BRITISH SCIENTIFIC ASSOCIATION. 471 



Emits, i, <?. could not diifer from 71 ° Fahr. by more than a single degree. Now, 

 from the Bible we learn that both plants were simultaneously cultivated in the cen- 

 tral valleys of Palestine in the time of Moses ; and its then temperature is thua 

 definitively determined. It is the same at the present time ; so that the mean 

 temperature of this portion of the globe has not sensibly altered in the course of 

 thirty-three centuries. 



a The future of physical science seems to lie in the path upon which three of 

 ■Our ablest British physicists have so boldly entered, and in which they have 

 already made such large advances. I may, therefore, be permitted briefly to 

 touch upon the successive steps in this lofty generalization, and to indicate the 

 goal to which they tend. It has been long known that many of the forces of na- 

 ture are related. Thus, heat is produced by mechanical action, when that is ap- 

 plied in bringing the atoms of bodies nearer by compression, or when it is ex- 

 pended in friction. Heat is developed by electricity, when the free passage of the 

 latter is impeded. It is produced whenever light is absorbed; audit is generated 

 by chemical action. A like inter changeability probably exists among all the other 

 forces of nature, although in many the relations have not been so long perceived. 

 Thus, the development of electricity from chemical action dates from the obser* 

 nations of Galvani; and the production of magnetism by electricity from the dis- 

 covery of Oersted. The nest great step was to perceive that the relation of the 

 physical forces was mutual ; and that of any two, compared together, either may 

 etand to the other in the relation of cause. With respect to heat and mechanical 

 force, this has been long known. When a bodyws cmnpressedby mechanical force 

 it gives out heat ; and, on the other hand, when itris heated, it dilates, and evolves 

 fower. The knowledge of the action of electricity in dissolving the bonds of 

 chemical union followed closely upon that of the inverse phenomenon ; and the 

 discovery of electro-magnetism by Oersted was soon followed by that of magneto- 

 Slectricity by Faraday. With reason, therefore, it occurred to many minds that 

 the relations of any two of the forces of nature were mutual — that that which is the 

 Cause, in one mode of interaction, may become the effect, when the order of the 

 phenomena is changed ; — and that, therefore, in the words of Mr. Grove, one of 

 the able expounders of these views, while they are "correlative," or reciprocally 

 dependent, " neither, taken abstractedly, can be said to be the essential cause of 

 the other." But a further step remained to be taken. If these forces were Dot 

 only related, but mutually related, 'was it not probable that the relation was also 

 a definite one ? Thus, when heat is developed by mechanical action, ought we not 

 to expect a certain definite proportion to subsist between the interacting forces, so 

 that if one were doubled or trebled in amount, the other should undergo a pro- 

 portionate change ? This anticipation, it has been already stated, has been real- 

 ized by Mayer and Joule. The discovery of the mechanical equivalent of heat 

 has been rapidly followed by that of other forces ; and we now know not only 

 that electricity, magnetism, and chemical action, in given quantities, will produce 

 each a definite amount of mechanical work, but we know further — chiefly through 

 the labours of Joule — what that relation is, or, in other words, the mechanical 

 equivalent of each force. The first step in this important career of discovery — 

 though long unperceived in its relation to the rest — was, undoubtedly, Faraday's 

 great discovery of the definite chemical effect of the voltaic current. The last 

 will probably be to reduce all these phenomena to modes of motion, and to apply 

 to them the known principles of dynamics, in such a way as not only to express 



