GUM AND MUCILAGE, 136 



"2. That when the proportion of hydrogen to that of ox- 

 ygen is greater than is necessary for the formation of water, 

 the substance is oily, resinous, alcoholic, or ethereal. 



3. That when the quantity of oxygen and hydrogen con- 

 tained in a substance is the same as in water, the substance 

 is analogous to sugar, gum, fibre, &,c. 



I shall in this work speak only of such products of vege- 

 tables as are most common, or of the most extensive use, 

 either for domestic purposes, or in the arts ; and I shall 

 endeavour as much as possible to follow the order prescribed 

 by the analogy of their constituent principles. 



ARTICLE I. 



Crum and Mucilage. 



Mucilage appears to be in the greater part of vegetables 

 the effect of the first change wrought upon the sap by the 

 §aws of vitality ; and the gums, which differ so little from it, 

 are generally formed upon trees by the extra;vasation of the 

 sap, during the period of most vigorous vegetation. This 

 first product of vegetation appears, however, to be perma- 

 nent through all its stages : the leaves of the marsh mallows, 

 the seeds of flax, lichens, and the bulbs of hyacinths fur- 

 nish it at all times ; so that it appears to be a constant and 

 inherent product of their composition. 



Gum exists in a liquid form in the cells of plants ; it 

 hardens by exposure to the air, loses a portion of its trans- 

 parency, experiences a greater or less change of color, and 

 becomes slightly brittle. Mucilage preserves its consistency 

 a longer time, though it has less affinity for water 



Gum and mucilage are soluble in water, from which they 

 may be precipitated by alcohol, and by sulphuric acid : they 

 burn with difficulty, and during ignition give out but little 

 flame, and produce a great deal of smoke ; their residuum 

 consists of bubbles of carbon. 



The gums that are most nsed in the arts, are gum Ara- 

 bic, gum Senegal, and the reddish gum of the country, 

 which forms in tears upon the branches and trunks of plum, 

 cherry, apricot, and many other trees. 



Crum and mucilage may he employed as food : mucilage 



