ELECTRIC LIGHT ELECTRO-MAGNETISM 511 the sparks are passed. Hence the electric light becomes an important means of chemical analysis. With carbon the lines are remark- able for number and brilliancy ; with silver they are intensely green ; with lead, of a violet tint ; and so on, varying with each different substance. Being producible in nitrogen, the electric light is not the effect of combustion, but of the intense heating and volatilization of ponderable matter. We say ponderable matter, because the electric spark cannot pass through a vacuum. A most intense and steady electric light is evolved be- Duboscq's Regulator. tween two points of coke, forming the poles of a battery, and brought into close proximity. Erom its great brilliancy and cheapness this light would seem to be well adapted for illu- mination, especially for lighthouses, and if in- troduced into mines it would apparently prove the most powerful illuminating agent, without tending, like other lights, to contaminate the purity of the air. But this light is deficient in the most penetrating ray of the spectrum, the red, and therefore does not produce the effect which was anticipated from it in the way of 289 VOL. vi. 33 penetrating fog. It is also so very intense, a large quantity of radiation being given off from a single point, that the eye is dazzled, and vi- sion becomes much more indistinct than with the same quantity of light given off from a lamp with large concentric wicks. It may however be used with much success in various optical experiments. But in this application one of the principal obstacles to be overcome is the continual separation of the charcoal or coke points as these are slowly consumed. Caution is required in experimenting with this light when of great intensity ; a single moment of exposure to the radiation from a battery of 600 couples produces violent headache and in- flammation of the eyes. For the purpose of illumination it is necessary to have the light constant and uniform. The galvanic current as well as the distance between the carbon points must not change ; and as the carbon slowly wears away, an apparatus is required to move them toward each other. The en- graving represents Duboscq's regulator. Both the points move, but with unequal velocities, so as to keep the light at a fixed point, the positive point wearing away twice as fast as the negative. The motion is produced by a coiled spring in the drum placed on the axis x y. This turns two wheels, a and &, which move the racks C and C', and by these the carbon points, one up and the other down. The current passes into the apparatus by the wire E and leaves it by E', without however passing through the rackwork, which is insu- lated. Before leaving it passes through the coil B, surrounding a piece of soft iron, which is a magnet while the current passes, and con- stitutes the regulator. When the carbon points approach too near the strength of the magnet is increased, and by means of a lever the rod d is made to turn in such a way as to stop the motion of a series of toothed wheels which are moved by the drum. In this way the mo- tion of the points is arrested until they have wasted enough to diminish the current, and consequently the strength of the magnet, when the stop is relieved, and the motion continues. ELETRO- MAGNETISM, that science which treats of the development of magnetism by means of voltaic electricity. In our article on electricity we have given an exposition of the facts of this branch of science, independent of any hypothesis as to the causes of the phe- nomena; but the present topic cannot be treated satisfactorily without giving some idea of the generalizations which have been in- vented to explain the phenomena, and to ex- press the laws of their mutual connection and dependence. It must be recollected that sci- ence does not consist in an accumulation of facts, but in a knowledge of principles, and it is impossible to arrive at a full comprehension of these principles without expressing them by means of some hypothesis from which logical deductions can be made, which will enable us at any time, independently of mere memory, to
