463 Notices respecting New feooks. . 



on these subjects be ultimately so effectually geueralized as to become 

 laws, they cannot avoid the necessityfor retaining different names for 

 these different affections ; or, as they would then be called, different 

 Modes of Motion. . . .Light, Heat, Electricity, Magnetism, Motion, 

 and Chemical Affinity are all convertible material affections ; assu- 

 ming either as the cause, one of the others will be the effect ; thus 

 heat may be said to produce electricity, electricity to produce heat, 

 magnetism to produce electricity, electricity magnetism ;' and so of 

 the rest. Cause and effect, therefore, in their abstract relation to 

 these forces, are words solely of convenience. We are totally un- 

 acquainted with the ultimate generating power of each and all of 

 them, and probably shall ever remain so ; we can only ascertain 

 the normas of their action ; we must humbly refer their causation to 

 one omnipresent influence, and content ourselves with studying 

 their effects and developing by experiment their mutual relations." 

 (Quoted in Preface to fifth edition.) These mutual relations are 

 the subject of an admirable experimental illustration which brings 

 the whole subject under view at a glance, and is thus described : — 

 "A prepared Daguerreotype plate is enclosed in a box filled with 

 water, having a glass front with a shutter over it ; between this 

 glass and the plate is a gridiron of silver wire ; the plate is con- 

 nected with one extremity of a galvanometer-coil, and the gridiron 

 of wire with one extremity of a Breguet's helix ; the other extremi- 

 ties of the galvanometer and helix are connected by a wire and the 

 needles brought to zero. As soon as a beam of either daylight or 

 the oxyhydrogen light is, by raising the shutter, permitted to im- 

 pinge on the plate, the needles are deflected ; thus, light being the 

 initiating force, we get chemical action on the plate, electricity cir- 

 culating through the wires, magnetism in the coil, heat in the helix, 

 and motion in the needles " (p. 28, 1st edit. ; p. 101, 6th edit.). We 

 may here pause to observe that this view leads to the conclusion 

 that light consists in the vibration of matter itself, and not of a di- 

 stinct setherial medium pervading the ordinary forms of matter. 

 This conclusion the author adopts ; and, in fact, he has argued the 

 point both in his lecture and still more fully in his essay ; indeed 

 his words are, "Although this theory has been considered defective 

 by a philosopher of high repute, I cannot see the force of the argu- 

 ments by which it has been assailed ; and therefore, for the present, 

 though with diffidence, I adhere to it " (p. 109). There can be no 

 doubt that there is great force in the remark that " at the utmost 

 our assumption, on the one hand, is that wherever light, heat, &c. 

 exist, ordinary matter exists, though it may be so attenuated that 

 we cannot recognize it by the test of gravitation ; and that to the 

 expansibility of matter no limit can be assigned. On the other 

 hand, a specific matter without weight must be assumed of the ex- 

 istence of which there is no evidence, but in the phenomena for the 

 explanation of which its existence is supposed. To account for the 

 phenomena the sether is assumed, and to prove the existence of the 

 aether the phenomena are cited. For these reasons, and others 



