UNDULATORY FORCES. ^ [INTBODUCWET. 



of the refraction of light, from the time of Galileo, wo are indebted for the accurate construction of our telescopes, 

 by means of which we can penetrate into space, and discover the laws of plim-tary motion, which, when applied by 

 the navigator, are of the utmost commercial importance to us. By means of the microscope, as another application 

 of the laws of light, we are enabled to detect differences of nature, quality, <fcc., utterly unapproachable by the nuked 

 eye. There is perhaps no law of nature which has so magnificent a range as that of the refraction of light, for in 

 its minute application we may detect the presence of individual insects, millions of which may be crowded into 

 ft space less than the size of a pin's point; and, in the next moment, may view by its agency groups of stars to count 

 which, is beyond the power of the most accomplished astronomer. Again, the generalisation of this law of light 

 is truly marvellous ; for we observe that the delicate colour of the flower, the gorgeous plumage of tropical birds, 

 the colours of the rainbow, and the infinite variety of colours seen throughout nature, are all produced by this one 

 cause. Proceeding to the polarisation of light, we enter into one of the most fascinating of all scientific pursuits, 

 and are enabled to detect the most brilliant and varied appearances which the imagination can depict, in bodies 

 which, to the unassisted eye, seem utterly destitute of beauty. 



Turning to another of the imponderable forces Electricity we find an agent which, although almost always 

 anperceived by our senses, is, nevertheless, only second to light in its universality : at one moment we find its 

 presence manifested in the vivid flash of lightning ; whilst, at the same instant, it is all but the animating power of 

 our existence, and is coursing unperceived through our nerves and muscles. In its applications, we have it at times 

 passing through wires, and enabling us, by its heating effects, to destroy masses of rocks as with the power of a giant ; 

 and yet, at our command, it silently travels by the path we choose for it, until, vibrating the little compass-needle, 

 it conveys instantaneously the expression of our wishes from one extremity of the world to the other commanding 

 the march of hostile armies, or conveying words of peace and good-will towards men. 



With respect to Magnetism, we need scarcely remind the reader, that, to an application of its laws, we are 

 indebted for the mariner's compass, by which our sailors may steer safely by day or night across the otherwise 

 trackless ocean. In the deepest darkness, without a star to be seen, in storm and tempest, the helmsman keeping 

 his eye on the oscillation of the needle, progresses boldly on his voyage, and pursues his course as free from error 

 as if a cloud of fire directed his path. With what a noble contrast does a modern voyage stand out when compared 

 with that of Ulysses, who, coasting round shore after shore in search of his beloved Ithaca, had nothing to guide him 

 in the absence of a clear sky. Besides the mariner's compass, there are various facts of the most interesting nature, 

 which have been brought to light by the researches of Faraday and others, as some of the effects of the magnetic 

 force. These have been arranged under the term Dia- Magnetism ; and we learn that nearly every substance in 

 nature can be made susceptible of the magnetic force, and not iron alone, as had previously been imagined. We 

 must, however, not be misunderstood as referring here to that pseudo-scientific nonsense termed Mesmerism, etc. , 

 wliich is merely the product of the ignorance and evil design of some men. The effect to which we refer, and of 

 which we shall fully treat hereafter, is the result of the agency of natural laws, wliich have been carefully studied 

 both as to cause and effect, by men whose names alone are a guarantee for truth and accuracy. 



We have hitherto refrained from mentioning the connection which each of the imponderable agents have with 

 others ; and the subject, of course, can only be dealt with hero in the most superficial manner. It will answer our 

 purpose, until we treat more fully on the matter in future pages, to remark, that each of the four forces, Heat, 

 Light, Electricity, and Magnetism, are mutually producible ; or, in other words, the presence of either agent may 

 be, or generally is, united with that of another. For instance, by heating a substance to a high temperature, it 

 becomes red or white hot, emitting rays of light just in proportion to the intensity of heat employed. By heating 

 a few bars of antimony and bismuth, arranged in a peculiar manner, electricity and magnetism may be produced. 

 Again, electricity continually produces heat, light, and magnetism, during its progress by conductors. Magnetism 

 not only produces electricity, <fcc. , but will, under certain circumstances, magnetise a ray of light ; whilst a ray of 

 light can be used to render a sewing-needle magnetic. 



We may remark, that with respect to all these forces, motion of some kind is necessary to their evolution ; and 

 that, in most cases, they exist in what is called a latent state. Tims, for instance, the well-known article of food, 

 sugar, may easily be made to afford heat, light, and electricity in a perceptible state, by means of chemical and 

 mechanical action. It is also remarkable, that whenever any of these forces is set free, motion is, generally 

 speaking, communicated to matter, as in heating, a body, which causes its parts to separate or expand. Motion and 

 these forces have therefore a mutual relation ; and philosophers have endeavoured to account for the existence of the 

 imponderable agents, and for the phenomena they produce, by supposing that an " ether," which is presumed to be far 

 more rare than matter, and to exist in the pores of bodies, is put into motion, and by its undulations, gives rise to 

 the varied effects we observe when heat, light, &c., become evident to our senses. Others have supposed, that each 

 of these forces is owing to the emanation of a peculiar kind of matter from bodies, wliich, by radiating from any 

 object, produces heating, lighting, and electric effects. We shall, however, not stop to discuss either of these 

 theories, because we must take them in detail hereafter, and endeavour to satisfy, as far as possible, in the present 

 state of scientific knowledge, all enquiries which may be made on tliis interesting question. 



We have stated, that most of the imponderable agents are mutually producible; but we must add, that 

 between two of them a most remarkable analogy exists in some respects. We refer to light and heat, the rays 

 of wliich can be both refracted, reflected, and polarised. Thus, if a heated or luminous body be placed in the focus 

 of a concave mirror, the rays of either force may be reflected from one, and received on another mirror placed at a 

 considerable distance from the first. The phenomena, as produced by either force, are found to be obe- 

 dient to the same laws ; and the only ditl'erouce we can detect, is in the nature or speciality of the force under 

 examination. 



