256 Analogy between the Prismatic Spectrum, &c. [Oct. 



2. The reasons for this hypothesis arise of course from the cele- 

 brated experiments of Dr. Herschel and Sir H. Englefield ; but 

 particularly from the latter gentleman's statement of the degrees 

 of heat exhibited by the thermometer placed in the several prisma- 

 tic rays from btue downwards. As the intensity of light in the spec- 

 trum is found in the region analogous to the situation of the musical 

 third ; so, if we imagine the rays of heat to be in a kindred key 

 with light — to bear the same relation that the key of E minor bears 

 to the key of A minor in music — we may immediately see a reason 

 why the greatest heating power is without the range of the prismatic 

 colours ; for there is the situation of the third of E minor. It is 

 not so easy to account why the deoxidating power should be great- 

 est beyond the violet ray. I am inclined to think (from analogy 

 only, for I have not the means of making experiments) that the 

 commencement of the violet colour must be the seat of its greatest 

 energy ; and if this is not the case, I can only suppose that this 

 faculty increases with the refrangibility. 



3. All the metals that I am acquainted with (gold and copper 

 excepted) display when most devoid of oxygen a pale blue colour — 

 or that point in the spectrum where heat is first acknowledged by 

 the thermometer. By a reference to your valuable Table of Me- 

 tallic Oxides, it will be seen that they obey the order just pointed 

 out in a very decided manner; that as they lose their heat and ac- 

 quire weight by the addition of oxygen, so they descend in the order 

 of colours (supposing spectrum to follow spectrum), — not in the 

 same gradations ; for some appear to take surprising leaps, while 

 others glide through almost every shade. Lead is an example of 

 the latter; and steel (respecting the oxidizement of which Sir H. 

 Davy has lately satisfied himself) in the tempering fairly and cleanly 

 passes through and completes the octave: — I say octave, because 

 the last colour is light blue, from whence it started ; and all this 

 before it becomes the Hack oxide, the first stage :— to make the 

 peroxide, it has to descend again to red (repeating, perhaps, all 

 that progress which we were before enabled to view, and forming 

 the distance of what is called a twelfth in music from the black 

 oxide). Copper has been observed also to display a variety of tints 

 before the black oxide is confirmed : I doubt not but that they pro- 

 ceed in the same line of march. Gold presents us with a purple 

 oxide. 



Hoping I have rendered myself intelligible, but much fear- 

 ing that I have not, 



I am, Sir, yours most respectfully, 



David Huston. 



