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AS'TRONOMIGAL PAPERS. 



139 



quantity of heat produced in any body by fridion, de- 

 pends greatly on the body being fit to preferve the motion 

 once communicated. Thus a faw fixed in a hand vice fo 

 that it may long retain its tremulous motion, will foon be 

 heated, whilft the file with which it is rubbed is not foon 

 heated, being held in the foft unelaftic hand, whereby the 

 vibratory motion of its particles are immediately deftroyed. 

 The facility with which fome bodies are heated before 

 others, and with which the fame body may be heated in 

 one pofition rather than in another, abundantly prove that 

 the quantity of heat produced in anybody by fridion will 

 not be as the motion communicated, but as the flrokes 

 communicated, together with the number of vibrations re- 

 tained and communicated in confequence of each ftroke, 



2. The heat which is produced by chymical mixture 

 has been the fubjed of much fpeculation. — There are fun- 

 dry bodies which joined together produce confiderable heat 

 as water with oil of vitriol; others produce cold, as fait of 

 nitre with water. Why fhould one union produce heat 

 the other cold? It appears in general that all mixtures, 

 properly (o called, produce heat, all folutions produce 

 cold. But in every mixture the bodies undergo a certain 

 change m their qualities, whereas bodies undergo no 

 change by folution. This may point out to us the true 

 origin of heat in one cafe, and cold in the other, and the 

 pores of the one body are fo conftituted as that the minute 

 particles or atoms of the other body may penetrate into 

 them, a general di/Tolutioa of the conflituent parts of the 

 body'mufl enfue, the minute particles being rent afunder 

 by the attradive force of the parts; fuch diffolution of the 

 conftituent parts of a body neceilarily alters the qualities 

 of that body. We may eafily perceive that in the rapid 

 union of fuch bodies by which the minuteft particles are 

 rent afiinder, the vibratory motion of thofe parts muft be 

 greatly in creafcd. Hence the generation of heat by mix- 

 tures. Hence too the heat in fuch mixtures, feems to be 

 in proportion to the number of particles, which in any 



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