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IX. Notices respecting New Books. 

 A Chemical Catechism, in which the Elements of Chemistry, with the re- 

 cent discoveries in the Science, are clearly and fully explained. By 

 Thomas Johx Graham, M.D. Member of the Royal College of Sur- 

 geons in London, Sfc. &c. 

 THE author of the present work may be cited in addition to one we 

 have lately noticed, (in vol. v. p. 54,) as an example of the small 

 quantity of knowledge which will suffice for the trade of book-making j 

 and we shall, without further remark, proceed to develop the very 

 simple and easy means which have been adopted by Dr. Graham for 

 the completion of his purpose. With this intention we shall copy, 

 making observations as we proceed, the whole of the chapter on 

 Light, except the notes : this we prefer to others on account of its 

 brevity, and because we apprehend that it exhibits a fair specimen 

 of Dr. Graham's powers in the adaptation of means to ends. It is to 

 be understood, that except the questions, and those portions of the 

 answers, which we shall print in italics, the whole of this chapter has 

 been copied, literally, and without the slightest acknowledgment, 

 from the authors whom we shall name. 

 " What is the nature of light ? 



Different opinions are entertained on this subject. The philosopher, 

 Huygens, considered it is a subtile fluid filling space and rendering 

 bodies visible by the undulations into which it is thrown. Sir Isaac 

 Newton, on the contrary, considered light as a material substance, 

 consisting of small particles constantly separating from luminous bo- 

 dies, moving in straight lines, and rendering bodies luminous by 

 passing from them and entering the eye. — Dr. Thomson's System of 

 Chemistry, vol. i. p. 12. 



What is the space called through which light moves? 

 A medium : thus air and other gaseous substances are called rare; 

 while water, and transparent liquids and solids are termed dense 

 media.— Dr. Fyfe's Manual of Chemistry, p. 98; gaseous being sub- 

 stituted for aeriform. 



Does a ray of light pass in the same direction through rare and 

 dense media 9 



No : when it passes through the same medium or perpendicularly 

 from one medium into another, it continues to move without changing 

 its direction ; but when it passes obliquely from one medium to an- 

 other of a different density, it always bends a little from its old direc- 

 tion, and assumes a new one. It is then said to be refracted. When 

 it passes into a denser medium, it is refracted towards the perpendi- 

 cular; but when it passes into a rarer medium, it is refracted horn 

 the perpendicular.— Dr. Thomson's System, vol. i. p. 13 ; into being 

 substituted for to. 



H'liut happens when light strikes a polished opaque body? 

 It is reflected, and at the same angle at which it fulls on the po- 

 lished object. — Dr. Fyfe's Manual, p. 93. 



What is meant by certain bodies refracting doubly 9 

 When a ray of light passes through a crystallized body, provided 



tlic 



