126 Mr. 0. Heaviside. On the Forces, Stresses, and [June 18, 



flame of an oil-flame cannot be altogether deprived of its yellow 

 luminous tip, without serious risk of total extinction ; and this faint 

 luminosity is sufficient to prevent pale caps from being seen. 



From further experiments made in the above testing chamber with 

 flames produced by alcohol and by hydrogen, it was found to be true in 

 practice, as might be inferred from theory, that, if the flame was pale 

 and practically non-luminous, the size and definition of the flame-cap 

 was augmented by increasing either the size or the temperature of 

 the flame. It is quite possible by attending to these conditions to 

 obtain a flame which, although it is very sensitive for low percentages 

 of gas, becomes unsuitable for the measurement of any proportion of 

 gas exceeding 3 per cent. This must, for the general purposes of the 

 miner, be looked upon as a defect; but it is not a fault of the lamp 

 already referred to. It is of interest to note that with the Pieler 

 spirit-lamp a flame-cap an inch in height was seen in air containing 

 only 0'5 per cent, of methane. 



V. "On the Forces, Stresses, and Fluxes of Energy in the 

 Electromagnetic Field." By OLIVER HEAVISIDE, F.R.S. 

 Received June 9, 1891. 



(Abstract.) 



The abstract nature of this paper renders its adequate abstraction 

 difficult. The principle of conservation of energy, when applied to 

 a theory such as Maxwell's, which postulates the definite localisation 

 of energy, takes a more special form, viz., that of the continuity of 

 energy. Its general nature is discussed. The relativity of motion 

 forbids us to go so far as to assume the objectivity of energy, and to 

 identify energy, like matter ; hence the expression of the principle is 

 less precise than that of the continuity of matter (as in hydrodynamics), 

 for all we can say in general is that the convergence of the flux of 

 energy equals the rate of increase of the density of the energy ; the 

 flux of the energy being made up partly of the mere convection of 

 energy by motion of the matter (or other medium) with which it is asso- 

 ciated localisably, and partly of energy which is transferred through 

 the medium in other ways, as by the activity of a stress, for example, 

 not obviously (if at all) representable as the convection of energy. 

 Gravitational energy is the chief difficulty in the way of the carrying 

 out of the principle. It must come from the ether (for where else 

 can it come from?), when it goes to matter; but we are entirely 

 ignorant of the manner of its distribution and transference. But, 

 whenever energy can be localised, the principle of continuity of 

 energy is (in spite of certain drawbacks connected with the circuital 

 flow of energy) a valuable principle which should be utilised to the 



