106 PRODUCTION OF VINEGAR FROM WOOD, AND THE 



3 atoms water 27, = 51 *. In consequence of the frequent 

 expression of the composition of bodies in atomic quantities, 

 in this Course of Lectures, the pupils had by this time ac- 

 quired, almost insensibly, a knowledge of the nature of the 

 atomic or definite-proportional constitution of substances in 

 chemistry ; and that the foregoing explanation was satisfactory 

 to their minds, was evinced by the observations upon it, which 

 were involved in the questions, proposed by them, after the 

 Lecture in which it was given. 



A resume was finally given of the chemical nature of the 

 vegetable principles, as consisting, essentially, of carbon and 

 water ; and the difference between Proximate and Ultimate 

 elements or constituents, was explained, by reference, (among 

 other compounds) to alcohol, the proximate elements of which 

 appear to be olefiant gas and water, as before stated, while 

 the carbon, hydrogen, and oxygen, of which that olefiant gas 

 and water themselves consist, are its ultimate elements or con- 

 stituents; some particulars being also related of the more 

 complicated vegetable compounds. 



* The atomic constitution here assigned to lignin, is deduced from Dr. 

 Prout's analysis of that substance (Phil. Trans. 1827.). fr m which it appears 

 to be composed, as stated above, of equal parts of carbon and water. For 

 this deduction I am alone accountable, as I have not found an atomic com- 

 position assigned to lignin in any chemical work, though all chemists have 

 of late regarded it as a definite principle. 



It has often been remarked that Teaching'^ one of the most effectual modes 

 of Learning, that the endeavour to adapt any branch of knowledge to the 

 apprehension of those who are ignorant of it, frequently leads to the dis- 

 covery of truths previously unknown to the instructor himself. This proved 

 to be the case with rne in the present instance : the consideration and com- 

 parison of the atomic constitution of lignin and that of acetic acid, into which 

 I was led by the desire of explaining to my young auditors how it was that 

 vinegar should be so readily producible from wood, first led me to perceive 

 that a class of bodies might probably exist, in which the proportions of the 

 elements were the same, but their atomic constitution radically different. 

 How long the existence of such bodies may have been known to chemists, 

 without having been distinctly enunciated, I am not aware ; but since the 

 probability of their existence occurred to me in preparing these Lectures, in 

 the spring of 1830, Ber/elius has noticed this class of bodies, in his paper on 

 what he has called isomeric substances, or those which consist,apparently,not 

 only of the same proportions of their elements, but of the same respective 

 numbers of atoms of them, but which, at the same time, possess entirely 

 different properties, both physical and chemical. In that paper he has ex- 

 pressed an opinion that it will be expedient, in the further progress of 

 chemical science, to designate, also, the class of bodies I have alluded to, 

 by some distinctive and characteristic appellation. 



[t will probably be found necessary, in the further investigation of this 

 subject, to ascertain how far Dr. Prout's doctrine of Merorganization bears 

 upon it. It will be requisite, 1 apprehend, to distinguish those substances 

 which truly belong to the class in question, from those, which, being mcr- 

 orgamzed, belong to it in appearance merely. 



