230 NATURE OF 



fine ourselves to the consideration of the manner in 

 which the blood is probably altered. Although the 

 precise nature of the alteration induced cannot be 

 determined, it may be worth while to consider which 

 of several changes that may be caused by the growth 

 and multiplication of the disease germs is most likely 

 to be the actual change which is occasioned. 



In the first place, some substance existing in the 

 blood prior to its infection may be taken up, assimi- 

 lated, and thus removed by the growth and multipli- 

 cation of the germs. This matter being necessary to 

 their growth, it follows that by its removal from the 

 blood further growth of the germs, should more be 

 introduced, would be prevented, or, in other words, a 

 second attack of the disease becomes impossible. 



Secondly, it is possible that by the growth and 

 multiplication of the disease germs in the blood, some 

 new material may be produced which is destructive, 

 and thus, as long as this material remains, further 

 growth is prevented. 



Neither of the above suggestions is, however, 

 satisfactory, because the materials supposed cannot 

 be proved to exist chemically ; and in order to 

 account for " protection " being only temporary, we 

 must further suppose that the material which we 

 know nothing about, may according to the first hypo- 

 thesis, recur at a future time, and according to the 

 second disappear. 



But, thirdly, it may be supposed that from the par- 



