him in visiting the falls of Niagara, was the distance at which persons might be heard even when speaking low in this district there is always a very dense state of atmosphere. The density of transparent bodies has also an influence upon light, as is seen in the laws of refraction. And that the medium through which the sense of feeling is affected has considerable influence, may be proved by the different degrees of heat which can be borne when applied through the different media of water, vapour, and air: the body will bear about one hundred degrees of heat through water, one hundred-and-thirty through vapour, and about two hundred through the rarer medium of air.

Now, as animals are so much more easily affected through a dense than through a rare medium, they whose natural element is the dense medium of water, will require, to place them in a proper relation with the laws of heat, light, sound, &c., a much lower degree of sensibility than those animals whose natural element is the rarer medium of air. If, then, the operation from which sensibility results be the source of heat, and animals dwelling in the dense medium of water require low degrees

of sensibility, it results as a necessary consequence that but little heat will be generated, and hence those animals will have blood of so low a temperature as to be termed cold-blooded, and to them lungs would be useless. The nature and functions of these animals accord with all that has been advanced upon the uses of organization; they have a languid circula, tion, generate but little heat, consume but small relative quantities of food, and soon undergo the putrefactive process after death: and their assimilative organs bear a very small proportion to their bulk. [These facts may be observed in a contemplation of the nature of fishes.]

There is another class of animals capable of living both in the dense medium of water and in the rarer medium of air. The natural history and habits of these animals (amphibia) further support these opinions of the uses of the assimilative functions, the source of heat, and the use of the lungs. In these animals, the lungs have been considered (without sufficient reason) to be under the influence of the will, but whether this be the case or not, it is clearly a matter of fact that they are used when dwelling in the rarer medium of air, and fall into disuse

in the dense medium of water. This can hardly be allowed to arise from caprice in the animal, and it is still more difficult to admit it an useless arrangement of the GREAT DESIGNER of their being. But, keeping in mind the known circumstance of sensations being more easily excited through the dense medium of water, and consequently the lesser degree of sensibility required in that situation, and the further consequence of a lesser evolution of heat, it becomes easy to perceive that the lungs become quiescent in the water because their use is no longer required. But when those animals are subjected to extraordinary excitement, even in this dense medium, they are under the necessity of occasionally rising to the surface to cool by breathing, their heated blood, as is observed in the otter when hunted, and in the whale: this breathing is termed blowing. The higher degree of sensibility required in the rarer medium of air, and the consequent greater generation of heat, renders a regular and unremitting breathing necessary on land. [Thus we have a rational explanation of the phenomena of amphibious animals].

It is difficult to estimate the breathing structure of insects by an appeal to anatomy,

but experiment and reason prove the extensive permeability of their bodies to air, without which they cannot live; and it has been proved by Spallanzani, that they deteriorate a greater relative quantity of air than the larger animals. These animals, dwelling in the rare medium of air, must of necessity, from the smallness of their bodies, have great degrees of sensibility: they must consequently generate great heat, breathe extensively, consume great relative quantities of food, have large abdominal development, and they will be late in the chemical decomposition of their bodies after death. [These facts may be observed in contemplating the nature of insects].

There is another property of living animals now claiming our attention as essential to the knowledge of individual life, and a necessary preliminary to an extended understanding of the general principles of animal existence: this property is muscular motion-the subject of the next chapter.

CHAPTER VI.

Muscular motion-consists in the simple property of contracting is dependent upon nervous agencyElectric fluid is capable of exciting muscles to contract-Dr. Ure's experiment-Argument and facts in support of the identity of electric fluid and the natural cause of muscular contraction- Concluding remarks.

THE multiplicity of actions in living bodies may at first appear to involve this subject in great difficulties, but when it is known that the immense variety of motions-the slow progression of the large unwieldy animals, the elephant, rhinoceros, &c.-the amazingly rapid progression of some of the smaller animalsthe easy and graceful actions of the dancerthe sweet smile of satisfaction-the laugh of merriment—the quivering lip of fear-the sarcastic sneer of scorn-the beating action of the heart-the ever-labouring motions of breathing -the peristaltic movement of the stomach and intestines, the absorbents and lacteals-are the result of one simple property of flesh, this rough and discouraging prospect becomes a path of comparatively easy ascent. Of the