LIGHT. 121 



substance, illuminated by the same spectrum, may present 

 different appearances to different persons, the spectrum ap- 

 pearing more elongated to the one than to the other, so that 

 what is light to the one is darkness to the other. A depend- 

 ence on the recipient body may also, to a great extent, be 

 predicated of heat. Let two vessels of water, the contents 

 of the one clear and transparent, of the other tinged by some 

 colouring matter, be suspended in a summer's sun ; in a very 

 short time a notable difference of temperature will be ob- 

 served, the coloured having become much hotter than the 

 clear liquid. If the first vessel be placed at a considerable 

 distance from the surface of the earth, and the second near 

 the surface, the difference is still more considerable. Carry- 

 ing on this experiment, and suspending the first over the top 

 of a high mountain, and the second in a valley, we may ob 

 tain so great a difference of temperature, that animals whose 

 organization is suited for the one temperature could not live 

 in the other, and yet both are exposed to the same luminous 

 rays at the same time, and substantially at the same distance 

 from the emittent body the substance nearer the sun is in 

 fact colder than the more remote. So, with regard to the 

 medium transmitting the influence : a green-house may have 

 its temperature considerably varied by changing the glass of 

 which its roof is made. 



These effects have an important bearing on certain cos- 

 mical questions which have lately been much discussed, and 

 should induce the greatest caution in forming opinions on 

 such subjects as "light and heat on the sun's surface, the tem- 

 perature of the planets, &c. This may depend as much upon 

 their physical constitution as upon their distance from the 

 sun. Indeed, the planet Mars gives us a highly probable ar- 

 gument for this ; for, notwithstanding that it is half as far 

 again from the sun as the earth is, the increase of the white 

 tracts at its poles during its winter, and their 'diminution dur- 

 ing its summer, show that the temperature of the surface of 

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