528 Notices respecting New Books. 



and perhaps induce some of them to pursue their own scientific 

 education after leaving school." 



The book, as a whole, has been carefully drawn up ; but here and 

 there are traces of a want of skill in the mechanical part of book- 

 making : e. g. on p. 121 there is a mysterious and unexplained dia- 

 gram ; it looks like an egg arrested on its way through a tube con- 

 tracted in the middle. What it really means we cannot say ; but 

 we suppose it is intended to illustrate the difference between the 

 orbits of a planet and a comet, though the sides of the contracted 

 tube are not in the least like a hyperbola. We must also demur 

 to the statement on the same page, that the fact that " two bodies 

 subject only to their mutual attractions .... will move round one 

 another in some curved orbit," is Newton's First Proposition ; that 

 proposition concerns an " immobile centrum." 



Again, in the article on Chemical Energy, the author gives a 

 careful account of the action of a galvanic cell, explaining the 

 principal points and the meaning of the technical terms as he goes 

 on ; e. g. he explains what is meant by " closing a circuit ; " but sud- 

 denly the reader comes upon the statement that a " sufficient num- 

 ber of such cells will work and decompose water in the electrolytic 

 cell" (p. 102). What the " electrolytic cell" is he must find out 

 for himself from the context. 



Another point may be noticed ; and it is a fault (to speak bluntly) 

 which is commonly to be found in books of science: we mean 

 that of sticking in here and there arbitrarily chosen historical 

 notices. The research required for a correct exposition of the 

 principles of a given science is very different from that required 

 for giving a correct account of the history of its formation. A 

 writer who merely expounds principles is under no obligation to 

 go into the history of his subject ; but if he do, the main points 

 should be noticed with some degree of completeness ; or if he re- 

 strict himself to one or two points, it should be because their his- 

 tory is not well known, and his researches enable him to throw 

 light upon them. Mr. Heath adopts the more usual course : his 

 historical notices are put in quite arbitrarily, and the longest of 

 them runs off into a conjecture which is quite a curiosity in its 

 way. " The very oldest speculation on Physical Science that we 

 know of is the Doctrine of Thales of Miletus, about 600 B.C., that 

 water is the substance of all things. Perhaps, could we cross-exa- 

 mine him, his essential meaning might be found to be that all 

 things can, like water, exist in the fluid, the solid, and the aeriform, 

 vaporous, or gaseous state" (p. 78). We must own that we should 

 be a great deal surprised if such were in fact the result of the 

 cross-examination ; at all events we incline to follow Mr. G-rote, 

 who tells us that "Aristotle says little about Thales, and that 

 little in a tone of so much doubt that we can hardly confide in the 

 opinions and discoveries ascribed to him by others " (Plato, vol. i» 

 P. 4). 



