MEMOIR XVIII.] THE FERRIC-OXALATE PHOTOMETER. 267 



carbonic acid produced, sometimes by volume, sometimes 

 by weight. It is to be understood that before any car- 

 bonic acid can be disengaged the solution must become 

 saturated therewith, and that before we can correctly 

 measure the quantity of light by the quantity of acid 

 produced this dissolved portion must be ascertained. 

 In one of my photometers the expulsion of the dissolved 

 gas is accomplished by exposure to a bath of boiling 

 water, in another by a stream of hydrogen. Both yield 

 satisfactory results. 



But this method by the determination of fche produced 

 carbonic acid is only one of numerous plans; for in- 

 stance, we miorht use the weight of certain metals which 



O O 



the solution after exposure will precipitate. Thus a por- 

 tion which has been made and kept in the dark may be 

 mixed with chloride of gold without any action ensuing, 

 but if it has been illuminated, the weight of metallic 

 gold precipitated is in proportion to the incident light. 

 On this principle I commenced an attempt to determine 

 the hourly and diurnal illumination of a certain locality. 

 At the bottom of a metal tube, arranged as a polar axis, 

 was placed a bulb containing a standard solution of the 

 iron salt, and at the close of the proposed periods the 

 weight of gold it could reduce was ascertained. There 

 is something fascinating in determining the quantity of 

 light which the sun yields by the quantity of gold it 

 can produce. Upon the whole, however, I would rec- 

 ommend those who are disposed to renew these at- 

 tempts to select a method depending on the volume of 

 carbonic acid, for it is always easier to make an obser- 

 vation than an experiment. 



Among the important results which may be expected 

 from these new modes of photometry are the hourly, 

 diurnal, and annual quantities of sunlight. These are 

 important not only in a meteorological point of view, 



