ADDRES8. 



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to a particular model or type, which impresses upon the aggregate formed 

 certain common properties, and also causes it to undergo change most 

 readily, through the substitution of some other element in the place of one of 

 those which already enters into its constitution. 



And this principle, having been established with regard to one class of 

 bodies, has since been extended to the rest; for it now begins to be main- 

 tained, that in every case of chemical decomposition a new element is intro- 

 duced in the place of one of those which constituted a part of the original 

 compound, so that the addition of a fresh ingredient is necessarily accom- 

 panied by the elimination of an old one. 



The same doctrine, too, has even been extended to the case of combination 

 with a body regarded as elementary, for here also the particles are considered 

 as being in a state of binary combination one with the other, owing perhaps 

 to their existing in opposite electrical conditions, and therefore possessing for 

 each other a certain degree of chemical affinity. 



Thus, when we unite hydrogen with oxygen, we substitute an atom of the 

 latter for one of the former, previously combined with the same element. 

 The type therefore remains, although the constituents are different. 



When, in the formation of alcohol, we combine the oxide of the compound 

 radical sethyle with water, there is still only a substitution of the former for 

 one of the atoms of water previously united together, two and two ; and 

 when we form aether, we eliminate the second atom of water, and replace it 

 by another atom of the same compound radical. Thus the type of water 

 still remains, although none of the materials of the original fabric continue ; 

 or, if I may adopt the metaphor of a building, although the original bricks 

 which composed the structure may have been all replaced by other materials, 

 the latter, however ditfering in their nature, always correspond, in point of 

 shape, dimensions, and number, with the parts of the edifice which have been 

 removed to make way for them. 



It is on this principle that Professor Williamson has propounded a new 

 theory of aetherification, regarding the process as resulting from the alternate 

 replacement of hydrogen by sethyle, and of sethyle by hydrogen, in the 

 sulphuric acid concerned, — a view, which best harmonises with the composi- 

 tion of the new aether he hit upon in the course of his investigations. 



The same principle may even be extended to bodies of the same type as 

 ammonia ; for inasmuch as this body is made up of a union of an atom of 

 nitrogen with three of hydrogen, it is easy to conceive that a variety of 

 different compounds might be formed by the substitution of one, two, or three 

 atoms of other radicals for the same number of atoms of the original 

 hydrogen. How beautifully this idea has been carried out in the recent 

 researches of Hofmann, and how happily it serves to elucidate the formation 

 of the various vegetable alkaloids, which, from their energetic action upon 

 the animal ceconomy, have of late excited so much interest in the public 

 mind, is sufficiently known to those who are chemists, and could not be 

 rendered intelligible to those who are not, without entering into details which 

 would be out of place on the present occasion. 



I must not, however, pass over this part of the subject without remarking, 

 that the adoption of Professor Williamson's othyle theory would establish a 

 still nearer analogy between the constitution of organic and of mineral com- 

 pounds than is at present recognized, since in that case alcohol and aether 

 would stand in the same relation one to the other, and belong to the same 

 class or series, as the acids and their salts. 



These views, however, and others having reference to the same subject, 

 are now under discussion, and I hope in progress of being worked out bj 



