Sir Henry Englefield compared these results, and obtained the following figures:——
| Blue | 56 deg. Fahr. |
| Green | 58 " |
| Yellow | 62 " |
| Red | 72 " |
| Beyond the red | 79 " |
Bérard obtained similar results, but he at first found that the maximum of heat was just at the end of the extreme red, and that beyond it the air was only about one-fifth warmer than the ordinary temperature. Sir John Herschel attributed these discordant results to Bérard having used a thermometer with too large a bulb; he accordingly repeated his experiments with other instruments with long narrow bulbs, and arrived at similar results to those obtained by the English philosopher.
We will now pass on to the physical properties of the other end of the spectrum. Towards the end of the last century, Scheele, a Swedish philosopher, remarked that chloride of silver was blackened more quickly by the violet portion of the spectrum than by any other. In 1801, Ritter of Genoa, in repeating certain experiments made by Herschel, found that a much stronger blackening effect was produced at a point beyond the violet, and that the discoloration was produced with less intensity by the violet and still less so by the blue, the change gradually decreasing till the red ray was reached. He also found that when slightly blackened chloride of silver was exposed to the effects of the red rays, or even in the space beyond, its colour was restored to it. From these facts he drew the conclusion that in the solar spectrum there existed two kinds of rays, one at the red extremity, which favoured oxygenation; the other, at the blue end, which possessed the contrary properties. He also found that when phosphorus was placed in the invisible rays beyond the red, it gave off fumes of oxide, which were immediately extinguished when it was transferred to the other end.
On repeating the experiment with chloride of silver, Lubeck found that the tint varied according to the colour in which it was placed. Beyond or in the violet ray it became brownish red, in the blue it became bluish or bluish grey, in the yellow it remained white, or became slightly yellow and reddish in or beyond the red ray. When he used prisms of flint glass, the chloride of silver was discoloured beyond the visible limits of the spectrum.
Without being aware of Ritter’s experiments, Dr. Wollaston obtained the same results by acting on chloride of silver with violet light. In continuing his researches he discovered that gum guaiacum was also influenced by the chemical rays of light.
The magnetic influence supposed to be exerted by the solar rays still remains without positive proof, although numbers of philosophers have experimented in this direction. More than fifty years ago Dr. Morichini announced that the violet rays of the solar spectrum possessed the property of magnetizing steel needles that were previously free from magnetism. He produced this effect by concentrating the violet rays upon one-half of each needle with a convex lens, taking care to keep the other half concealed beneath a screen. After having continued this experiment for more than an hour, the needles were found to be quite magnetic.
Dr. Somerville tested Morichini’s experiments by covering one-half of an unmagnetized needle an inch long with a piece of paper, and exposing the uncovered half to the violet rays of the spectrum, and found that the needle became magnetic in the course of a couple of hours, the exposed end being the north pole. The indigo rays produced almost the same effect, but the blue and green rays were much less powerful. When the needle was exposed to the yellow, orange, red, and invisible rays beyond the red, no magnetic effect was produced, although the experiment was continued for three days. Pieces of chronometer and watch springs were submitted to the same influences with a similar result; but when the violet rays were concentrated upon the needles and pieces of spring with a lens, the time necessary for magnetizing them was greatly reduced.
Baumgartner of Vienna and Christie of Woolwich also repeated these experiments. The latter philosopher found that when a needle of magnetized steel, copper, or even glass, vibrated by force of torsion in the rays of the sun, the arc of vibration diminished much more quickly than when the experiment was conducted in the shade. The sun’s rays appeared to have the greatest effect upon the magnetized needle. From these results Christie concluded that the solar rays were capable of exerting a certain amount of magnetic influence.
These experiments were afterwards fully confirmed by those of Barlocci and Zantedeschi. The former found that a natural magnet which was capable of supporting a pound weight, had its power almost doubled by exposure to strong sunlight for four-and-twenty hours. Zantedeschi exposed a magnet which would carry fifteen ounces to the sun for three days, and increased its power two and a half times. These experiments seem almost to decide the fact of the power of white and violet light to induce magnetic force; but a series of researches by a philosopher who without doubt is greater than any of those already mentioned, seems to throw some doubt on the facts we have related above.