TO PHISIOLOGY AND PATHOLOGY. 15 



OBSERVATION. 



To see and perceive by the senses is a condition of observation, but sight and 

 perception do not characterize observation. 



Observation is not limited to seeing the thing itself, but likewise the parts of 

 which it is composed ; thus a good observer must perceive and seek to become 

 conscious of the mntnal^onn.ection existing between the several parts among 

 themselves, and c^nsiSeredwith reference to the whole. 



EXAMPLES OF ERRONEOUS OBSERVATIONS THE SUPPOSED INFLU- 

 ENCE OF THE MOON UPON THE FORMATION OF DEW. 



One of the most familiar illustrations of erroneous observation, is the influence 

 ascribed to the moon in reference to the cold felt in moonlight nights, and to the 

 formation of dew and hoar frost, while the moon in these cases is a mere spectator 

 of their formation. 



In a work, in other respects very good, published at Dresden last year, on the 

 influence of the moon upon earth, the following passage occurs : 



THE INFLUENCE OF THE ATMOSPHERE UPON EVAPORATION. 



" In the absence of an atmosphere we cannot conceive the existence of water, 

 or any similiar fluid in a liquid form. If our globe were suddenly deprived of air, 

 its rivers and seas must evaporate, and the whole earth would in a short time dry 

 up, as we see exemplified on a small scale by experiments under the air pump." 

 Here, as we see, a connection between the atmosphere and evaporation is presup- 

 posed, which does not exist in nature. Without an atmosphere it is true no clouds 

 would be formed, liquid water would not be converted into vesicles of vapor, and 

 aqueous vapor would not rise to so great a height; but the atmosphere has no 

 effect upon evaporation, and an equal quantity of aqueous vapor is produced under 

 the receiver of the air pump, whether or not the air be exhausted." 



DILUTION OF THE OXYGEN OF THE ATMOSPHERE BY NITROGEN. 



We find in many physiological works the view advanced, that the nitrogen of 

 the atmosphere contributes to the dilution of the oxygen, and the modification of 

 its action upon the organism ; while in fact the quantity of oxygen in a given 

 space would not in any respect be changed, were we to assume that the nitrogen 

 had suddenly been removed from the earth. Two gases varying in their nature 

 exercise a certain pressure upon the human body and the surface, with which it is 

 brought in contact; but the particles of the one gas do not compress those of the 

 other. If we take two bottles, one filled with nitrogen, and the other exhausted, 

 and bring them in contact by a glass tube, the nitrogen will distribute itself through 

 both vials; if again both bottles are of equal volume, both will contain an equal 

 amount of gas, and the same thing happens when one vial instead of being 

 exhausted is filled with oxygen at an equal pressure ; the nitrogen will distribute 

 itself in the bottle as if no oxygen were present the action of oxygen similar to 

 that of nitrogen. 



