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combustibility; they are formed in large quantity bymany plants, and are very frequently seen adhering inlarge transparent drops to the bark of trees -which pro-duce them. Resins very frequently look very like gum,but they may readily be distinguished from gum by thefact that they are quite insoluble in water, whilst gumeasily dissolves or softens in that fluid (233). Mostof the common resins dissolve in spirit of wine.
249. There are likewise found a number of gum-resins, as they are called, which have some of the pro-perties of gum, and yet resemble resins; these aremixtures of gum and resin. They dissolve, though butimperfectly, in both water and spirit of wine.
250. Plants contain an immense variety of differentcolouring matters, all of which, however, when ana-lyzed, are found to consist of nothing but oxygen,hydrogen, and carbon, or these three with the additionof nitrogen. The nature and properties of these sub-stances are as various as their colours. They are alldestroyed by heat, which burns them, like all the othervarieties of organic matter. When burnt, the resultsare water, carbonic acid, and frequently ammonia.
251. Lastly, there are a number of peculiar vegetablesubstances, to the presence of which in plants theirmedicinal properties are in many cases due. Chemistshave obtained from various plants a great number ofsubstances called “ active principles,” some of which arehighly poisonous, whilst others in small quantities arevaluable medicines. Amongst this class of substances
g 2