264 THE VITAL FUNCTIONS. 



with their aeration ; for we find that insects, whicli 

 have but an imperfect and partial circulation of 

 their blood, still require the free introduction of air 

 into every part of their system. The necessity for 

 air is more urgent than the demand for food ; many 

 animals being capable of subsisting for a consider- 

 able time without nourishment, but all speedily 

 perishing when deprived of air. The influence of 

 this element is requisite as well for the production 

 and developement, as for the continuance of or- 

 ganized beings in a living state. No vegetable 

 seed will germinate, nor will any egg, even of the 

 smallest insect, give birth to a larva, if kept in a 

 perfect vacuum. Experiments on this subject have 

 been varied and multiplied without end by Spal- 

 lanzani, who found that insects under an air pump, 

 confined in rarefied air, in general lived for shorter 

 periods in proportion to the degree to which the 

 exhaustion of air had been carried. Those species 

 of infusoria, which are most tenacious 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 sur- 

 face, or diving to the bottom of the fluid, and pro- 

 ducing 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 indications 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 infusions of animal or 

 vegetable substances, which, under ordinary cir- 



