288 Mr. Powell on Solar Light and Heat. [Oct. 



been above sufficiently proved, the non-existence of any rays of 

 heat (at least in a free and separate state) in the solar beam. 



(48.) It is obvious that by several different applications of the 

 differential thermometer, we may examine the validity of such 

 views, and decisively ascertain whether this heating power is of 

 such a nature as to affect the bulb of black glass alone, or also 

 to produce some effect on the plain one, which is equally absorp- 

 tive for simple heat. With this view several of the following 

 experiments were tried. In Experiments, Nos. 1, 2, and 3, I 

 observed the effect which would arise when each of the bulbs 

 was respectively placed just without the rays, as compared with 

 the indication when they were both equally exposed, by having 

 the focus thrown between them (they being at nearly 0*75 inch 

 distance), and when both away from the rays at an equal dist- 

 ance under the shadow of an opaque screen which surrounded 

 the lens on all sides. In this disposition of things, it is evident 

 that if the effect were due to simple radiant heat, the indication 

 when the focus was between the bulbs, should not differ from 

 that displayed when they weie at a distance. Again the effect 

 when the plain bulb was nearest should have been as much below 

 the point at which the instrument stood when away, as the effect 

 with the black bulb nearest was above it. 



(49.) The first of these conditions took place only in Exp. 3 ; 

 a difference is perceptible in 1, 2,4,5, 6, and 7. 



The second is observed in some degree in Nos. 4, 6, 7, though 

 not to the extent which the supposition would require; it is, 

 however, completely shown in No. 12. An effect on the plain 

 bulb is shown in Nos. 1, 2, and 6, as well as subsequently in 

 No. 16. The effect also when the focus was between the bulbs 

 is in general greater than on the black bulb when the other was 

 at a greater distance. 



Thus far then we can only conclude, that though simple heat 

 may be in action, it is not the sole cause of the effect : light 

 unquestionably contributes to it. The aperture of the lens was 

 varied in the two sets of experiments. This does not seem to 

 have at all altered the effect, yet it must have altered any effect 

 which was immediately dependent on the convergence of the 

 rays. 



From some other trials I was convinced that the light of the 

 focus reflected in different ways from the inner suri'ace of the 

 glass case affected the results. Hence Exp. 6 and 7 were tried 

 without the case ; but the effects were still nearly the same rela- 

 tively to each other, the total intensity of each being of course 

 diminished. 



(50.) In prosecuting the inquiry, my next idea was to present 

 a surface absorptive for simple heat, but only covering the bulb 

 in part in order that the increased radiation might not counter- 



