426 JOHN WILLIAM DRAPER. 



simple influence of heat, it is not exclusively conditioned on tempera- 

 ture, but it is caused by changes in the body : if these changes, be 

 they chemical or of any other kind, cease, then the phosphorescence 

 must also vanish." This quotation is introduced because it has been 

 thought that Kirchhoff did not sufficiently recognize the value and the 

 priority of Draper's work. 



Dr. Draper's experiments on the spectra of various flames, proving 

 that the occurrence of lines, bright or dark, was connected with the 

 chemical nature of the substance producing the flame, brought him to 

 the threshold of spectrum analysis, as now understood. His words are 

 prophetic : " For this reason these lines merit a much more critical 

 examination than has yet been given to them, for by their aid we may 

 be able to ascertain points of great interest in other departments of 

 science. Thus, if we are ever able to acquire certain knowledge re- 

 specting the physical state of the sun and other stars, it will be by an 

 examination of the light they emit." If he did not himself fulfil the 

 prophecy. Angstrom and Stewart also failed, though they had come so 

 near as to know that a gas when luminous emits rays of light of the 

 same refrangibility as those which it has the power to absorb. 



Dr. Draper's later papers (1872) on the Distribution of Calorific and 

 Chemical Activities in the Solar Spectrum reveal a mind luminous in 

 thought and fertile in devising experiments. His final statement is 

 that the different rays of the sun are only distinguished by varieties of 

 wave-length or rapidity of uuduliition. Whatever other differences 

 appear do not belong to the rays, but to the bodies on which they fall, 

 by which their energy is converted into other forms of energy. The 

 excessive heat at and beyond the red end of the spectrum is the work 

 of the prism, which condenses comparatively the red end and scatters 

 the violet end. As the heat of the diffraction-spectrum was insufficient 

 for experiment, he equalized the dispersion by collecting the rays in 

 the focus of a curved mirror. The superiority in the chemical action 

 at the violet end belongs to the bodies submitted to it, and disappears 

 when they are properly chosen. 



These are the fruits, not of richly-endowed scientific research, but 

 of the intervals of leisure left by professional duties. In 183G Mi: 

 Draper was appointed Professor of Chemistry, Natural Philosophy, 

 and Physiology in Hampden Sidney College, Virginia. In 1839 he 

 was made Professor of Chemistry and Natural History in the Univer- 

 sity of New York. In 1841 he assisted in the establishment of the 

 Medical Department of the University, was Professor of Chemistry in 

 it, and afterwards President. A Treatise on Chemistry, first published 



