NATURAL HISTORY. 9 



of bodies bears some proportion to the resistance of the same bodies 

 in a solid form — e. g. the resistance of melted lead to that of water 

 bears some proportion to what solid lead bears to that of ice. 



Although matter in general may be rendered fluid, yet it is an un- 

 natural state, and requires other properties in matter to effect it. But, in 

 some, it is so easily effected, that the common operations in the general 

 system are in most parts capable of effecting it ; therefore we have 

 that substance, called water, preserved in that state almost everywhere 

 on the globe, or easily rendered so by art. 



Fusion implies solidity. If an impalpable powder is fused it will 

 form a solid when cold ; therefore, when quick lime is so heated as to 

 take on the properties of a fluid, yet it cannot be considered as such, 

 because it does not become a solid on cooling, but remains a powder as 

 before. 



Yapour is another state of matter where both the attraction of cohe- 

 sion and the centripetal is destroyed. [It is a state dependent on a 

 force] which may be called centrifugal or repulsive ; but most jnobably 

 is not [due to] a property inherent in the matter itself, but [to its] 

 being joined with matter which has this power in a great degree, as 

 fire. The resistance of vapour is as the quantity of matter hi a given 

 space, perhaps bearing no proportion to the solid form of the same 

 matter ; e. g. perhaps the vapour of mercury may make as little resist- 

 ance as that of water. Matter being thrown into vapour is only an increase 

 of the power which produced fluidity, so that the same principle which 

 produces the one effects the other. 



All matter may be reduced to such small parts as to float in any 

 medium ; for their cohesion to that medium may be increased so as to 

 destroy the power of gravity. 



Does solidity depend upon there being no matter situated between 

 the particles of matter, such as water, air, or heat ? 



We may observe that in Natural Things nothing stands alone ; that 

 everything in Nature has a relation to or connexion with some other 

 natural production or productions ; and that each is composed of parts 

 common to most others but differently arranged. Therefore, in every 

 natural production there is an appearance of affinity in some of the parts 

 of its composition [with those of another natural production] ; and 

 where there are the greatest number of these affinities or [correspond- 

 ing] parts, as also the closer the correspondence or affinity between 

 those of one production with those of another, the nearer are those 

 [natural productions] allied 1 . 



1 [The principle of ' unity of plan ' and of ' homologous parts ' is here ex- 



