544 THE PRINCIPLES OF SCIENC:^. [chap. 



Sir W. Thomson was enabled to anticipate the following 

 curious effect, namely, that an electric current passin<T in 

 an iron bar from a hot to a cold part produces a cooling- 

 effect, but in a copper bar the effect is exactly opposite in 

 character, that is, the bar becomes heated.^ The action 

 of crystals with regard to heat and electricity was partly 

 foreseen on the grounds of theory by Poisson. 



Chemistry, although to a great extent an empirical 

 science, has not been without p)rophetic triumphs. The 

 existence of the metals potassium and sodium was fore- 

 seen by Lavoisier, and their elimination by Davy was one 

 of the chief cxpcriyncnta crucis which established Lavoi- 

 sier's system. The existence of many other metals 

 which eye had never seen was a natural inference, and 

 theory has not been at fault. In the above cases the 

 compounds of the metal were well known, and it was 

 the result of decomposition that was foretold. The dis- 

 covery in 1876 of the metal gallium is peculiarly inter- 

 esting because the existence of this metal, previously 

 wholly unknown, had been inferred from theoretical con- 

 siderations by M. Mendelief, and some of its properties 

 had been correctly predicted. No sooner, too, had a 

 theory of organic compounds been conceived by Professor 

 A. W. Williamson than he foretold the formation of a 

 complex substance consisting of water in which both 

 atoms of hydrogen are replaced by atoms of acetyle. 

 This substance, known as the acetic anhydride, was after- 

 wards produced by Gerhard t. In the subsequent progress 

 of organic chemistry occurrences of this kind have become 

 common. The theoretical chemist by the classification of 

 his sjjecimeus and the manipulation of his fornnike can 

 plan out whole series of unknown oils, acids, and alcohols, 

 just as a designer might draw out a multitude of patterns. 

 J'rofessor Cayley has even calculated for certain cases the 

 ])0ssible numbers of chemical compounds.^ The formation 

 of many such substances is a matter of course; but there 

 is an interesting prediction given by Hofmann, concerning 

 the possible existence of new compounds of sulphur and 



• Tait's Thermodynamics, p. 77. 



2 On the, Arialytir.al Forms adled Treea, with Afplication to the 

 Tlieory of Chemical Gombinatiois. Rept)it of the BritLsh Associiiti^ia, 

 1875, p. 257. 



