3S2 JT. D. Dana on Cohesive Attraction. 



Through the preceding pages I have intentionally avoided al- 

 lusions to the actual nature of molecules, as the conclusions are 

 independent of any views on that subject. Even form and size 

 are not essential to the deductions, as what has been designated 



ths of the axes, may be viewed as the inverse ratio of the 

 attracting force in the axial directions. If the existence of an 

 ether be insisted upon as surrounding the molecules, the relations 

 are none the less correct. 



The facts however prove that in the action of cohesive attrac- 

 tion there is a limit to penetrability, fixed in different directions 

 for given temperatures ; and this limit is essentially a limit of 

 form and size ; and as the phenomena of light are dependent on 

 molecular forces, we cannot deny to molecules color and other 

 qualities of sensible objects. 



The ether appealed to in order to explain the phenomena of 

 light, — admitted to have none of the qualities of matter and yet 

 often spoken of as a real existence, — is a kind of machinery, sum- 

 moned for the sake of an explanation ; and since we may now 

 believe that instead of such an ether capable of pulsations, pul- 

 sating molecular force itself will afford as perfect an explanation 

 of the phenomena of light, the necessity of the ether even as an 

 hypothesis is done away with* There is therefore no reason 

 from this source for doubting the conclusion that the forms of 

 molecules and their relative dimensions, as ascertained from crys- 

 tals, are their real forms and relative dimensions. 



The phenomena of heat as explained by received theories 

 seem to present a similar objection to the view we here take, since 

 an ether or a mysterious imponderable agent is supposed to inter- 

 vene between the molecules in the expansion of solids, and by 

 its arrangement to cause the change of axial directions. But the 

 assumption of this tertium quid gives us no aid in understanding 

 the change of axial direction, and the general law with regard to 

 attraction, on which we must fall back in either case, will be 

 much simpler without it. Neither, as my friend Mr. J. D. Whelp- 

 ley has argued, is this hypothesis necessary in order to explain 

 expansion. Since molecules may undergo all the various modi- 

 fications of condition and form which have been pointed out, it 

 is not improbable that they should also admit of change of size. 

 Size is known to be directly related to temperature : every de- 

 gree of temperature in a given substance is connected with a spe- 

 cific size. To effect a change of size in molecules, attraction^ 

 the same which has been shown to vary in concentration and otn- 

 er particulars, must also vary in radial force. The variations 

 which have been pointed out are caused by induction, according 



* See note, page 379. This principle, like the theory of gravilation wto* ' J* 

 presented, rose into view to simplify, just when theoretical science was Dec" 

 encumbered with rapidly increasing perplexities. 



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