102 



DISCOVERY 



as big as the biggest filter-passer. We can, however, 

 see bodies as small as six-millionths of a millimetre 

 by appropriate means. \A'hen we look through an 

 ordinary microscope, we see objects as black specks on 

 a bright background — some of the rays of light from 

 the reflecting mirror are obstructed by the object 

 looked at. An alternative method is to transmit the 

 light at an angle through the liquid examined. In 

 that case every little particle will reflect ra\-s of light 

 to our eye, and we wUl see the object as a bright 

 speck on a dark background. It is by this method 

 that we can see the smallest objects yet described, and 

 it is obvious that a filter-passer is a giant compared 

 with the particles that may be seen. 



Filter-passers, then, are not too small to be seen. 

 The trouble is that when a fluid, such as a nasal 

 secretion from a dog with distemper, is looked at by 

 these special means, so much is visible that it is not 

 possible to distinguish the filter-passer from other 

 unimportant particles. 



Even a bacterium is not easy to identify if examined 

 in a state of nature. It must first be " stained " with 

 aniline dyes ; and the aniline dyes themselves consist 

 of relatively large particles. Staining filter-passers 

 has therefore not proved helpful — the particles of dye 

 and the organisms themselves are too nearly of the 

 same size. 



In spite of these difficulties, many claims have been 

 made that filter-passers have been both seen and 

 identified. In some cases — notably in measles — a tiny 

 bacterium, 2 fx in diameter, is described from America. 

 Again, in the pneumonia of cows a bacterium is showii 

 to pass a filter, and to be associated with the disease. 

 We see, therefore, that at least in some instances the 

 fact that the organism of a disease passes a filter does 

 not show it to be of necessity other than a bacterium. 



On the other hand, in distemper, influenza, and 

 rabies, to mention only a few diseases, the objects seen 

 do not resemble bacteria. They are tiny specks— that 

 is aU that can be said of them. 



Are Filter-passers Alive ? 



We must now turn to a rather important question. 

 How do we know that these little objects are alive ? 

 And if alive, how can we be sure that they cause the 

 disease ? It is probable that the answers to these 

 questions would not satisfy everyone. Bernard Shaw, 

 in the preface to The Doctor's Dilemma, thinks it 

 possible that all bacteria may be only sjTnptoms, and 

 not the cause, of disease. It is worth while to spend 

 some time in the consideration of the reasons given 

 for a belief in the life of a filter-passer, and its relation 

 to disease. 



Life is always difficult of definition. But one very 

 significant property of living matter is its power of 



reproduction. It is on the establishment of that 

 feature in filter-passers that the whole theory of their 

 nature rests. It is not enough to say that if we filter 

 a nasal secretion from a sick man, and then inject it 

 into another living creature and produce similar 

 symptoms, we have proved that there is a living 

 organism in the filtered fluid. All bacteria produce 

 poisons as the\' carry on their business of living ; 

 these can be filtered off and will produce severe symp- 

 toms. But if, having produced symptoms, we can 

 filter off another fluid, and produce sjTnptoms again 

 in this third animal, and so on many times over, we 

 must come to the conclusion that an organism is re- 

 producing itself on each occasion in the body of the 

 patient. No bacterial poison could produce symptoms 

 after dilution to many thousand times its original 

 volume, and this dilution is involved in the process 

 described. 



Artificial Culture 



Again, if we can grow the filter-passer, take a tiny 

 morsel of the growth, and grow it again and again on 

 suitable soil, we can prove that it is reproducing itself 

 and is alive. Both these processes have been carried 

 out %\ith many filter.-passers. But very special 

 methods must be adopted to cultivate a filter-passer 

 in a test-tube. It will onl}- grow out of reach of the 

 air ; only in special fluids, and only if a piece of 

 rabbit's kidney, or another organism, be included in 

 the tube. Volpino, an Italian scientist, has gro\m the 

 filter-passer which causes cow-pox by growing a harm- 

 less bacterium with it. Noguchi, a Japanese bac- 

 teriologist, was the first to grow a filter-passer, in 1913 

 — the organism of infantile paralysis. The influenza 

 filter-passer has been grown in America, in England, and 

 in South Africa, and at one time or another many other 

 successful attempts have been reported. There have 

 been many failures too. Even Polar bears cannot 

 rear their young in captivity ; the leprosy bacillus 

 has never been grown, and so we cannot be surprised 

 if filter-passers, too, do not seem to love unnatural 

 surroundings. They wilt and die, like rare orchids. 



Do Filter-passers Cause Diseases ? 



When Koch, the great discoverer of the tubercle 

 bacillus and the father of modern bacteriology, first 

 described his success, he laid down the law that in 

 order to establish a definite relationship between an 

 organism and a disease, the organism must be dis- 

 covered in the body of an animal into which it has been 

 injected, and which has suffered, as a result, from the 

 disease. This has proved verj' difficult in many 

 bacterial diseases. Leprosy has never been success- 

 fully transmitted in this way. It is doubtful whether 

 scarlet fever, in spite of many attempts with human 



