THIRD SERIES.— VARIABLE POLARIZABILITY OF HEAT. 183 



of M. Melloni's mica plates (4 inclies long and 2 wide), that they could be less than 

 one fifteen-hundredth of an inch thick each ; a pile of ten would then be ten 

 times as thick as my pile of equal energy, and at an incidence of 55° the thick- 

 ness traversed would not be much shorter than that of the mica plate alluded to 

 in art. 20, which we have there seen to be sufficient to obliterate all distinctive 

 character as to polarizability between an Argand-lamp and dark heat. 



23 Being now fully aware of the importance of the construction of piles of 

 mica which I had adopted, I thought it worth while to examine the proportions of 

 heat from different sources, which these very delicate laminae were capable of trans- 

 mitting, which, I presumed,* would be found far less variable than when plates 

 of the usual thickness are employed. My expectations were more than realized, 

 as is seen in the following Table, the second column of which shews the propor- 

 tion, to the whole incident heat, of that transmitted by the two mica piles I and K 

 placed parallel to each other ; by far the greater proportion of the loss being that 

 due to the obliquity of reflection and the number of surfaces, f By way of con- 

 trast, I have placed in the third column the proportion of the whole incident heat 

 transmitted at a vertical incidence by a plate of mica .016 inch thick. 



Rays out of 100 transmitted by 

 Source of Heat. Plates 1 and K parallel. Mica Plate .016 inch thick. 



Locatelli-lamp, . . . . , . . 18.8 57 



Ditto, with plate of glass .06 inch thick interposed, 16.2 72 



Incandescent platinum, ..... 17.6 50 



Dark hot brass (700°), 15.5 15 



Heat from boiling water, ..... 10. 8 



24. It is very evident that, for the first four sources of heat at least, the 

 transmissive power of the plates I and K varied little, and in no sort of propor- 

 tion to the characteristic action of mica even in moderate thicknesses. This will 

 be more evident, if we compare the ratios of the heat from different sources 

 transmitted in the two cases, taking the heat from the lamp sifted by glass as the 

 standard for each column. 



Plates I and K. Mica .016 inch. 



Locatelli with glass, 100 100 



Locatelli, 116 79 



Incandescent platinum, 108 70 



Brass at 700°, 96 21 



Heat of 212°, 62 11 



25. I need hardly add, that so remarkable a result as that the heat sifted by 

 glass should be less readily transmitted by the thin mica laminae, than the direct 

 heat from a lamp, was carefully verified. 



* I do not state this as a new idea ; it has been repeatedly remarked by M. Melloni, that, in pro- 

 portion as substances are thinner, they possess a more equable diathermancy for heat of different qualities. 



f The part of the effect due to reflection, T had previously established to be nearly the same for 

 different kinds of heal. 



