24 CHEMISTRY OF PLANTS. 



which he calls Protein ; and that the combination of this substance 

 with sulphur (lOPr + S) forms Casein; with phosphorus and sulphur 

 (10 Pr + 1 P +1 S), Fibrin ; and with more sulphur (10 Pr + 1 P 

 + 2 S), Albumen. There is no means of distinguishing these substances 

 in the cells of plants ; and they are all so variable in their peculiarities, 

 that they can only be regarded as groups of substances. Through 

 Liebig's observation*, tliat these substances cannot be formed in the 

 animal body, but must be taken into it from without, they have obtained 

 a new and peculiar importance. According to the researches of Rochleder 

 and Hruschauer (Liebig's Ann. vol. xlv. p. 253., and vol. xlvi. p. 348.), 

 these substances, when pure, have the power of acting as weak acids. 

 Very important in this relation is their constant union with alkalies and 

 earths, especially the phosphatic salts (perhaps double salts) in the 

 vegetable and animal organism. 



11. The substances mentioned in 9. constantly pass from one 

 into the other, and the presence of mucus in the cells appears 

 necessary for this object. They appear to go through, successively, 

 all the forms from sugar, the most soluble, to cellulose, the most 

 insoluble. 



From the preceding remarks it will be seen, that the substances men- 

 tioned in 9. are not so well defined forms of matter as sulphuric and 

 sulphurous acids, or as the protoxide and peroxide of iron, but that a 

 pretty constant series of changes occur in the passing of one substance 

 into the other. Artificially we may produce this series of bodies by 

 mixing them with mucus, or acting upon them with sulphuric acid or 

 alkalies, or even by slighter chemical processes, as repeated solutions and 

 evaporations. The property possessed by mucus, sulphuric acid, &c., of 

 producing chemical changes without themselves becoming changed, has 

 been called by Berzelius catalysis, by Mitscherlich the action of contact 

 (Contactwirkung), and by Liebig by another name, but without any ex- 

 planation. In the first place, we ought to satisfy ourselves that it is so. 

 In those plants where the first-named bodies are in contact with mucus, a 

 constant metamorphosis seems to be going on, and only rests for a short 

 time at one point. Almost all these changeable bodies are compounded 

 according to the same chemical formula, and vary sometimes in the 

 quantity of oxygen, but mostly in the quantity of water, they contain. 

 Does it not appear very probable that they possess a common basal prin- 

 ciple, and that, through varying proportions of water, and through phy- 

 sical conditions, as cohesion, &c., they assume so many appearances ? It 

 appears to me that there is here a great field for chemical inquiry. 



The mysterious property of the physical processes which is called life, 

 and which is supposed to depend on an especial vital principle, has been 

 made use of from the fact of certain chemical actions and re-actions 

 going on which have escaped observation, but which all allow to go on 

 in the commencing combinations. We know now with certainty, because 

 these changes go on out of the plant, the transition of cellulose into 

 starch, of starch into dextrin, of dextrin and cane sugar into grape sugar, 

 and of grape sugar into gum (as in the fermentation of beet-root juice). 

 All these metamorphoses, with the exception of the first, which is 

 effected only through sulphuric acid, can be produced by the agency of 

 nitrogenous substances (mucus). With great probability it may be con- 



* This is not Liebig's, but Mulder's, observation. TRANS. 



