necessary by the “ Emission Theory This celebrated man 
entertained the view that light was produced by vibrations 
similar to those of sound ; but it was not till the era of Dr. 
Thomas Young that the theory of undulations had any chance of 
coping with the rival theory of emission ; so slow is the progress 
of truth against a current of error upheld by great names. 
Tor T must hold that the Emission Theory is false science, and 
the Undulatory Theory is the true explanation. 
30. Young was led to his discoveries regarding light by a 
series of investigations on sound. He rose from the known to 
the unknown, from the tangible to the intangible. 
31. I conclude then that heat is indeed a mode of motion, and 
as Sir Humphrey Davy said long ago, that “ it seems possible to 
account for all the phenomena of heat if it be supposed that in 
solids the particles are in a constant state of vibration, those of 
the hottest bodies moving with the greatest velocity ; and that 
in liquids and elastic fluids, besides the vibratory motion the 
particles move round their own axis with different velocities. 
This refers to three states of matter, the solid, — the fluid, the 
gaseous or aeriform ; but when heat becomes radiant we can only 
explain its complete analogy to light by supposing that motion 
is communicated to the particles of a luminiferous ether ” To 
this statement I shall have to return, but, before concluding 
the consideration of the ether in question I must request those 
gentlemen from whom on this point I venture to differ, kindly 
to remember that I do not consider that I have sufficiently 
proved the views to which I have given in my adhesion. My 
examination tends chiefly to show that the amount of 'proven 
scientific truth is much less than is supposed, and that the 
belief in scientific facts depends chiefly on the training which 
the mind has previously received. Thus it is probably the 
amount of attention which I have been compelled to give to the 
practical phenomena of chemistry which induces me to entertain 
convictions on evidence which I can only partially produce, and 
beg that it may be understood that their establishment is not the 
object of this paper. 
Part III. — Luminiferous Ether. 
32. The preceding remarks belong especially to the subject of 
this part, which, though rendered necessary to the completeness 
of my argument, involves me of necessity in a measure of 
controversial discussion. I have expressed my belief in the 
