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the whole horizon : it passes from body to bodys even the most densely transparent ^ with as much velo c ity as through void space . Its retrograde marches , its refractions and reflections from one body to another , present no apparent difference in velocity from that by which it comes from the sun . Its motion is entirely dissimilar to that of caloric . Light proceeds in straight
lines , with a velocity perfectly conformable to our idea of a power darting from planet to planet , and from , sun to sun . Caloric is as slow in its motion as the other is rapid : it passes equally by curved as by straight lines ; and its whole character is altogether unlike that © f a traveller into foreign lands . Its motion is not uniform ; there is a difference in the time it requires to pass through different bodies . In short , it appears rather to be dragged by the different attractions of body , than to fly with any inherent velocity of its own . "
The experiments of Pictet , which have been supposed to prove the projection of caloric in straight lines with extreme velocity , are not overlooked by our sagacious Philospher ; but even in this case he adduces some very cogent reasons for supposing that tight , is the agent in transporting it ; and indeed the circumstance of the temperature of the thermometer being raised by blackening its bulb , v ( ry much strengthens the supposition . Again ,
" If caloric be communicated by the nun , it is impossible to account for the great degree of cold which prevails in the upper regions of the atmosphere-. It is vain to talk of the power of transmission in order to account for it . No body in nature permits another for which it has a strong attraction to pass through , until it is at the point of saturation ; and if the strength of affinity be always in proportion to its distance from this point , it is absurd to suppose that the colder parts of the air shoufd communicate caloric to the warmer without supplying ; itself . This is contrary to all the laws of- affinity , and albo con-
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trary to the capacity of caloric for prc * serving an equality of temperature . " " This view of the nature of caloric is more correspondent with the general character of matter than those gencralljr entertained , ( t explains the grand principle by which it acts ; and , instead of making caloric an exception to all ^ the other kinds of matter , shews that its superior power is only owning to its more general attractions , and that it has no independent and inherent source qf action ' different from the rest . Among ; members of the same family , there is something- preposterous in the idea of
one being paramount to a 1 others , and having nothing in its nature common with them ; but by adopting this view we are enabled to trace its genealogy , and unite it with its brethren . It explains the whole phenomena of heat . It sti ' . l represents the sun as the great agent in the production of heat , ¦ without sup * posing it to be an enormous mass of fire > , the existence of nvhich woul < J neces arily destroy itself and all fiaturc with it . We have only to consider the sun as the great storehouse of light ; a power indeed the most active in nature ,
but noways destructive , like that terrible body , fire . It shew » that light produces heat merely by exciting an insensible ac ion betwixt caloric and the particles of matter contained in bodies . It accounts for the want of heat in the upper regions of the atmosphere by the want of sufficient matrer to produce the action of caloric ; and for the cold of the ni ^ ht and the polar regions , by the want of that action which the sun
produces . It destroys the absurd op :-nion concerning transmitted and conducted heat , so contrary to the invariable character of all the ^ material powers . It shews that caloric is not an exception to all the other kinds of matter , but that , like its fellows , it exists in other , haracters be ides heat . It enables us to solve the great difficulty concerning the distribution of heat ainon ^ the different planetary bodies ; according : to this
view , those nearest the sun may have no more than thoge at the most remo ' c distance . We have only to suppose the quantity of caloric to be proportioned to the distance ; and if a small quantity exists in Mercury , no more heat may be excited than is done by a large quantity in Saturn . " [ To be concluded in our next , J
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Physical and Metaphysical Inquiries . 159
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), March 2, 1807, page 159, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct2378/page/47/
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