LIFE, CAEBON'S OUTSTANDING PROPERTY 71 



a filtrate is not a carrier of a disease, since the bacteria are 

 too large to pass through the filter. 



In spite of this evidence some scientists still hesitate to 

 call a filterable virus alive, chiefly on account of its diminu- 

 tive size. A vague opinion prevails in some quarters that 

 a living thing proper must have "organs" of some sort, 

 which means some visible feature about it: for example, a 

 skin. Not so long ago the requirements for what was really 

 deemed alive were set considerably higher than now. Thus 

 the famous chemist, J. Liebig, held that yeast is not a 

 living thing. He fought a long and bitter controversy on 

 this issue with his renowned contemporary Louis Pasteur, 

 who had a more enlightened insight. The basis for Liebig's 

 erroneous assumption was that the too simple make-up of 

 yeast cells did not conform with his rather fastidious idea 

 of what a living organism ought to be. But his error in 

 this instance has been demonstrated. At present, scientists 

 have become less dogmatic in that respect. So it is perhaps 

 permissible to state that those who refuse to recognize 

 viruses as living may later modify their opinion. 



The most important feature, however, about these most 

 minute living things is that their size approaches the size 

 of the smallest known particles of matter in general. It 

 has long been known that no material — alive or not — can 

 be indefinitely divided and subdivided. A limit is reached 

 at a diameter of about li0 oo,ooo of a millimeter. Thus, no 

 oil can be spread out on a large water surface thinner 

 than this; the thinnest gold leaves are of about the same 

 thickness. Numerous other observations show that no 

 material can be stretched, dissipated, or broken up by any 

 mechanical force into particles below that size. These tiny 

 indivisible particles are the molecules. That they actually 

 exist is shown by the fact that their size is found to be the 

 same if determined by independent methods. 



