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NATURAL HISTORY OF ORGANIZED BODIES. 



Ill returning' to the synthetic reproduction of the phenomena which accom- 

 pany life, I shall present but one other example of synthesis. The uses of the 

 natatory bladder of fishes have been very much controverted ; most naturalists, 

 liowever, have considered this organ as capable of modifying the volume of the 

 fish, and consequently its density, so as to render it sometimes lighter than the 

 water, thus causing it to ascend to the surface ; and sometimes heavier, thereby 

 enabling it to plunge to great depths. More recently, M. Moreau resumed the 

 stud}- of this subject, and pursued it much further than Ixad previouslj^ been done. 

 His attention was first arrested by the circumstance that a fish drawn at sea from a 

 great depth swells and sometimes bursts when brought to the surface of the water, 

 and in this condition floats helplessly, because it has become much less dense than 

 the water. The elastic force of the air of the bladder, resisted under normal con- 

 ditions by the weight of a column of water extremely high, 

 brings on a great distension of the animal if the pressure is 

 diminished, so that, having become lighter than the water, it 

 floats on the surface. Hence it follows that a fish which lives 

 normall}' at great depths in the sea cannot rise above a certain 

 altitude, under penalty of being borne to the surface by the 

 expansion of tlie gas of its air-bladder. And this theoretical 

 deduction involves a converse one : that the fish cannot descend 

 to a depth greater tlian that for which its natatory bladder is 

 adapted. If it ventures to a greater depth the gases of its 

 bladder will undergo greater comj)ression, the density of the 

 animal will be augmented, and it will be precipitated indefi- 

 nitely, even to the bottom of the sea; whence it can rise no 

 more, unless it could secrete within its bladder a quantity of 

 gas sufficient to distend it notwithstanding the enormous 

 pressure to which it is subjected. 



Theoiy teaches us, then, that a fish is not fitted to live 

 except at a certain depth; that it cannot all of a sudden 

 transfer itseif from a certain zone to which the state of its air- 

 bladder assigns it; that if it emerges from that zone in which 

 it possesses nearly the same density with the water, it must 

 be impelled indefinitely, whether to the surface or towards 

 the bottom of the sea. It ma}", moreover, be inferred that 

 the animal can within certain limits extend this zone to which 

 it is assigned, if it has the power of compressing or relaxing 

 its air-bladder ; that is to say, of modifying spontaneously its 

 own density, whether in one direction or the other. It is to 

 be understood, finally, that the fish has the faculty of con- 

 tending to a certain extent, b}^ the movements of its fins, 

 against the effects of its own density, and thus still further 

 enlarges the zone in which it can subsist. 



The whole of these theoretical deductions can scarcely 

 seem evident at the first glance, hence experimental control 

 Avould appear to be indispensable. AVe knoAv, by experience, 

 that a fish drawn Ixom a certain depth to the surface of the 

 sea floats in spite of itself; but the inverse phenomenon, a 

 fish precipitated to the bottom of the sea, is Avhat no one 

 has witnessed. Yet a very simple scheme will render this 

 phenomenon perfectly evident. The apparatus for this pur- 

 pose {Fig. 5) is analogous to the lud'ton, an instrument with 

 which we are familiar. It is formed of a bladder of caout- 

 chouc filled with air, and sustaininof a weio-ht o-i'aduated in 

 such manner as to give to the whole system a density equiv- 

 alent to that of water. This apparatus is placed in a glass 



lig./, 



