331 



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 curreut of error upheld by great names. 

 For 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 J' 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 



