Prof. J. C. Draper on a new Photometric Process. 91 



connected with the expansion of water near the freezing-point, 

 it will be perceived that the volume diflPers so little up to 100°, 

 that the fact of there being any difference would have been 

 doubtful, if the experiments had been limited to that tempera- 

 ture. At higher temperatures, however, the volume diminishes 

 very rapidly, so much so, that if the formulae already given for 

 the expansion of water and the saline solution hold true up to 

 300°, the chloride of potassium would enter into solution with 

 only about ro^hs of the volume it would occupy when not dis- 

 solved. It may, indeed, be doubted whether the above be really 

 the true explanation of the small amount of the expansion of 

 saline solutions as compared with that of water. If it be not 

 owing to this cause, it must be that the presence of the salt 

 exerts such an influence on the expansion of the watei', that the 

 saline solution has a rate of expansion of its own, independent 

 of that of its constituents, since it is quite certain that the ex- 

 pansion of the mixture differs in a remarkable manner from the 

 united expansion of the separate constituents. 



XVI. On a New Photometric Process for the Determijiation of 

 the Diurnal Amount of Light by the Precipitation of Gold. By 

 John C. Draper, ilf.D., Professor of Analytical Chemistry in 

 the University of New York *. 



THE influence of light on a solution of the peroxalate of iron, 

 and the use of such a solution as a photometric agent, has 

 been noticed by my father. Prof. J. W. Draper, in the Philoso- 

 phical Magazine for 1857. 



The adaptation of the peroxalate to this purpose may be 

 greatly improved by the addition of perchloride of iron ; and I 

 have used such a mixture for the determination of the diurnal 

 amount of light, being thus enabled to compare the diffuse light 

 of one day with that of another, or of one portion of a day with 

 that of another poi'tion. 



In these experiments the exposure of the sensitive solution 

 was to the north, or rather to the region about the pole. The 

 amount used on each occasion was 10 cubic centimetres, placed 

 in a thin glass tube -j^jj^Qths of an inch in diameter, and 

 graduated to cubic centimetres. 



In order to protect the exposure-tube and its contents from 

 stray light, it was placed in a box darkened in the interior, so 

 that only rays which entered by an opening 1'5 inch square, and 

 1"3 inch distant from the tube, could reach its contents. 



The sensitive solution was prc|)arcd as follows : — In the expo 

 sure-tube, mentioned above, 3 cubic centimetres of a solution of 



* Coininunicated l)v tlic Author. 



