114 The Inside s and the Wordings of Living Things 



protein to which sensitization has been brought about. 

 Then the normal serum remains clear, whereas the serum 

 from the sensitized animal becomes turbid. An insoluble 

 solid is formed which settles down; this is called a precipi- 

 tate. There is something present in the serum of the sensi- 

 tized animal, but not in normal blood. This something 

 combines with the protein in question to form the precipitate. 

 This unknown something is called a precipitin, 



'Precipitin Tests 



We may use a few drops of serum from the blood of a 

 sensitized rabbit to reveal the presence or absence of a sus- 

 pected protein. So sensitive is this precipitin test that a 

 blood stain on a piece of cloth can be positively identified 

 as being human in origin or hen's blood or whatever it is, 

 whereas the microscope or the methods of a chemist can tell 

 us nothing with certainty — except that there is a blood 

 stain. The blood stain is washed out with a few drops of salt 

 solution and a drop of this solution is added successively to 

 each of several glass tubes containing serum from several 

 sensitized animals. These animals had been previously sensi- 

 tized, each to a different kind of protein. For example, one 

 to hen's blood, one to horse, one to dog, one to human, and 

 so on. All of these samples of serum will remain clear ex- 

 cept the one that corresponds with the protein which is be- 

 ing tested; this will become cloudy. In legal practice this 

 procedure is often of great importance, and has been the 

 means of giving the court needed evidence as to the source 

 of a drop of blood. 



When this method is applied to different animals, we 

 find that the sensitive serum has what we may call a margin 

 of error. The blood of a wolf and the blood of a dog, for 

 example, might both show precipitations in the same serum 

 — there is a certain closeness of chemical nature between 

 the dog proteins and the wolf proteins. If we take samples 

 of other carnivorous animals and apply them to the serum 



