NATURAL PHILOSOPHY. 159 



plants decompose the carbonic acid of the air, assimilate the carbon, and 

 give off the oxygen for the use of the animal creation. The intensity of the 

 chemical rays is measured by an instrument which contains equal volumes 

 of chlorine and hydrogen, and enables the quantity of hydro-chloric acid 

 formed by their combination to be accurately ascertained. The mixed gases 

 do not combine in the dark, and the quantity of acid formed is directly pro- 

 portional to the incident light. In order to institute a comparison, it i$ neces- 

 sary to agree upon a unit or standard flame. This is obtained by a jet of 

 ignited carbonic oxide gas of known dimensions. The unit amount of chem- 

 ical action is that effected by such a flame acting for one minute, at a distance 

 of one meter, upon the mixed gases. Ten thousand such units are called 

 one chemical degree of light. In measuring the sun's action, the chemical 

 photometer must not be exposed to the full blaze of its beams, or the effect 

 would be too violent. A known portion of solar rays are therefore admitted 

 through a small aperture, and by means of Silbermann's heliostate the solar 

 image is reflected all day upon the same spot. When the effect of a given 

 portion of the light is known, the effect of the whole can be calculated. A 

 cloudless day should be selected for these observations. In one experiment, 

 made 011 the 15th of September, 1858, the total action of the sun, at 7h. 9m., 

 A.M., was 5.54 degrees; and by 9h. 14m. it reached 67.61 degrees. The larger 

 action, as the sun rises higher, is occasioned by the diminution of the col- 

 umn of air through which its rays pass, and a consequent lessening of their 

 absorption. Knowing the law according to which this effect takes place, the 

 action of solar rays upon different parts of the globe, or upon different plan- 

 ets, can be calculated. Thus Mercury experiences an action equal to 2125.0 

 degrees, while Neptune enjoys only 0.4. The differences upon the earth are 

 very striking. Thus, at noon, at the A ; ernal equinox, Melville Island has 3.51 

 degrees solar chemical action; Manchester, 47.15, and Cairo, 105.3. Chem- 

 ical action of this kind is greater at elevations than on the sea-level. In the 

 highlands of Thibet, where wheat flourishes at 12,000 and 14,000 feet above 

 the sea, the sun's chemical action is one and a half times as great as in the 

 adjacent lowlands of Hindostan. It is to be regretted that at present there 

 is no easy and portable instrument for making these observations. 



PHOTOGRAPHING COLOR. 



A short paper, by Sir John Herschel, in the London Photographic News, 

 contains some important remarks on this subject. Sir John expresses his 

 firm bqlief that the problem will one day be solved; and mentions a photo- 

 graph in his own possession, in Avhich green foliage is unmistakably 

 distinguished as green from the sepia tints of other portions of the picture. 

 He'considers it important, in attempts to obtain color, that the non-luminous 

 rays should be cut off, which can be done by quinine. In the absence of a 

 perfect positive photography, he reminds experimenters that they must work 

 through the intervention of a negative ; and that they ought not to expect 

 this negative to be colored, either as the objects are, or with their comple- 

 mentary colors, in order to yield a colored picture by the process of 

 photographic painting. The effect of colors in the object would thus be to 

 convert the negative into absorbent media, which would reproduce the colors 

 from whose action they were derived. 



