LIFE, CARBON'S OUTSTANDING PROPERTY 75 



tion would have changed the properties of the material. 

 Since this was not the case, the only conclusion left is that 

 the virus is a chemically pure substance or, to put it in more 

 familiar language, that it consists of only one kind of matter. 



Stanley prepared his crystallized virus at first from in- 

 fected leaves of Turkish tobacco. Later he repeated his 

 experiments starting from "mosaic" infected tomato plants. 

 "Mosaic diseased spinach and phlox plants were next 

 studied and, although the yields were still less than for the 

 tomato plants, it was found possible to isolate the virus 

 protein from these plants," said Stanley in an article pub- 

 lished in 1937. In the same article Stanley describes the 

 appearance of the crystals obtained from tobacco mosaic: 

 "The suspension of the crystals has a very characteristic 

 appearance. When stirred it has a satin-like sheen. The 

 crystals are very small and may best be observed by means 

 of a microscope at a magnification of about 400 times. The 

 crystalline protein may be removed by centrifugation or 

 by means of nitration on filter paper. The crystallized 

 proteins obtained in this way had the same properties in 

 every regard, hence were obviously identical." (Figs. 28 

 and 29.) 



All these findings suggest that other types of virus might 

 prove to be similar in nature. In fact, by similar methods, 

 other members of The Rockefeller Institute, Drs. C. Ten 

 Broeck and Th. J. G. Wykoff, have prepared in crystallized 

 form a certain virus causing warty growths in wild rabbits 

 and still another virus which causes an inflammation of the 

 brain, usually in horses. These viruses causing animal 

 disease have been found to be somewhat larger than the 

 viruses of plant diseases, yet are still quite invisible. "All 

 of the facts available at present may be explained reason- 

 ably, simply and without effort, on the assumption that 

 the different virus proteins are in fact the different viruses," 

 Stanley concludes. 



