1 14 Dr. Brewster on new properties of heat , &c. 
1 cannot conclude this paper without expressing my obliga® 
tions to the Rev. Dr. Milner of Cambridge, for the very 
handsome manner in which he transmitted to me a quantity 
of thick plate glass, which I found it impossible to procure from 
any other quarter. I was thus enabled to obtain several new 
results, and to complete many experiments that had been left 
imperfect.* 
I have the honour to be, &c. 
DAVID BREWSTER. 
To the Right Hon. Sir Joseph Banks, Bart. 
G. C. B. P. R. a S. Sfc. Sfc. 8 fc. 
domestic purposes, should be carefully examined by polarised light before they are 
purchased. Any irregularity in the annealing, or any imperfections analogous to 
what workmen call pins in pieces of steel, will thus be rendered visible to the eye, by 
their action upon light. The places marked out by these imperfections, are those 
where the glass almost always breaks when unequally heated, or when exposed to a 
slight blow. Hence, glass-cutters would find it of advantage to submit the glass to 
this examination before it undergoes the operations of grinding and polishing. 
* Since the preceding letter was written and sent to Sir Joseph Banks, I have 
learnt that M. Seebeck has published in a German Journal for Dec. 1814, an account 
of some experiments similar to those contained in Sect. II. of this Paper. As there is, 
so far as I know, only one copy of this Journal in England, in the possession of Dr. 
T homson, I have not been able to obtain a sight of it, in order to compare M. See- 
beck’s results with mine. I understand, however, that he has discovered the fact, 
that a plate of red hot glass often acquires, in cooling, the depolarising structure, and 
that the tints depend upon the mode of cooling the glass. This result, however, has 
no connection whatever with the new properties of heat unfolded in the first Section 
of the preceding Paper, and does not anticipate the developement of the phenomena 
contained in the Second Section. The discovery of the new property of heat was made 
by me early in 1814, and an account of it was read before the Royal Society on the 
39th of May, 1814. See PbiL Trans . 18:4, p. 436. 
