524 TEE POPULAR SCIENCE MONTHLY 



As mentioned above^, we have but few data from Tucson and, therefore, 

 at present we can scarcely make a comparison, but doubtless when a 

 longer series of observation is at hand we shall discover that there are 

 many days in Tucson showing a maximum as high or even higher than 

 Manila, and an average of about the same. The fact is also observed 

 that two days apparently equally clear will show marked differences 

 between each other during corresponding hours, so that the proportion 

 of the rays reaching the earth and lying between 550 fi/x and 291 ju,;it 

 varies from day to day. A comparison of the total effect of these rays 

 with the measurements obtained by the black bulb thermometer a? so 

 demonstrates that the two are not functions of each other. Of course 

 it is clear that there must be a certain relationship because, obviously, 

 on clear days both black Imlb readings and photocatalytic measurements 

 will be high, but they need not necessarily be high in the same propor- 

 tion. The sun's rays which lie in the portion of the spectrum under 

 discussion, on reaching the atmosphere, suffer molecular scattering, re- 

 fraction and other changes which modify the proportion of direct sun- 

 light that reaches the earth's surface, and, of course, these changes vary 

 with the condition of the atmospliere. Nevertheless, it is very interest- 

 ing to observe that even in the tropics the shortest wave-lengths appear- 

 ing in the spectrum of the sunlight are very close to 291 [xjx and that no 

 shorter rays reach the surface of the earth. The same observation was 

 made in northern latitudes, so that it can confidently be stated that the 

 range of the spectrum everywhere is the same, the difference, if any, 

 being in the intensity of the light. 



The average of the measurements made at Baguio, which lies a little 

 north of Manila at an altitude of 1,432 meters, shows that the photo- 

 catalytic action of the sun in that locality is much the same as it is in 

 the lowlands. The maximum is slightly higher, being practically iden- 

 tical with that of Honolulu, and the average is 1.75 per cent, more than 

 in Manila, and 1.09 less than at Kuala Lumpur, and 0.39 more than 

 Honolulu, so that the ascent of 1,432 meters has produced the same 

 effect upon the photocatalysis as would a transfer from Manila to 

 Honolulu. 



The above is a very brief summary of the results so far obtained in 

 a study of photocatalytic reaction brought about between the rays lying 

 between 550 and 291 /i,,u, and in view of the results the more extensive 

 remainder of the spectrum, which extends upward from the point men- 

 tioned into the red and infra-red and which would include the heat 

 rays, must be considered. It is self-evident that comparative measure- 

 ments in this field involve much greater difficulties, as, at present, no 

 photocatalytic reaction is available. The best means at hand is by a 

 comparison of a series of readings with the Angstrom pyrheliometer of 



