1824.] 



Mr 



Powell on Solar Light and Heat. 



325 



Thermometers. 



Rise in two 

 minutes cen- 

 tigrade. 



Mean 



A. 



Black. 



7 



6 



10 



7 



7-5 



Thickness 

 of glass. 



Inch. 



0-06 

 0-44 



(9.) These results exhibit a very close agreement in the ratio 

 of the risings of the two thermometers, when exposed, and when 

 screened, and this with glasses of different thickness, at different 

 times, and with different absolute intensities of the sun's rays ; 

 as also when the colours of the bulbs were mutually changed. 

 The mass of the bulb A was somewhat greater than B ; the glass 

 acquired no heat sufficient to interfere with the results ; and the 

 thermometer was always placed so that the bulbs were not near 

 any object which might radiate heat. The temperature of the 

 air affecting both surfaces equally would tend to diminish the 

 ratio. To its variation, I conceive, the trifling difference in the 

 ratios may fairly be ascribed. 



(10.) Hence, I think, we are entitled to conclude, that there 

 do not exist in the solar beam in its natural state any rays of the 

 description just alluded to ; but that the whole emanation 

 consists of one sort of rays distinguished by the two characteris- 

 tics of affecting substances with heat in proportion to the dark- 

 ness of their colour, and being wholly transmissible through glass 

 without heating it ; and that these same rays when they infringe 

 on the eye are capable of producing the sensation of vision ; 

 and by the absorption of some, and the reflexion of others, of 

 their constituent parts, at the surfaces of bodies, produce the 

 phenomena of colours. 



(11.) The heating effect maintains an intimate relation to 

 light both in respect to the substances which it traverses without 

 interception, and to those by which it is absorbed, and to the 

 degree of absorption. It is found to accompany the rays of 

 light in the most constant and inseparable manner : through 

 whatever substance, and in whatever direction it takes its course, 

 this is strikingly exemplified in one of Sir W. IlerschePs experi- 

 ments (Phil. Trans. 1800, No. 13, Exper. 11), m which the heat- 

 ing effect is shown to accompany the rays of light in all the 

 alterations of its course through a Newtonian telescope with 

 four lenses. 



Speaking in general terms, within ordinary limits, and for 

 light of the same colour, we may say, that the healing effect 

 increases or decreases in proportion to the intensity of illuminat- 



