INVISIBLE ASSAILANTS OF HEALTH. 807 



They increase with such amazing rapidity that, unless limited 

 by want of nutriment and favorable environment, a single species 

 would in a few years occupy the earth's surface to the exclusion 

 of all other life. 



Many species are of wonderful vitality and tenacity of life, 

 and resist the extremes of temperature, of boiling and freezing. 

 Others may be dried to an entire suspension of vitality for 

 months and years ; wafted here and there by the winds until, un- 

 der favoring circumstances, they renew their wonted activity. 

 Scattered on the snow of the hill-side, and carried down with the 

 spring freshet, miles away, they may be swallowed with the 

 water by some unfortunate individual, and perhaps prove their 

 presence and their source by inducing in him an infectious dis- 

 ease of their specific kind. 



The microbes may be captured, and cultivated on beds of gela- 

 tin, albumen, sugar, and in broth of meats ; and under skillful 

 management be made to furnish flourishing colonies and speci- 

 mens of the highest degree of development. Or they may be 

 starved and chilled to such helpless weakness and attenuation as 

 to seem to lose their specific characteristics. Those we are con- 

 sidering are among the most minute bodies within the possible 

 scope of microscopy. This, and their perfect transparency, have 

 heretofore seemed an insurmountable hindrance to our further 

 knowledge of them. But the discovery of their strong affinity 

 for certain of the intense coloring matters has been fortunate 

 and timely, furnishing a key to brilliant developments, since it 

 is found that certain species show a predilection for special col- 

 ors ; and a particular part of the microbe, as its membrane, or its 

 contents, may unite with the color, while other parts may totally 

 reject it — thus giving, not only outlines, but illuminated inter- 

 nal structure, otherwise invisible and unknown. 



The extreme minuteness, then, of these bodies has heretofore 

 been the bar and hindrance to our better knowledge of them. 

 But already we have been able to peer downward and inward, 

 from gross visible matter, through organs, tissues, cells, nuclei, 

 nucleoli, and granules, until, in the so-called structureless proto- 

 plasm, our present hunting-ground and limit, we seem to have 

 reached the confines of the inorganic molecule and atom, which 

 are subject to chemical instead of physiological law. The modern 

 discoveries in this microcosmic realm, and the demonstration of 

 the causative relation of micro-organisms to disease, upon which 

 the "germ theory of disease " depends, stands so conspicuously 

 as a scientific success, and is a step so important toward the alle- 

 viation of suffering, the prolongation of life, and enhancement of 

 human happiness as to be the subject of universal congratulation. 



In order to comprehend the importance of this subject, it must 



