the animal and means of seizing its prey and of its defence.  And speculations of this kind must be ranked with those belonging to some of the more superficial followers of the Newtonian philosophy, who explained the properties of animated nature by mechanical powers, and muscular action by the expansion and contraction of elastic bladders; man, in this state of vague philosophical inquiry, was supposed a species of hydraulic machine.  And when the pneumatic chemistry was invented, organic structures were soon imagined to be laboratories in which combinations and decompositions produced all the effects of living actions; then muscular contractions were supposed to depend upon explosions like those of the detonating compounds, and the formation of blood from chyle was considered as a pure chemical solution.  And, now that the progress of science has opened new and extraordinary views in electricity, these views are not unnaturally applied by speculative reasoners to solve some of the mysterious and recondite phenomena of organised beings.  But the analogy is too remote and incorrect; the sources of life cannot be grasped by such machinery; to look for them in the powers of electro-chemistry is seeking the living among the dead:  that which touches will not be felt, that which sees will not be visible, that which commands sensations will not be their subject.

Phil.—­I conclude, from what you last said, that though you are inclined to believe that some unknown subtle matter is added to the organised system by respiration, yet you would not have us believe that this is electricity, or that there is any reason to suppose that electricity has a peculiar and special share in producing the functions of life.

The Unknown.—­I wish to guard you against the adoption of any hypothesis on this recondite and abstruse subject.  But however difficult it may be to define the exact nature of respiration, yet the effect of it and its connexions with the functions of the body are sufficiently striking.  By the action of air on the blood it is fitted for the purposes of life, and from the moment that animation is marked by sensation or volition, this function is performed, the punctum saliens in the ovum seems to receive as it were the breath of life in the influence of air.  In the economy of the reproduction of the species of animals, one of the most important circumstances is the aeration of the ovum, and when this is not performed, from the blood of the mother as in the mammalia by the placenta, there is a system for aerating as in the oviparous reptiles or fishes, which enables the air freely to pass through the receptacles in which the eggs are deposited, or the egg itself is aerated out of the body through its coats or shell, and when air is excluded, incubation or artificial heat has no effect.  Fishes which deposit their eggs in water that contains only a limited portion of air, make combinations which would seem almost