174 Reflections on Heat. 



them, we should be dazzled by an insupportable flood 

 of light, even in the darkest night. 



It is well known that, if the vibrations of a sonorous 

 body be less frequent than 30 in a second, or more 

 frequent than 3000 in a second, the undulations of 

 the air caused by these vibrations do not perceptibly 

 affect our organs of hearing ; and it is very probable 

 that the range of our organs of sight is still more 

 limited. 



When we have found strong reasons for suspecting 

 the existence of agents which fail to manifest themselves 

 to our senses, we ought to employ all our skill in devis- 

 ing means for compelling them to discover themselves 

 and to unveil the mysteries of their invisible operations. 



By means of an instrument which I have called a 

 thermoscope, and which is extraordinarily sensitive, I have 

 found not only that all bodies at all temperatures emit 

 rays, but also that the rays emanating from cold bodies 

 are as effectual in cooling warm bodies as the rays from 

 the latter are effectual in warming cold bodies. 



The principal part of the instrument of which I have 

 made use in these delicate experiments consists of a 

 long glass tube bent at both ends, and having at each 

 extremity a very thin glass bulb an inch and a half in 

 diameter. The middle portion of this tube, which is 

 straight, is placed in a horizontal position, while the two 

 end portions, whose extremities are the two bulbs, are 

 turned upwards in such a way as to form right angles 

 with the horizontal portion of the tube. The hori- 

 zontal portion is from 15 to 16 inches in length, and 

 each of the two end portions, which are vertical, is 

 from 6 to 7 inches long. The internal diameter of 

 the tube should be about half a line. 



