MAXWELL RECEIVES THE "HISTORIC SKETCH" 215 



" 3. What was their chief end ? To show that the and Law of Thermodynamics 

 has only a statistical certainty. 



"4. Is the production of an inequality of temperature their only occupation? 

 No, for less intelligent demons can produce a difference in pressure as well as 

 temperature by merely allowing all particles going in one direction while stopping 

 all those going the other way. This reduces the demon to a valve. As such value 

 him. Call him no more a demon but a valve like that of the hydraulic ram, 

 suppose." 



Maxwell, writing on Dec. 23, 1867, acknowledged receipt of the 

 two-chaptered pamphlet in these words : 



" I have received your histories of Thermodynamics and Energetics, and will 

 examine them, along with Robertson on the Unconditioned who holds that our 

 ultimate hope of sanity lies in sticking to metaphysics and letting physics go down 

 the wind. I have read some metaphysics of various kinds and find it more or less 

 ignorant discussion of mathematical and physical principles, jumbled with a little 

 physiology of the senses. The value of the metaphysics is equal to the mathematical 

 and physical knowledge of the author divided by his confidence in reasoning from 

 the names of things. 



" You have also some remarks on the sensational system of philosophising (sensation 

 in the American not the psychological sense). Beware also of the hierophantic or 

 mystagogic style. The sensationalist says, ' I am now going to grapple with the Forces 

 of the Universe, and if I succeed in this extremely delicate experiment you will see 

 for yourselves exactly how the world is kept going.' The hierophant says, ' I do not 

 expect to make you or the like of you understand a word of what I say, but you may 

 see for yourselves in what a mass of absurdity the subject is involved.' 



"Your statement however seems tolerably complete considering the number of 

 pages. One or two ideas should be brought in with greater pomp of entry perhaps 



" There is a difference between a vortex theory ascribed to Maxwell at page 57, 

 and a dynamical theory of magnetism by the same author in Phil. Trans. 1865. 

 The former is built up to show that the phenomena are such as can be explained 

 by mechanism. The nature of the mechanism is to the true mechanism what an 

 orrery is to the Solar System. The latter is built on Lagrange's Dynamical Equations 

 and is not wise about vortices. Examine the first part which treats of the mutual 

 action of currents before you decide that Weber's is the only hypothesis on the 

 subject.... 



"You wrote me about experiments in the Laboratory. There is one which is 

 of a high order but yet I think within the means and powers of students, namely, 

 the determination of Joule's Coefft. by means of mercury. Mercury is (i3'S7/O'O33) 

 times better than water so that about 9 feet would give 10 F....[Plan described for 

 obtaining a vertical fall of mercury and measuring temperatures above and below]... 

 I think it a plan free from many mechanical difficulties, and in a lofty room with 

 plenty of mercury and strong ironwork, and a cherub aloft to read the level and 

 the thermometer and a monkey to carry up mercury to him (called Quicksilver Jack), 



