CHAP. XI.] CONSERVATION OF ENERGY. 335 



To EEV. LEWIS CAMPBELL. 



8 Palace Gardens Terrace, 

 Kensington, W., 21st 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 purga- 

 tory. 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, 1 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 pro- 

 secutes 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 m 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 Leibnitz. 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 



