152 M. MELLONI ON THE POLARIZATION OF HEAT. 



the piles, and traverse in a greater or smaller proportion the 

 sheets of mica. We will suppose, for the sake of perspicuity, 

 that each pile is composed of five laminaj, and that the planes of 

 these laminae are all parallel, vertical, and inclined 45° upon the 

 axis of radiation. After emergence, the pencil of heat passes 

 over another interval of from twenty to thirty centimeters, pene- 

 trates the envelope of the thermomultiplier, and, lastly, arrives 

 at the anterior surface of the thermoscopic pile which transmits 

 the impression received to the galvanometer. The indicating 

 needle of the instrument commences its movement, and describes 

 a certain angle, say of 35°'92. 



Before proceeding to experiments of polarization, we must 

 ascertain, 1 . That the heat absorbed by the sheets of mica has 

 no sensible influence upon the thermoscope ; 2, That the effect 

 observed is independent of the vertical direction of the planes of 

 the two piles during their parallelism. 



We may easily satisfy ourselves that these conditions are 

 really fulfilled in the circumstances of the experiment, by first 

 removing the thermoscopic body out of the space occupied by 

 the pencil of transmitted heat, without increasing its distance 

 from the last laminae of mica, and still keeping the opening of 

 its envelope directed towards them ; and then reinstating the 

 thermoscope in the direction of the calorific pencil, and turning, 

 by means of the drums, the two piles of mica quite round the 

 axis of the tube, without altering either their parallelism or 

 their inclination. In effect, in the first case the needle of the 

 galvanometer returns exactly to the zero of the dial*; and in the 

 second, it gives constantly 35°*92 of deviation. The heating of 

 the mica laminae and the assumed vertical direction of their 

 parallel planes have therefore no influence upon the results ; and 

 the deviation observed in any case of parallehsm is produced 



• In the supposed arrangement of the apparatus, the planes of the mica 

 lamhise are vertical : the axis of the thermoscopic pile, which at first formed an 

 anele of 45° with these planes, may, therefore, become perpendicular to them 

 during its lateral movement. The thermoscope then receives anteriorly this 

 same action, caused by the heating of the laminae, which was previously ex- 

 erted obliquely, and yet the needle of the galvanometer always returns to 0°. 

 Therefore, the proper radiation of the piles of mica does not produce any ap- 

 preciable effect. 



It is evident that this demonstration should be repeated each time that the 

 source of heat is changed, or its position relatively to the piles and the thermo- 

 scope altered. 



