TO REV. LEWIS CAMPBELL.

                                                                 8 Palace Gardens Terrace,
                                                                 Kensington, W, 2lst April 1862.

     It is now a long time since I wrote half a letter to you, but I have never since had time to write or to find the scrap.   I suppose, as it was more than a good intention, but less than a perfect act, it may be regarded as destined to   paper purgatory. This is the season of work to you, when folks visit shrines in April and May, but I get holiday this   week. I have been putting together a large optical box, 10 feet long, containing two prisms of bisulphuret of  carbon, the largest yet made in London, five lenses and two mirrors, and a set of movable slits. Everything  requires to be adjusted over and over again if one thing is not quite right placed, so I have plenty of trial work to    do before it is perfect, but the colours are most splendid.

     I think you asked me once about Helmholtz and his philosophy. He is not a philosopher in the exclusive sense, as   Kant, Hegel, Mansel are philosophers, but one who prosecutes physics and physiology, and acquires therein not  only skill in discovering any desideratum, but wisdom to know what are the desiderata, e.g., he was one of the   first, and is one of the most active, preachers of the doctrine that since all kinds of energy are convertible, the first   aim of science at this time should be to ascertain in what way particular forms of energy can be converted into
     each other, and what are the equivalent quantities of the two forms of energy.

     The notion is as old as Descartes (if not Solomon), and one statement of it was familiar to Lebnitz. It was wholly   unknown to Comte, but all sorts of people have worked at it of late,—Joule and Thomson for heat and electricals,   Andrews for chemical combinations, Dr. E. Smith for human food and labour. We can now assert that the power  of our bodies is generated in the muscles, and is not conveyed to them by the nerves, but produced during the transformation of substances in the muscle, which are supplied fresh by the blood.

     We can also form a rough estimate of the efficiency of a man as a mere machine, and find that neither a   perfect heat engine nor an electric engine could produce so much work and waste so little in heat. We therefore   save our pains in investigating any theories of animal power based on heat and electricity. We see also that the   soul is not the direct moving force of the body. If it were, it would only last till it had done a certain amount of    work, like the spring of a watch, which works till it is run down. The soul is not the mere mover. Food is the   mover, and perishes in the using, which the soul does not. There is action and reaction between body and soul, but
     it is not of a kind in which energy passes from the one to the other,—as when a man pulls a trigger it is the   gunpowder that projects the bullet, or when a pointsman shunts a train it is the rails that bear the thrust. But the  constitution of our nature is not explained by finding out what it is not. It is well that it will go, and that we remain in   possession, though we do not understand it.

     Hr. Clausius of Zurich, one of the heat philosophers, has been working at the theory of gases being little bodies   flying about and has found some cases in which he and I don't tally, so I am working it out again. Several   experimental results have turned up lately, rather confirmatory than otherwise of that theory.

     I hope you enjoy the absence of pupils. I find that the division of them into smaller classes is a great help to me   and to them; but the total oblivion of them for definite intervals is a necessary condition of doing them justice at the    proper time.