On the Structure and Functions of the Organs of Respiration. 279 



the hills bounding the valley, and in situations quite beyond the reach of 

 any existing water course ; and in such positions the rock itself in situ is 

 often found to be similarly worn. 



Fragments nearly resembling these lie scattered over the carboniferous 

 limestone and oolitic downs of Bristol and Bath, are familiar to every one 

 under the name of rockwork, and are in great request among ornamental 

 gardeners. These difiFer slightly in figure from the sandstone rockwork, 

 because limestone being neither so hard nor so brittle as sandstone, admits 

 more readily of being pierced through and through in various directions, 

 and of being worn into a number of acute and grotesque points and figures 

 of which the friability of the sandstone renders it incapable. 



These holes should not be confounded with those smaller ones in flints, 

 produced generally by the washing out of some fossil of softer material 

 than its matrix. 



ON THE STRUCTURE AND FUNCTIONS OF THE ORGANS 



OF RESPIRATION, 



IN THE ANIMAL AND VEGETABLE KINGDOMS. 



( Concluded from p. 228.^ 



Having now glanced at the means by which the function of respiration 

 is accomplished, and witnessed their great uniformity, as well as their 

 universal diffusion, we may pass on to the inquiry as to the precise changes 

 effected by tliis process, on the animal and vegetable oeconomy. There is 

 now good reason to believe, that the analogy which we have seen to exist 

 between the forms of the respiratory apparatus in these two great classes, 

 holds good with regard to their function also. AVe well know that if an 

 animal be placed in a limited quantity of air, so much carbonic acid will 

 be produced, that life can no longer be supported ; and that if, on the con- 

 trary, we expose ^ plant to the sun, in an atmosphere charged with car- 

 bonic acid by combustion or the respiration of animals, the carbon will be 

 absorbed, and oxygen only will remain. At first sight, therefore, the 

 effects produced by respiration in the two kingdoms, appear quite contrary ; 

 and it has often been cited as a very beautiful provision of nature, that the 

 air vitiated by the respiration of one class of beings, should be renovated 

 by that of the other. The effect cannot be denied ; but another explana- 

 tion may be given as to its cause. 



The experiment just alluded to, which was first performed by Priestley, 

 does not give a fair estimate of the effects produced on the air by vegeta- 

 bles ; for it was soon afterwards proved by Mr. Ellis, that plants, not 

 exposed to light, absorb oxygen and give off carbonic acid j and a consider- 

 able doubt for some time existed, whether the quantity of carbonic acid 

 given off during the night and in shade, does not equal or even surpass that 

 which is absorbed in open day-light. The recent experiments of Dr. 



