Thomson's Syatetn of Chemistry. 127 



different objects which surround him on all sides naturalhj en- 

 gage his attention. He cannot fail to be struck with their 

 number, diversity, and beauty ; and naturally feels a desire to 

 be better acquainted with their properties and uses." His 

 division of science into two great branches is awkwardly copied 

 from Professor Robison*; " the first comprehending all those 

 natural events, which are accompanied by sensible motions ; 

 the second, all those which are not accompanied by sensible 

 motions. The first is Natural Philosophy ; the second is Che- 

 mistry." " Chemistry, then," he subjoins, " is that science 

 which treats of those events or changes in jm^waZ bodies, which 

 are not accompanied by sensible motions." Are chloride, iodide 

 of azote, and the detonating metals, natural bodies ? And, are 

 fusion, evaporation, effervescence, combustion, explosion, S)C. 

 events, 7iot accompanied by sensible motions ? On the other 

 hand, the whole doctrines of statics are independent of sensible 

 motions. Change of interior constitution is the criterion which 

 distinguishes chemical, from mechanical, action. When this 

 change does not supervene, the event should be referred to 

 natural philosophy. Thus the study of doubly-refracted and 

 polarized light, belongs to physics ; that of the oxidating and 

 hydrogenating rays, to chemistry. A similar distinction ought 

 to be observed with regard to the distinct actions of both 

 electricity and caloric. 



" The science," says Dr. T, " therefore naturally divides 

 itself into three parts: 1st. A description of the component 

 parts of bodies, or of simple substances, as they are called ; 

 2d. A description of the compound bodies, formed by the union 

 of simple substances : 3d. An account of the nature of the power 

 which produces these combinations. This power is known in 

 chemistry by the name of affinity. These three particulars will 

 form the subject of the three foUoAving books." The above 

 enunciation constitutes nearly the whole of one head, entitled 

 Principles of Chemistry, which stands at the beginning of the 

 work. Now, one might certainly expect some general ideas on 

 the principles of chemistry, to usher in the details of simple and 

 compound substances, whose elimination and combination are 

 to occupy the first two volumes. Yet no preliminary explana- 

 tion whatever is offered, to guide the student through this 

 intricate maze. To supply this grievous omission, should he 

 have recourse to the third volume, where aflSnity is discussed, he 

 will derive little benefit ; because the subject is treated in a 

 manner unintelligible by the beginner, and unsatisfactory to 

 the proficient. Nothing can be more untrue, therefore, than his 

 assertion in the preface, of " its being better adapted to convey 

 a clear idea of the present state of the science, in all its bearings, 



* Fncyclnp(rdia Hritannica, Article Philosophv. 



