ORGANIC SUBSTRATA OF THE ANIMAL ORGANISM. 29 



stances of the animal organism. We must, however, submit the 

 facts before us to a careful and critical enquiry, if we would employ 

 zoo-chemistry as the firmest support of physiological chemistry. 

 For there is scarcely any department of scientific enquiry in which 

 truth and error, suppositions and facts, acquired and presumed 

 results, and positive and hypothetical deductions, have been more 

 confounded. We need only refer to the fanciful trifling with che- 

 mical formulae which, from bearing the impress of the words and 

 symbols of an exact science, have deceived many unaccustomed to 

 such characters. The cause of the many erroneous views which 

 have passed from physiological chemistry to physiology and medi- 

 cine, mainly depends upon the inadequate knowledge of what is 

 necessary for the establishment of a formula for the chemical consti- 

 tution of a body. It seems, therefore, not wholly inappropriate, in 

 an introduction to zoo-chemistry, to refer to the points in pure che- 

 mistry, from which alone the chemist is able to deduce a formula. 



We might indeed draw some conclusions regarding the atomic 

 composition of a body from the mere result of one or more 

 elementary analyses, or, in other words, we might, from the per- 

 centage composition of a body, construct an empirical formula 

 which would serve to exhibit the relation of the separate elements 

 to one another. But this method can alone possess any scientific 

 value when, on the one hand, we are convinced that the substance 

 under consideration is chemically pure, and when, on the other hand, 

 after the former fact has been fully proved, the errors incidental 

 to every analysis are considerably smaller, (i. e. when the varia- 

 tions in the percentage results of the analysis are less,) than would 

 be afforded by any other formula than the one calculated. Such 

 variations by which an entire analysis may be rendered unavailable 

 are of common occurrence in the determination of hydrogen ; the 

 atomic weight of this element being so small that the slightest 

 variations in the percentage composition derived from the individual 

 analyses may cause the formula of a body to differ by one or 

 more atoms of hydrogen. Moreover, another reason why element- 

 ary analyses often exhibit the most marked variations in the quan- 

 tity of hydrogen, is that the drying of an organic substance is only 

 relative, and as many of these substances are extremely hygro- 

 scopic, it is impossible, even with the greatest care, to prevent them 

 from condensing water from the atmosphere during the process of 

 weighing. We call this drying relative, because in many substances 

 we are unable to determine at what degree of temperature, and after 

 what time they should be regarded as dried, as decomposed, or as 



