22 Vital Energy of Plants. [Book VII L 



of the juices i and if there is, the veiTels which perform 

 it, we may reafonably believe, accompany each other 

 through every part of their co.urfe,' 



By what force the juices of plants are propelled in 

 their circulation, remains yet one of the fecrets of 

 nacure. It has been attributed to capillary attraction,. 

 bur this caufe leems inadequate to the effect j nor is 

 it poiTible on ihat principle to explain why the fap of 

 the vine flows from an incifion made in the fpring, 

 and not from one made in the fummer. The capillary 

 attraction ought not to be lefs powerful in the latter 

 than in the former fcafon j indeed it ought to be more 

 fo, as the heat is greater. Befides, capillary tubes do 

 not difcharge their contents when broken acrofs j but 

 from the ftem of a plant cut tranfvcrfelyj a large quan- 

 tity of fluid is difcharged. The more probable opi- 

 nion is, that plants are endued with fomeuhing of a 

 vital power or energy, which impels the juices through 

 the whole vafcular fyftem ; and this opinion is ftrength- 

 cntd by an obfervation of Dr. Bell, which wa* the re-, 

 full of experiment, namely, that there are particular 

 fubftanres which increafe the growth of plants, by 

 acting asftimilbnts on their fibres. 



The expeiiments of Dr. Prieftley have fufficiently 

 fhewn that vegetables have the power of correcting 

 bad air; and Dr. Ingenhouz has proved chat they nave 

 the faculty of producing vital air only when acted on 

 by the rays of light. If a vegetable is immerfed in 

 water, and the rays of the fun directed on it, air-bub- 

 bks will be obferved to collect on the leaves, and at 

 length rife to the furface of the water. This appear- 

 ance is moft remarkable in the morning, as the leaves 

 have not then been previoufly exhaufted by the action 

 of light. Vital air of a great degree of purity may 

 bfe obtained in the fummer time, by inverting a jar 

 i filled 



