Prof. Daubeny’s Reply to Dr. John Davy. —- 249 
periment which I have made to show that conduction has no 
influence in producing the appearance of polarization. For a 
statement of the objection I refer to my note inserted in this 
Journal for November last. It is thus obviated: 
poe ewes 
ee 
T had a tin vessel constructed of the shape shown at A, which 
had one surface a similar in size and position to the first or 
polarizing plate used in my experiments, and of which the se-~ 
condary radiation to the analysing plate was supposed to pro- 
duce the variations observed. ‘The analysing plate B was 
laced between the thermo-electric pile P and the vessel A, 
which was filled with bozling water, and which therefore pro- 
duced on an enormously exaggerated scale the effects attributed 
to my mica plate. The vessel A was then turned into the 
various rectangular positions as regarded B, without any de- 
cided difference of effect on the pile being observable; indeed, 
if any, that effect indicated a maximum of heat reaching the 
pile when the position of the surfaces was unsymmetrical, or 
when in polarizing, it is least. This experiment was also re- 
peated for the case of polarization by reflexion. 
By a particular process (which I will take another oppor- 
tunity of describing,) I have been enabled to prepare mica 
plates, which, whilst they polarize more effectively than my 
former ones, are of extreme tenuity, so that they are almost 
incapable of becoming sensibly heated. With such plates I 
can polarize about 50 per cent. of heat wholly unaccompanied 
by light, and readily polarize the heat of boiling water. 
I am, Gentlemen, yours truly, 
Edinburgh, 12th Feb. 1836. James D. Forses. 
ee 
L. Reply to some Remarks contained in Dr. John Davy’s Life 
of Sir Humphry Davy. By Cuartes Davseny, M.D., 
Professor of Chemistry, 5¢-5 Oxford. 
Mr. Eprror, 
N Dr. Davy’s lately published book, entitled “* Memoirs of 
the Life of Sir Humphry Davy,” occurs a passage reflecting 
on myself, on which I feel myself called upon to offer a few re- 
marks. 
