NITROGENOUS HISTOGENETIC SUBSTANCES. 325 



yet have no guarantee that there is only one simple, organic sub- 

 stance deposited in the remaining mass of tissue ; and both micro- 

 scopic and microscopico-chemical investigations have rendered it 

 probable that several chemical substances are mechanically depo- 

 sited by the side of one another in many of the animal tissues, 

 as quartz, mica, and feldspar, occur together in granite, and 

 cellulose and the incrusting matter, in vegetable cellular tissue. 

 It is often impossible to determine whether, after treating animal 

 tissues with the more powerful solvents, the dissolved matter was / 

 originally only mixed with the undissolved, or whether it must be 

 regarded as the product of decomposition of a body having a more 

 complicated composition. 



We might perhaps succeed in exhibiting these substances in a 

 chemically pure condition, and in acquiring a more accurate know- 

 ledge of their chemical constitution, if they could only be united 

 with other substances in definite proportions, and admitted, if 

 possible, of a single neutral combination ; but such, unfortunately, in 

 very few instances is the case. Many, it is true, obviously enter into 

 chemical combination with alkalies, with the oxides of heavy metals, 

 and even with acids, but as these combinations are mixed with other 

 bodies and other compounds, we are hindered from establishing by 

 analysis any definite relation between any two of these substances. 

 Moreover, putting out of the question the alkaline and earthy salts 

 that are blended with them, we find that no definite conclusions can be 

 formed from the combinations of such animal matters with oxide of 

 lead ; for this oxide (which, with oxide of silver, we prefer to 

 the other metallic oxides, since it almost always forms anhydrous 

 compounds with organic substances, or compounds that can be 

 readily deprived of their water) is found to combine with these 

 bodies in more than one proportion ; these compounds are then 

 simultaneously formed^ and cannot be separated from one another. 

 The analysis exhibits more or less oxide of lead, according as 

 the neutral compound is mixed with more or less of the basic 

 compound. Hence we can readily understand the cause why 

 chemists have succeeded in so few instances in determining with 

 any certainty the saturating capacity and the atomic weights of 

 these animal substances. 



In the arrangement of these bodies we are again compelled to 

 have recourse to a physiological principle of classification, which 

 is the more admissible from the circumstance that chemistry here 

 affords us no assistance. Our deficient knowledge regarding the 

 chemical properties of the bodies included in this class, does not 



