ETH 



EVA 



dium would therefore have the advantage 

 of simplicity in the original law of its ac- 

 tion, since the repulsive force, which is 

 known to belong to all matter, would be 

 sufficient, when thus modified, to account 

 for the principal phenomena of attrac- 

 tion. 



It may be questioned whether a me- 

 dium, capable of producing the effects of 

 gravitation in this manner, would also be 

 equally susceptible of those modifications 

 which we have supposed to be necessary 

 for the transmission of light : in either 

 case it must be supposed to pass through 

 the apparent substance of all material 

 bodies with the most perfect freedom, 

 and there would, therefore, be no occa- 

 sion to apprehend any difficulty from a 

 retardation of the celestial motions; the 

 ultimate impenetrable particles of matter 

 being perhaps scattered as thinly through 

 its external form as the stars are scatter- 

 ed in a nebula, which has still the dis- 

 tant appearance of an uniform light, and 

 of a continuous surface : and there seems 

 no reason to doubt the possibility of the 

 propagation of an undulation through the 

 Newtonian medium, with the actual ve- 

 locity of light. It must be remembered, 

 that the difference of its pressure is not 

 to be estimated from the actual bulk of 

 the earth, or any other planet alone, but 

 from the effect of the sphere of repul- 

 sion of which that planet is the centre; 

 and we may then deduce the force of 

 gravitation from a medium of no very 

 enormous elasticity. 



A similar combination of a simple pres- 

 sure with a variable repulsion is also ob- 

 servable in the force of cohesion ; and 

 Dr. Young, in his Lectures, remarks, that 

 supposing two particles of matter float- 

 ing in such an elastic medium, capable of 

 producing gravitation, to approach each 

 other, their mutual attraction would at 

 once be changed from gravitation to co- 

 hesion, upon the exclusion of the portion 

 of the medium interveningbetweenthem: 

 this supposition is, however, as he adds, 

 directly opposite to that which assigns to 

 the elastic medium the power of passing 

 freely through all the interstices of the 

 ultimate atoms cohering in this manner; 

 but that, as we see some effects so nearly 

 resembling them, which are unquestion- 

 ably produced by the pressure of the at- 

 mosphere, we can scarcely avoid sus- 

 pecting that there must be some analogy 

 in the causes. 



Two plates of metal, which cohere e- 

 nough to support each other in the open 

 air, will often separate in a vacuum. When 



a boy draws along a stone by a piece of 

 wet leather, the pressure of the atmo- 

 sphere seems to be materially concerned. 

 The well-known experiment of the two 

 exhausted hemispheres of Magdeburgh 

 affords a still more striking instance of 

 apparent cohesion, derived from atmo- 

 spherical pressure : and if we place be- 

 tween them a thick ring of elastic gum, 

 we may represent the natural equilibri- 

 um between the forces of cohesion and 

 of repulsion ; for the ring would resist 

 any small additional pressure, with the 

 same force as would be required for se- 

 parating the hemispheres, so far as to al- 

 low it to expand in an equal degree ; and 

 at acertain point the ring would expand 

 no more, the air would be admitted, and 

 the cohesion destroyed in the same 

 manner as when a solid of any kind is 

 torn asunder, 



But all suppositions founded on these 

 analogies must be considered as merely 

 conjectural; and our knowledge of every 

 thing which relates to the intimate con- 

 stitution of matter, partly from the intri- 

 cacy of the subject, and partly for want 

 of sufficient experiments, is at present in 

 a state of great uncertainty and imper- 

 fection. 



ETHICS, or MORALITY, the science of 

 manners or duty, which it traces from 

 man's nature and condition, and shews to 

 terminate in his happiness : or, in other 

 words, it is the knowledge of our duty 

 and felicity, or the art of being virtuous 

 and happy. See MORAL PHILOSOPHY. 



ETHULIA, in botany, a genus of the 

 Syngenesia Polygamia JEqualis class and 

 order. Natural order of Composite Dis- 

 coid ex. Corymbiferae, Jussieu. Essen- 

 tial character : receptacle naked; down 

 none. There are six species. 



ETYMOLOGY, that part of grammar 

 which considers and explains the origin 

 and derivation of words, in order to ar- 

 rive at their first and primary signification, 

 whence Quintilian calls it originatio. See 

 GRAMMAR. 



EVAPORATION, in natural philoso- 

 phy, is the conversion of water into va- 

 pour, which, inconsequence of becoming 

 lighter than the atmosphere, is raised 

 considerably above the surface of the 

 earth, and afterwards by a partial conden- 

 sation forms clouds. It differs from exha- 

 lation, which is properly a dispersion of 

 dry particles from a body. When water 

 is heated to 212, it boils, and is rapidly 

 converted into steam ; and the same 

 change takes place in much lower tem- 

 peratures ; but in that case the evaporn- 



