Drechsler, has for its proper object the testing of facts obtained by pot experiments or by other scientific methods, as to their direct applicability to practice. It can never be a means of investigation itself, but it is indispensable to a proper utilization of the results of investigation, 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 precautions 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 agriculture are nil. 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.
By Dr. MAX VON PETTENKOFER.
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 maladies 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 disease 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 Naëgeli, hardly weigh one milligramme! Infective material is derived partly from sick individuals, 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."