustion is therefore 72 = 10 millims. 
in length. The wire burning at a distance of 2440 millims. from the insolation- vessel 
of our photometer, effected an action of 181 *7 units of light, or of 0 '01 81 7 degree of 
light per minute. The light producing this action emanated from a cylinder of 
magnesium, having a radius 0T485 miliim. and a height of 10 millims. The surface 
of this half cylinder gives out as much light as an equally intensely illuminated 
rectangle whose base is equal to the diameter, and whose height is the same as the height 
of the cylinder. This rectangle had, in our experiment, the area 2r72=2'9/ square 
millimetres, or a circular area having a radius of 0‘9725 miliim. radius. If this 
surface had been placed at a distance of 208*7 millims. from the insolation-vessel, 
