THE TROPICAL SUNLIGHT. O 



a spark caused by a current of high tension. If we compare the spec- 

 trum, of the sun's light with that of a metal, such as cadmium, we discover 

 that the latter shows lines extending into the ultra-violet field far beyond 

 that of the former, yet it is evident, if we consider the elements known to 

 be present in the sun's mass and chromosphere, by a study of the spectrum 

 of the sunlight, that outside our atmosphere, or at least in the central body 

 of the sun, such ultra-violet rays must be present. It follows that the 

 air surrounding our globe must be able to absorb or otherwise modify a 

 large proportion of the latter. Measurement can show whether, as a rule, 

 there is a greater or less absorption in the Tropics than in temperate 

 climates. 



The theory was prevalent some years ago that the spectrum was 

 divided into classes of rays, of which those in the blue, violet and ultra- 

 violet- were considered as "actinic," namely, as producing chemical reac- 

 tions, those in the visible field as causing light, and those in the ultra-red 

 as heat, although even as long ago as 1859, Bunsen and Eoscoe 2 indicated 

 that no sharp lines could be drawn between the rays which give rise to the 

 thermic phenomena in the atmosphere and on the earth, and those causing 

 chemical changes. The power of seeing is only a subjective symptom, 

 some persons may have a range of vision for light rays beyond others ; all 

 rays may produce chemical changes, yet it is true that some of the latter 

 are brought about much more rapidly by the portion of the spectrum 

 lying toward the violet and beyond, than by the other part toward the 

 infra-red; indeed, in many changes the latter can not act as a catalyzer at 

 all. However, we must not lose sight of the fact that the total energy 

 derived from the sun, in which must be included rays of all lengths, is 

 an essential factor in maintaining chemical changes on the earth, regard- 

 less of the fact that recent investigations have made us cognizant of other 

 sources of energy which are supplied by the mass of the globe itself. The 

 quantity in which the former reaches the earth's surface at different places 

 may have much to do with the effect of local environment. 



In studying the manifestations of energy in other branches of science, 

 we .endeavor to divide such manifestations into two factors, one of inten- 

 sity or stress, and the other of capacity. So, in electricity we have 

 volts (stress) and coulombs (capacity) ; in the effects of gravity, distance 

 (stress) and mass (capacity). Light can be treated in the same way. 

 Let us suppose we have an area of 1 square centimeter through which 

 rays of a given wave length are passing from a constant source of light. 

 If we double the opening, we would have twice the light passing through, 

 but if we concentrate the total number of rays entering the opening of 

 2 square centimeters, by means of an ideal lens, on the one we would 

 obviously have doubled the amount of light in the same time passing 

 through the slit, the area of which had not changed, in other words, we 



2 Aim. d. Phijs. u. Chem. ( Poggendorf ) (1859), 108, 193. 



