204 SCIENTIFIC APPARATUS. 



organic compounds, and those of Wurtz and of Hofmann upon 

 organic compounds containing nitrogen, led to the theory of types 

 which regarded the former set of bodies as derived from water, 

 and the latter from ammonia, by the substitution of radicals, or 

 groups of atoms of carbon and hydrogen, for the hydrogen of 

 the typical substances. The extent to which this substitution 

 may be carried is perhaps most conspicuously illustrated by the 

 researches of Hofmann on the ammonia derivatives, by which he 

 has shown that the hydrogen of ammcnia may be replaced 

 by other elements and radicals ; that the nitrogen may be re- 

 placed by phosphorus, arsenic, or antimony; and that in am- 

 monic chloride the hydrogen and nitrogen may be similarly 

 replaced, and the chlorine by bromine or iodine, the resulting 

 bodies possessing the characteristic properties of those from which 

 they are derived. All these results indicate a general similarity of 

 construction of compounds, and this has been considerably eluci- 

 dated by the doctrine of atomicity. The first indication of this 

 doctrine is contained at the conclusion of a paper on " A New 

 Series of Organic Bodies containing Metals," by Frankland, in the 

 "Philosophical Transactions" for 1852, in which he points out 

 that there is a limit in the power of certain elements in uniting 

 with others. Before this limit is attained the resulting compound 

 is still unsatisfied, and additional atoms can be assimilated, but 

 when the limit is reached the body becomes saturated and inca- 

 pable of further combination. This doctrine was further extended 

 to the carbon compounds by Koibe, and the general application 

 by Kekule of the idea thus originated has enabled chemists, 

 in a large number of cases, to give explanations of the atomic 

 constitution of chemical compounds. 



The application of these ideas to mineral chemistry has also 

 been made, notably by Frankland, and the results arrived at, now 

 that the atomic weights of the elements have been altered so as 

 to stand in a simple relation to their specific heats, exhibit a 

 remarkable degree of regularity. 



