RETURN OF THE SAP. 37 



which arrives at the leaves, and which is termed 

 the ascending sap. 



The returning sap still contains a considerable 

 quantity of water, in its simple liquid form; which 

 was necessary in order that it might still be the 

 vehicle of various nutritive materials that are 

 dissolved in it. It appears, however, that a large 

 proportion of the water, which was not ex- 

 haled by the leaves, has been actually decom- 

 posed, and that its separated elements, the oxygen 

 and the hydrogen, have been combined with 

 certain proportions of carbon, hydrogen, nitrogen, 

 and various earths, metals, and salts, so as to 

 form the proximate vegetable products, which 

 are found in the returning sap. 



The simplest, and generally the most abundant 

 of these products, is that which is called Gum* 

 From the universal presence of this substance 

 in the vegetable juices, and more especially 

 in the returning sap, of all known plants, from 

 its bland and unirritating qualities, from its great 

 solubility in water, and from the facility with 

 which other vegetable products are convertible 

 into this product. Gum may be fairly assumed 



* According to the investigations of Dr. Prout, 1000 grains 

 of gum are composed of 586 grains of the elements of water, that 

 is, of oxygen and hydrogen, in the exact proportions in which 

 they would have united to form 586 grains of water ; together 

 with 414 of carbon, or the base of carbonic acid. This, accord- 

 ing to the doctrine of chemical equivalents, corresponds to one 

 molecule of water, and one molecule of carbon. Phil. Trans, 

 for 1827, p. 584. 



