OF THE OX. Ill 



land and other countries, how, it will be asked, 

 is it that the disease has not broken out more 

 frequently, for it has never been seen in Great 

 Britain, at least, during the former part of the 

 nineteenth century? 



This question is not devoid of a certain 

 degree of importance, and deserves to fix our 

 attention for a moment. 



Now the conditions in which the animals 

 were exhibited in 1863 and 1864 were pre- 

 cisely the same as those of 1865, before the 

 outbreak of the disease ; and yet the contagion 

 has been possible in 1865, whilst it was not so 

 in 1863. 



We do not presume to explain the myste- 

 rious phenomena which govern the develop- 

 ment of epidemics and epizootics ; but it 

 seems to us not altogether impossible to give 

 a rational and satisfactory elucidation of the 

 facts. 



In general, in epizootics, and I might even say 

 in some particular epidemics in that of the 

 typhus, for instance three connected and in- 

 separable facts form the condition sine qua 

 non, of the generation of the disease. First, 

 a focus for producing the virus ; secondly, for 



