270 the president's address. 



simpler molecules, they usually present themselves to us in 

 aggregates which are large enough to be at once recognisable, 

 and one would naturally suppose the same to be true of the 

 multi-molecules, biophors, or whatever we like to call them, of 

 which living matter consists. As a matter of fact, however, the 

 chief objection that I can see to the Monera theory is the 

 almost ultra-microscopical size of the simplest organisms actually 

 known to us. Indeed, it' we take into account the so-called filter- 

 passers, or Chlamydozoa, which are believed to be the germs of 

 certain diseases, but most of which we know only by inference, 

 we are justified in saying that the simplest known organisms are 

 actually ultra-microscopic. 



It seems impossible to obtain any precise information as to the 

 size of the smallest particles that can be seen with the microscope. 

 Since this address was delivered, Dr. Spitta has been kind enough 

 to inform me that he has been able to see and photograph a 

 particle only 1/9 7,000th of an inch in diameter, and it will be 

 remembered that at a recent meeting of the Club Mr. Brown 

 claimed to have seen in the frustule of a diatom a pore the 

 diameter of which he estimated at 1/200, 000th of an inch. As the 

 filter-passing organisms are ultra-microscopic, they must be smaller 

 than this. Indeed, most of them have never yet been seen even 

 with the aid of the ultra -microscope, which, by a special method 

 of illumination, enables us to recognise the presence of particles 

 having a diameter of certainly not more than 1/2, 500,000th of an 

 inch and possibly a good deal less, though such particles cannot 

 be seen at all in the ordinary way by transmitted light. 



It is only by inoculation experiments that we can prove the 

 existence of these ultra-microscopic parasites. Thus we are told 

 that if even so small a quantity as 0'005 of a cubic millimetre of 

 lymph from an animal suffering from foot and mouth disease 

 be inoculated into a healthy calf, the latter will in due course 

 contract the same disease, although the lymph, so far as micro- 

 scopic examination enables us to judge, is entirely free from 

 organisms. 



Yellow fever, cattle plague, rabies and many other diseases are 

 believed to be caused by ultra -microscopic parasites. That such 

 diseases are due to living organisms and not to lifeless toxins is 

 indicated sufficiently clearly by the fact that a period of incu- 

 bation always follows infection, during which the poisonous matter 



