384 RADIANT HEAT. 



On the 27th of March M. Matthiessen placed at the focus of a con- 

 cave mirror of one metre diameter an air -thermometer, which showed 

 a similar rise above its indication in other positions, when the axis of 

 the mirror was directed to the zodiacal light. 



He next ^substituted for the thermometer a thermo-electric pile 

 with great precautions; which, having its condensing cone 



Directed to the nucleus of the comet, gave a deviation of 2° 



Directed to the zodiacal light near its summit 10° 



Directed to the zodiacal light at base 12° 



Directed to the part of sky over the sun 3° 



In other directions 0° 



On removing the condensing cone to try whether the effect was due 

 to atmospheric causes, he still found towards the base of the zodiacal 

 light 2° or 3°. 



Instead of the mirror he next used a flint-glass lens, 56 centim. 

 diameter, 16 centim. focal length. 



With the thermo-electric pile, as before, this gave, 



Directed to zodiacal light, summit 2° 



Directed to zodiacal light, base 4° 



Directed to sky over sun ' 0° 



With a tallow candle at 10 metres distance (whether with the lens 

 is not stated, but probably with it, as the experiments would not 

 otherwise be comparable) — 



With the condensing cone, deviation 15° 



This, he observes, shows "combien est minime la quantite de 

 chaleur envoyee par la lumiere zodiacale, et que T influence de la 

 comete doit etre reellement imperceptible par notre temperature.' 7 



It was perhaps this somewhat ambiguous sentence which led Hum- 

 boldt to represent these experiments as showing no sensible effect due 

 to the zodiacal light. — (" Cosmos," Note 98, p. 394, vol. 1, Sabine's 

 translation.) 



In the face of the experiments, however, we must adopt another 

 interpretation; and perhaps what the author means is the distinction 

 between the radiant heat affecting the thermoscope, and the temperature 

 communicated to the atmosphere; which are manifestly different things. 



The experiment with the candle, if made (as seems to be implied) 

 with the lens, is an important verification of the fact of the heating 

 power belonging to light from terrestrial sources. 



The result with the zodiacal light also shows that at least a portion 

 of its effect is of this species, and not dependent on its mere loss of 

 heat as a hot body cooling. 



It is, however, but right to add, that indications of such extreme 

 delicacy as those here referred to have been looked upon by some 

 physicists as almost too liable to uncertainty to be entirely trust- 

 worthy. 



