RESPIRATION. 209 



tion to the degree to which, the exhaustion of air had been 

 carried. Those species of infusoria, which are most tena- 

 cious of life, lived in very rarefied air for above a month: 

 others perished in fourteen, eleven, or eight days; and some 

 in two days only. In this imperfect vacuum, they were 

 seen still to continue their accustomed evolutions, wheeling 

 in circles, darting to the surface, or diving to the bottom of 

 the fluid, and producing vortices by the rapid vibration of 

 their cilia, to catch the floating particles which serve as their 

 food: in course of time, however, they invariably gave in- 

 dications of uneasiness; their movements became languid, a 

 general relaxation ensued, and they at length expired. But 

 when the vacuum was rendered perfect, none of the infu- 

 sions of animal or vegetable substances, which, under ordi- 

 nary circumstances, soon swarm with millions of these mi- 

 croscopic beings,ever exhibited a single animalcule; although 

 these soon made their appearance in great numbers, if the 

 smallest quantity of air was admitted into the receiver. 



Animals which inhabit the waters, and remain constantly 

 under its surface, such as fishes, and the greater number of 

 mollusca, are necessarily precluded from receiving the di- 

 rect action of atmospheric air in its gaseous state. But as 

 all water exposed to the air soon absorbs it in large quanti- 

 ties, it becomes the medium by which that agent is applied 

 to the respiratory organs of aquatic animals; and the oxy- 

 gen it contains may thus act upon the blood with considera- 

 ble effect; though not, perhaps, to the same extent as when 

 directly applied in a gaseous state. The air which is pre- 

 sent in water is, accordingly, as necessary to these animals 

 as the air of the atmosphere is to those which live on land: 

 hence, in our inquiries into the respiration of aquatic ani- 

 mals, it will be sufficient to trace the means by which the 

 surrounding water is allowed to have access to the organs 

 appropriated to this function; and in speaking of the action 

 of the water upon them, it will always be understood that 

 I refer to the action of the atmospheric air which that water 

 contains. 



VOL. II. 27 



