Notices respecting New Books. 469 



above given, I think that the assumption of the universality of or- 

 dinary matter is the least gratuitous " (p. 124). This, however, 

 does not amount to much more than an allegation that this part of 

 the subject is obscure, and that for the present we must acquiesce 

 in the conclusion that, " before this obscurity can be perfectly 

 cleared up, we must know something of the ultimate or molecular 

 constitution of the bodies, or groups of molecules, at present known 

 to us only in the aggregate"*. 



The present edition differs in many respects from the first, not 

 in substance, but in the way of additional illustrations ; e. g. on 

 comparing the parts treating of Heat in the two editions, it will be 

 found that the original eight pages exist almost word for word in 

 the new edition, but are expanded into forty-four pages by addi- 

 tions, which serve, for the most part, to mark the progress of science 

 during the last thirty years. 



The second essay, on " Continuity," is probably known to inos^ 

 of our readers as the President's Address to the British Associa- 

 tion at Nottingham in a.d. 1866 ; its leading idea is thus enunci- 

 ated : — " One word will give you the key to what I am about to 

 discourse on : that word is continuity — no new word, and used in 

 no new sense, but perhaps applied more generally than it has 

 hitherto been. We shall see, unless I am much mistaken, tl at the 

 development of observational, experimental, and even deductive 

 knowledge is either attained by steps extremely small and form- 

 ing really a continuous march, or, when distinct results, appa- 

 rently separate from any coordinate phenomena, have been at- 

 tained, that then, by the subsequent progress of science, interme- 

 diate links have been discovered uniting the apparently segregated 

 instances with other more familiar phenomena. We shall see that 

 the more we investigate, the more we find that in existing pheno- 

 mena graduation from the like to the seeming unlike prevails, and 

 in the changes which take place in time gradual progress is, and 

 apparently must be, the course of nature " (p. 186). This idea 

 serves as a thread by which to connect the parts of an interesting 

 survey of the state of science and its recent progress at the time of 

 the composition of the essay ; but it is something more than this ; 

 it is a just and even a profound conception, though it is not sus- 

 ceptible of so exact a treatment as that of the Correlation of Phy- 

 sical ".Forces. Thus continuity may (perhaps) be predicated with 

 exactness of the different forms of animal and vegetable life. If 

 we had before us the whole series of past and present existences, 

 and could trace out the tangled meshes of the various lines of their 

 descent, we should probably see all existing gaps filled up and any 

 two given forms connected in some more or less complicated way 

 by a succession of intermediate forms differing almost imperceptibly 

 from each other and derived by descent from some primitive form. 

 Continuity in this sense is plainly something very different from that 

 in which planets, planetoids, and meteorites can be regarded as ex- 

 emplifying a continuous series. In fact there is no probability that 



* Thomson and Tait, 'Treatise on Natural Philosophy,' vol. i. p. 311. 



