672 Prof. Draper on the Chemical Action of Light. 



But as with heat, so with light ; there are two distinct qua- 

 lities to be considered, — quantity and intensity. A very brilliant 

 beam may possibly break up a given combination, which a far 

 greater quantity of light, acting through a long period, might be 

 inadequate to touch. There are cases of the kind on recoi'd : 

 thus Sir Pi. Kane states, that he Avith M. Dumas could remove 

 two atoms of hydrogen from acetone by the action of chlorine in 

 the sunshine at Paris, but in Dublin only one. 



I have endeavoured to collect a scries of facts which might set 

 this part of tlie question in its true light. IMy iirst experiments 

 were made \\\\.\\ a lens of very fine and thin French plate-glass, 

 twelve inches in diameter, the goocbiess of which is such, that 

 on a fine day jilatinum may be melted in its focus. It was 

 mounted on a suitable support, but required to be guided by the 

 hand as the sun moved. The use of it was therefore attended 

 with considerable risk to the eyes, on account of the excessive 

 brilliancy of the focus. Screens and dark spectacles were found 

 to be very unsatisfactory ; and an illness which I consequently 

 contracted admonished me cither to abandon the subject or 

 pursue it in some other way. 



The following experiments were made with a smaller glass, 

 consisting of two lenses, the diameter being five inches, and the 

 focal distance eight. It is fixed upon a polar axis, and by the 

 aid of a clock, follows the motions of the sun with such accuracy, 

 that when once set in the morning, an object may be exposed to 

 its focus, if desirable, for a whole day. There is a contrivance on 

 the frame whicli carries the lens for su.pporting small crucibles, 

 glass matrasses, charcoal supports, &c. at the proper point, 

 which may be cither at the focus, or at any other distance from 

 the lens, as the circumstances of the experiment may require. 



The lens being five inches in diameter, and the space covered 

 by the solar image, owing to want of achromaticity and aberration, 

 one-fifth of an inch, the multiplying effect would be 6.25 times if 

 the glass were perfectly transparent, and there were no loss by re- 

 flexion from its surfaces. On a svnnmer's day of average bright- 

 ness, with the thermometer at 68° in the shade, and, the bulb 

 not being blackened, at 108° in the sun, the lens will fuse copper 

 instantly, the bead oxidizing only superficially, and cutting readily 

 after fusion. Black oxide of copper, in a little crucible of pla- 

 tinum foil, melts into a slaty-looking substance at once. Wrought 

 iron does not melt alone ; but if exposed on a charcoal support 

 in a globule of microcosmic salt previously fused by the lens, it 

 gives a clear round bead, which readily extends when beaten 

 upon the anvil. The globule (?f flux turns black. The specimen 

 employed was cut from a piece of good iron wire ; and though 

 it might be thought that exposure on the charcoal would tend to 



