136 CONTEMPORARY SCIENCE 



mentation and putrefaction, and a little later of the mi- 

 crobic origin of disease, is obviously bound up with the 

 perfection of the compound microscope, and the useful- 

 ness of that instrument for the purpose is as obviously 

 bound up with the ultimate size of bacteria and related 

 organisms. And yet without the fortunate conjunction 

 of an optical device and the degree of magnitude of liv- 

 ing objects, we should still be groping in outer darkness 

 in the search for the origin of disease, and still strug- 

 gling with the phantoms of spontaneous generation. But 

 the great men who proved the connection between micro- 

 scopic life and the biological processes mentioned, in- 

 cluding the source of the infectious diseases, did more 

 than describe the phenomena revealed by the microscope 

 and otherwise. They established methods with principles 

 so clearly enunciated and rigidly based that it has been 

 found possible to penetrate into an inhabited territory in 

 which thus far the most powerful microscope has not al- 

 ways enabled us to discern the living forms. 



Thanks to their labors we know now, first, that the 

 faculty of setting up disease in successive individuals is 

 a property only of matter which can itself increase indefi- 

 nitely, and all matter thus constituted is possessed of life; 

 and second, that certain disease-producing parasites can 

 be separated mechanically from the soluble products of 

 their growth, by passage through earthenware filters, in 

 which the interstices or pores are smaller even than the 

 size of the microbes themselves. By varying the density 

 or porosity of these filters, we arrive at a way of roughly 

 estimating the size of the microbic cells. 



Thus it came about that in 1898 two German bacteriolo- 

 gists, LoefHer and Frosch, who were engaged on the study 

 of the very highly communicable foot and mouth disease 

 of cattle, "discovered that after diluting the contents of 

 the unbroken vesicles which arise in that disease, with 20 

 to 40 times their volume of water and passing them 

 through such earthenware filters, the filtrate not only 



