CHOLERA. 503 



Drechsler, has for its proper object the testing of facts obtained by 

 pot experiments or by other scientific methods, as to their direct appli- 

 cability to practice. It can never be a means of investigation itself, 

 but it is indispensable to a proper utilization of the results of investi- 

 gation, as well as in suggesting new directions for research. For these 

 purposes it can not be too exact, and it would be well if those who are 

 called to the conduct of such experiments would make themselves 

 thoroughly acquainted with the difficulty of obtaining results which 

 will endure careful criticism, and with the almost numberless precau- 

 tions necessary thereto. 



Other and ruder forms of the field experiment are omitted here. 

 They are, or may be, of much practical value when carefully made 

 and rightly interpreted, but their contributions to the science of agri- 

 culture are oiil. The two methods whose general features have been 

 described, however, are really means of scientific research. They are 

 laborious because the subject is a difficult and complicated one, but by 

 their conjoined aid we may hope to make sure if slow progress. The 

 thing of prime importance is a clear recognition of the possibilities 

 and of the limitations of each method. 



CHOLERA.* 



By De. max von PETTENKOFEE. 

 I. ITS HOME AND ITS TRAVELS. 



CHOLERA is an infectious disease. By infectious diseases are meant 

 those diseases which are caused by the reception from without of 

 specific infective material into healthy bodies, which material acts like 

 a poison. To the list of infectious disorders belong such different mal- 

 adies as small-pox and intermittent fever. Infective material differs 

 essentially from lifeless chemical poison in being composed of the 

 smallest possible units of living matter which when taken into healthy 

 bodies rapidly increase and multiply under certain conditions and by 

 their life-growth disturb the health of the body. These germs of dis- 

 ease belong to the smallest units of life, to the schizomycetes, which 

 lie on the border-land of the invisible, and which, according to their 

 form, are known as cocci, bacteria, bacilli, vibriones and spirilla, and 

 thirty millions of which, according to Naegeli, hardly weigh one 

 milligramme ! Infective material is derived partly from sick individ- 

 uals, in which case the disease is termed " contagious " and partly 

 from locality (earth), in which it has developed, in which case the 

 resulting disease is termed "miasmatic." It is obvious that when 

 derived from both sources the resulting affection was, and even now 

 is, designated " contagio-miasmatic." I am of the opinion that the 

 * Reprint of a special translation made for the London " Lancet." 



