TO PHYSIOLOGY AND PATHOLOGY. 41 



sciences, as well as in physiology, rest upon the conviction that similar laws, based 

 upon conditions in the properties of bodies reciprocally depending upon each 

 other, may be obtained. 



THE WAY TO ATTAIN TO A KNOWLEDGE OF THE RELATIONS OF 



DEPENDENCE. 



There is no other method in natural investigation by which we can arrive at a 

 knowledge of the relations in which the properties of bodies stand to each other, 

 than by first seeking to learn these properties themselves, and next, the cases in 

 which they vary. It is a law of nature, that the deviations in a property are, 

 without exception, accompanied by entirely similar changes in another property ; 

 and it is perfectly clear, that the knowledge of the laws of these deviations, will 

 place us in a position to decide concerning the one property, without any further 

 observation of the other. The knowledge of the one, will lead to the explanation 

 ef the other. 



A few examples will be sufficient to prove the truth of these statements. 



EXAMPLES OF THE LAWS OF DEPENDENCE-PRESSURE AND THE 



BOILING POINT. 



It is known that every fluid passes into a state of ebullition under the same 

 circumstances, and at unvarying degrees of temperature ; this is so constant, that 

 we designate the boiling point as a characteristic property of fluids. 



One of the conditions of the constant temperature at which air bubbles are 

 formed in the interior of these fluids, is external pressure ; the boiling point varies 

 with this pressure in all fluids, according to an especial law, increasing or dimi- 

 nishing with the increase or diminution of the pressure. Every boiling point of 

 temperature has a corresponding and definite pressure, and every pressure a defi- 

 nite temperature. It is known that an acquaintance with the law of the mu u;tl 

 dependence of the boiling point of the water, and the pressure of the atmosphere 

 has led to our being able, by means of the thermometer, to decide altitudes above 

 the level of the sea, and thus to measure one property by deviations in the other. 



THE BOILING POINT. 



The relation in which the boiling point of fluids stands to their properties is less 

 known. Pyroligneous spirit, alcohol, and the fousel oil of potato spirit are three 

 fluids, possessing very different boiling points. Pyroligneous spirit boils at 138 

 F., alcohol at 172 F., fousel oil at 274 F. The comparison of these three 

 boiling points shows that the boiling point of alcohol is 34 higher than that of 

 pyroligneous spirit (138 +34 = 172), while that of fousel oil is four times 34 

 higher (138 +4x34 =274). Each of these three fluids yields an acid upon 

 oxidation under like circumstances : pyroligneous spirit gives formic acid ; alcohol, 

 acetic acid ; fousel oil, valerianic acid. Of these three acids, each has its own 

 boiling point : formic acid boils at 210 F., acetic acid at 249 F., and valerianir 

 acid at 347 F. If we compare these three points, we find that they stand in the 

 same relation to each other, as do the boiling points of the fluids from which thesf 

 acids have been obtained. The boiling point of acetic acid is 34 higher than thai 

 of formic acid, while the boiling point of the valerianic acid is four times 34 higher. 



A similar deviation in one property shows, as we have remarked, a similar 

 deviation in another. The property to be considered here, is the composition. F 

 we compare the composition of the six several bodies (of the three acids, and th? 

 three fluids,) from which they originate by the influence of oxygen, we find as 

 follows: the composition of the pyroligneous spirit is designated by the formula, 

 C z H 4 O 2 ; that of spirits of wine by C 4 H 6 O 2 ; that of fousel oil by C 10 H 12 O,. 



