ON THE STUDY OF PHYSIOLOGY. 69 



The only reason we have for asserting in any case that 

 any property belongs to any substance, is the certainty or 

 universality with which we find the substance and the pro- 



sion the effect of the fipplication of fire ; but, in all tliis variety of words, 

 we mean nothing more than our belief, that when a solid metal is sulyected 

 for a certain time to the application of a strong heat, it will begin after- 

 wards to exist in that different state which is termed liquidity — that, in 

 all past time, in the same circumstances, it would have exhibited the same 

 change — and that it will continue to do so in tlie same circumstances in all 

 future time. We speak of two appearances which metals present ; one be- 

 fore the application of fire, and the other after it ; and a simple but uni- 

 versal relation of heat and the metallic substances, with respect to these two 

 appearances, is all that is expressed. 



'* A cause, therefore, in the fullest definition which it philosophically ad- 

 mits, may be said to be, that which immediately precedes any change^ and 

 which-, existing at any time in similar circumstances, has been ahvays, and tvill 

 be always, immediately folloiccd by a similar change. Priority in the se- 

 quence observed, and invariableness of antecedents in the past and future 

 sequences supposed, are the elements, and the only elements, combined in the 

 notion of a cause. By a conversion of terms, we obtain a definition of the 

 corelative effect; and power, as I have before said, is only another word for 

 expiessing abstractly and briefly the antecedence itself, and the invariable- 

 ness of the relation. 



" The words property and quality admit of exactly the same definition ; 

 expressing only a certain relation of invariable antecedence and conse- 

 quence, in changes that take place, on the presence of the substance to 

 which they are ascribed. They are strictly synonymous with power ; or, at 

 least, the only diflerence is, that property and quality, e^s, commonly used, 

 comprehend both the powers and susceptibilities of substances — the powers 

 of producing changes, and the susceptibilities of being changed. We say 

 equally, tliat it is a property or quality of water to melt salt, and that it is 

 one of its qualities or properties to freeze or become solid, on the subtrac- 

 tion of a certain quantity of heat ; but we do not commonly use the word 

 power, in the latter of these cases, and say that water has the power of being 

 frozen." — " Power, property, and quality, are in the physical use of these 

 terms, exactly synonymous. Water has the power of melting salt : — it is a 

 property of water to melt salt ; it is a quality of water to melt salt :— all 

 these varieties of expression signify precisely the same thing — that when 

 water is poured upon salt, the solid will lake the form of a liquid, and its 

 particles be dilTused in continued combination through the mass. Two parts 

 of a sequence of physical events are before our mind; the addition of water 

 to salt, and the consequent liquefaction of what was before a crystalline solid. 

 When we speak of all the powers of a body, we consider it as existing in n 



