MECHANICS AND USEFUL ARTS. 23 



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 there- 

 fore 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 stih 1 only a substitution of the former for one of the atoms of water 

 previously united together, two and two; and when we form ether, 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 meta- 

 phor of a building, although the original bricks which composed the structure 

 may have been all replaced by other materials, the latter, however differing 

 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 Prof. Williamson has propounded a new theory of 

 setherification, regarding the process as resulting from the alternate replace- 

 ment of hydrogen by sethyle, and of sethyle by hydrogen, in the sulphuric acid 

 concerned, a view which best harmonizes with the composition of the new 

 sether 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 hj'dro- 

 gen, 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 Hofmanu, and how happily it 

 serves to elucidate the formation of the various vegetable alkaloids, which, 

 from their energetic action upon the animal economy, 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 Prof. Williamson's sethyle theory would establish a still nearer 

 analogy between the constitution of organic and of mineral compounds than 

 is at present recognised, since in that case alcohol and ether would stand in 

 the same relation one to the other, and belong to the same class or series, as 

 the acids and their salts. 



ARTIFICIAL FORMATION OF NATURAL PRODUCTS. 



To some, however, it may be more interesting to consider those practical 

 results bearing upon the arts of life, which have either been actually deduced, 

 or may be anticipated as likely to accrue, from the discoveries in question. Of 

 these perhaps the most important is the possibility of forming by art those com- 

 pounds, which had been formerly supposed to be only producible by natural 

 processes, under the influence of the vital principle. The last two years have 

 added materially to the catalogue of such bodies artificially produced, as in 

 the formation of several species of alcohol from coal gas by Berthelot, that of 

 oil of mustard by the same chemist, and the generation of taurine, a principle 



