250 THEORIES AND MODELS 



played an important role in other fields as well. In approaching 

 thermal phenomena we may imagine heat a subtle fluid ("caloric") 

 and— passing beyond the analogue— we imagine that fluid self -repel- 

 lent and essentially weightless. It is through this caloric theory that 

 John Dalton arriv^es at the set of conceptions that have developed into 

 the modern chemical atomic theory. Moreover, if we conceive "quan- 

 tity of heat" as measure of the volume of caloric fluid present, and 

 "temperature" as measure of its pressure or hydrostatic head, the 

 hydraulic analogue furnishes us with a suggestive explanation of 

 many simple thermal phenomena. Observe that this analogue and 

 explanation constitute the bridge over which Carnot travels to his 

 celebrated heat theorem, ultimately basis for the second principle of 

 thermodynamics. Note, finally, that electric phenomena (demanding 

 an exactly parallel distinction between charge and potential ) are also 

 first attacked successfully through the same analogue— today still 

 commemorated in the term "electric current." 



Of course the caloric theory is now rejected in favor of the kinetic 

 theory. It was an explanation that arose "only to be quelled," and 

 perhaps one must concede that this displacement was retarded by 

 the notable plausibility of the explanation. But surely this delay was 

 no disaster. Throughout its lifetime the caloric theory functioned as 

 a highly eflFective tool of scientific discovery— simply by supplying 

 an explanatory framework in which thermal phenomena become 

 readily conceivable. Ideas of corpuscularity perhaps present the most 

 striking general illustration of the immense heuristic power thus de- 

 veloped. Parmenides made bold to deny the reality of qualitative 

 change, on the ground that he found it essentially inconceivable. 

 Today we laugh at this apt illustration of the follies of rational- 

 ism. But observe that it is only by way of Democritus' speculative 

 explanation of qualitative change that we first acquire the power 

 to concei\^e and work upon it. Duhem, and others of the positivist 

 persuasion, pronounced anathema all atomic hypotheses (and all 

 like speculations about unobservables ) and declared vain illusions 

 the explanations provided by them. But those hypotheses have been 

 the prime instruments of an immense scientific progress demonstra- 

 bly no illusion. 



The model taken seriously. Ilutten emphasizes that the model 

 must not be taken as a complete "explanation." 



