Itut then how- do we know which of these com pounds is ortho, which is pieta, and which is part)? In other words, being given three different compounds and three different formulas corre sponding to them, 110'X do we decide for mula is to be assigned to which compound? Several different methods are employed in decid In order, therefore, to determine whether a given di-substitution produet is ortho, meta, or the given compound may be transformed into a tri-substitution product. if it is then found that the tri-substitution product is capable of existing in two isomeric modifications, the con clusion is that the particular di-substitution product from which it is derived corresponds to the ortho-formula. Similarly, if the number of isomeric tri-substitution products is three, the corresponding di-substitution product is seen to he necessarily a meta-compound. Amid if only one tri-substitution product can possibly be ob tained from a given di-substitution product. the latter is concluded to be a para-compound.
OanANto REAcnoNs. Now, in reality it would he a tremendous undertaking to go through a demonstration of the kind just described, in the ease of every newly discovered compound. In many cases years would have to be spent in de termining the constitution of a single compound.
Chemists therefore make use of an additional working principle —a generalization made from the study of a large number of organic reactions. .,\ccording to this, when one compound is formed into another by some simple reaction. the molecul•—at least the principal part of unchanged. Once, therefore, the con stitution of a curtain number of substances has been determined, say, by Kt rner's method, the constitution of any new substance derived flout one of them by a simple reaction becomes evident by the very fact that it is derived limn a substance of known constitution, or by the fact that it may be transformed into a sub stance of known constitution: and it is in this mintier that the constitution of a great ma jority of carbon compounds hes been brought to light. Thus, if the di-chloro-benzene, COLCI„ which is shown by Khmer's method to be an ortho-compound, were transformed into dioxy benzene, the latter, too, would be assunied to be en ort ho-compound. It is east- to see that this principle is at the basis of Ki4 ner's method itself, and it may be considered is one of the fundamental principles of organic re search.